1185 49.Magnetic Magnetic Information-Storage Materials Info Charbel Tannous, R. Lawrence Comstock†

49.1 Magnetic Recording Technology...... 1186 The purpose of this chapter is to review the cur- 49.1.1 Magnetic Thin Films...... 1187 rent status of magnetic materials used in data 49.1.2 The Write Head...... 1189 storage. The emphasis is on magnetic materials 49.1.3 Spin-Valve Read Head...... 1192 used in disk drives and in the magnetic random- 49.1.4 Longitudinal Recording Media (LMR) ... 1199 access memory (MRAM) technology. A wide range 49.1.5 Perpendicular Magnetic Recording ...... 1205 of magnetic materials is essential for the advance of magnetic recording both for heads and me- 49.2 Magnetic Random-Access Memory ..... 1215 dia, including high-magnetization soft-magnetic 49.2.1 Tunneling Magnetoresistive Heads ...... 1218 materials for write heads, antiferromagnetic al- 49.3 Extraordinary Magnetoresistance loys with high blocking temperatures and low (EMR) ...... 1220 corrosion propensity for pinning films in giant- 49.4 Summary...... 1220 magnetoresistive (GMR) sensors and ferromagnetic alloys with large values of giant magnetoresis- References...... 1220 tance. For magnetic recording media, the advances are in high-magnetization metal alloys with large values of switching coercivity. A significant lim- recording in order to progress steadily toward areal itation to magnetic recording is found to be the densities well above 1012 bit=in2 (1 Tbit=in2 or superparamagnetic effect and advances have been 1000 Gbit=in2). While an MRAM cell exploits some made in multilayer ferromagnetic films to re- of the materials used in GMR sensors, its basic duce the impact of the effect, but also to allow component is the magnetic tunneling junction in high-density recording have been developed. Per- which magnetic films are coupled by a thin in- pendicular recording as compared to longitudinal sulating film and conduction occurs by quantum recording is reviewed and it is shown that this mechanical tunneling. The status of MRAM cell technology will soon be replaced first by heat- technology and some closely related key problems assisted and later by bit-patterned magnetic are reviewed.

The purpose of this chapter is to summarize the sta- that we are about to reach 1 Tbit=in2, a value that is con- tus of magnetic materials used in high-capacity disk sidered as a formal limit to (perpendicular) recording

drives and magnetic-semiconductor memory devices. and requires a paradigm shift in order to keep further 49 | E Part The technology of disk drives is selected since these increasing areal density as explained below. devices have experienced the largest increase in data The total data capacity of a disk is approximately capacity over time and this has made disk drives the the areal density times the recording area, which is preeminent storage system for digital data. To illustrate twice the number of platters since both faces are used this point, consider Fig. 49.1, which is a plot of the areal and depends on the size of the disk (2:5and3:5in(64 density (number of data tracks per inch times the num- and 90 mm)) diameter being the most common. ber of bits per inch recorded on a track) for disk drives For a 3:5 in disk, a simple rule of thumb implies that over time [49.1]. 900 Gbit=in2 areal density means about 900 Gbyte stor- The increase in areal density is more than 100% per age per platter face. year up to about 2002, when it reduced to about Many technologies have contributed to this rapid 2030%, then increased steeply by 600% between increase in areal density, including advances in the tech- 2006 and now evolving from 150900 Gbit=in2,the nology of flying heads with reduced spacing to the current value at the time of writing. Practically it means disk surface, data codes and error detection and cor-

© Springer International Publishing AG 2017 S. Kasap, P. Capper (Eds.), Springer Handbook of Electronic and Photonic Materials, DOI 10.1007/978-3-319-48933-9_49 ]. ]. Fig- 2 5 ](1or0). 5 , 4 is a schematic of a digital LMR system. The 49.2 The discussion covers longitudinal magnetic recor- Consequently, the operating system and application MRAM is a possible replacement for the famil- MRAM technology combines a magnetic storage In the case of disk drives the write and read heads ding (LMR), perpendicular magnetic recordingand (PMR) the transitionincrease process in between areal themremanent densities allowing magnetization in an is hardsurface disks. perpendicular reducing In to the PMR impact thelimit. the of disk Technology the underlying superparamagnetic MRAMdom-access memory) (magnetic is ran- also discussed sincenonvolatility it to provides RAM, blurringing the permanent storage borderline and separat- RAM. programs will always be present in RAMputer’s since very the com- first boot, inthe sharp contrast electronic with RAM respect to whoseevery contents time totally the disappear computerjammed and is not switched responding. off or when itiar is semiconductor memories used indynamic microcomputers – and staticmodules are random-access placed on memory theto computer (DRAM central motherboard processing close unit (CPU) whereas staticaccess random- memory (SRAM)memories). is used inside CPUtechnology cache together with(MOS) metal-oxide devices semiconductor to resultmemory in devices. fast The and technologynetic high-density on part data which of thetechnology MRAM mag- used is in based magneticmagnetic is recording tunneling an devices junction extension – (MTJ).ogy of the will MRAM the also technol- be discussedwork in have been this previously chapter. Parts publishedMaterials of in Science: this the Materials Journal in of Electronics [49. recording (write) and read elementswith are the shown magnetic recording together surface, whichtechnology in disk is drive a thinbe metallic discussed). The film digital of data are anetic recorded film cobalt in as the alloy transitions mag- (to betweenof the the two possible magnetization states (pointing towith the the width left approximately or equalwrite right) to and the head width and of the the width of a data track. The transi- are separate thin-film structures depositedof on a the mechanical slider, back whichbearing uses to a fly hydrodynamic over air the surface of the disk [49. ure store digital data, in whichthe case write the head current is suppliedresent in to the the digital form data of [49. pulses encoded to rep- Deskstar 120GXP Ultrastar 73LZX 100% CGR × increase is the number Production year 60% CGR N 17 Million ~ is the magnetic flux  Travelstar 40GN Travelstar Microdrive II 1st AFC media ), where t 1st GMR head d 25% CGR 1st MR head = ]) d 1 ) N 2 ] in 1999. The fundamental concept 3 D V 1st thin film head IBM disk drive products IBM disk drive Industry lab demos IBM RAMAC (first hard disk drive) Historical variation of areal density from an IBM 1960 1970 1980 1990 2000 2010 6 5 4 3 2 –3 –1 –2 1 10 10 10 10 10 10 Areal density (Mbit/in Areal density 10 10 10 49.1 Magnetic Recording Technology The technology ofdred magnetic years recording old [49. was one hun- Fig. 49.1 rection, advanced servo-controlcontrol systems of for magneticand accurate recording improvements heads inprising the on mechanical a data structures tracks, diskused com- to drive, drive the includingonly disks. advances the However, this fundamental in paper technologyital discusses associated motors magnetic with recording, dig- includingrecord the and devices read used backon to the which recorded the data datarestricted and to is the the recorded. materials media and Theical not discussion to structures is any of associated also the mechan- disks. with the recording heads or of magnetic recording(the is write to head) usedata driven a by to magnetic be current structure can recorded that change represents to the the generate statespaced a of magnetic the magnetic recording magnetization field medium,liest in that which realization a in was closely magnetic thethe wire, ear- familiar and magnetic today tape or is adisk either magnetic layer substrate. on The a rigid datation are of recovered an by outputthe the voltage magnetization genera- in the inFaraday’s read the law head ( recording by medium, sensing e.g., by perspective. (After [49. of turns on the read head and coupled to the read head fromrecording the system media. The to magnetic be discussed here is that used to Novel Materials and Selected Applications

Part E

Part E | 49.1 1186 Magnetic Information-Storage Materials 49.1 Magnetic Recording Technology 1187

to align with the local magnetization. In nickel-iron a films the atomic pairs are the iron atoms and the in- duced uniaxial anisotropy energy density is typically 3 3 Ku  13kerg=cm (0:10:3kJ=m ), where the uniax- GMR Read Inductive ial energy density is Sensor Write Element D = 8 nm 2 Ek D Ku sin ; (49.1)

where is the angle of the magnetization with respect to the direction of the induced anisotropy. To induce d the anisotropy it is necessary to saturate the magneti- W zation of the film with a small magnetic field, typically = t NSSNNSSNNSSNNS 50100 Oe (40008000 A m), since it is the magneti- zation not the magnetic field that is responsible for the B Recording Medium magnetic annealing. If the anisotropy energy density is Fig. 49.2 Schematic illustration of a longitudinal mag- positive the energy is minimum is along the direction of netic GMR read sensor recording system showing a giant the anisotropy, which is referred to as an easy axis. magnetoresistive (GMR) read sensor, write element and A second method of introducing uniaxial anisotropy the recording medium. The parameters are: t the record- in thin films is by stress. If the magnetostriction con- ing medium thickness (ı is also used), W the width of the stant is isotropic with value s for a polycrystalline recorded track, B the length of the recorded magnetiza- material then the magnetoelastic energy density is given tion or bit length and d the head-media separation (HMS) by or magnetic spacing (includes overcoats on the disk sur- face and undercoats on the slider surface). Shown in the 3 E D   sin2 : (49.2) inset: the transition region between the two directions of me 2 s the magnetization; D is the media grain size and a is the transition parameter that characterizes the length of the The angle is the angle of the magnetization in the transition region (length l D a). (After [49.1]) film plane with respect to the direction of the stress (). If a sample is unmagnetized it will strain by s as tion region between the oppositely directed directions the sample is magnetized to saturation. A typical value 6 of the magnetization is similar to that between magnetic of s is  10 . If the magnetostriction constant and domains and has a length l. We will discuss the differ- the stress are both positive the direction for the min- ı ent parts of the recording system starting with the write imum energy and hence the easy axis is for D 90 . head and including the read head, which in Fig. 49.2 is The magnetization, permeability, crystalline anisotropy a giant-magnetoresistive sensor. First, we will discuss constant and magnetostriction constant of nickel-iron some aspects of thin magnetic films that are relevant to alloys [49.6]NixFe.1x/ are shown in Fig. 49.3.The magnetic recording components. All magnetic record- maximum permeability is for the Permalloy composi- ing components used in disk drives today are fabricated tion Ni80Fe20. Increasing the atomic percentage of iron from thin films to allow mass production and for tech- over that of Permalloy results in increased magnetiza- nology reasons, as we will discuss. tion since the magnetic moment for iron is 2:2 B (Bohr

magnetons) per atom in the metal, while the value for 49.1 | E Part 49.1.1 Magnetic Thin Films nickel is 0:6 B per atom in the metal.

Induced Uniaxial Anisotropy Hysteresis in Soft-Magnetic Films In most applications of soft-magnetic films, a uniaxial If we impose an external magnetic field He along the anisotropy in the plane of the film is required. One way direction of the induced anisotropy (easy axis) of the ı of introducing this uniaxial anisotropy is to induce it thin film ( D 0 ) the total energy density is given by applying a magnetic field in the plane of the film by and the effect is referred to as magnetic annealing. The 2 anisotropy can be induced during deposition of the film Ek D Keff sin  MHe cos : (49.3) or induced by a subsequent annealing step, using tem- perature and a magnetic field to modify the anisotropy. Keff is the sum of the uniaxial crystalline anisotropy The mechanism of the induced anisotropy is that of di- (Ku) and the stress anisotropy .3=2/s. The second rectional order, in which atomic pairs in an alloy tend term is the magnetostatic energy. The equilibrium angle 1 ) 3 K 100 –20 –40 –60 20 0 erg/cm 3 Anisotropy constant ]) (10 0 D ]) Applied field (Oe)

8 Weight percent Ni Weight cos e ı : 70 s 1 c MH T K M sin  . e alloys. (After7 [49. /

x M  MH 90 or 270 1 = C) cos  ( eff Fe D

c x

K T

2 sin =.2 sin e eff ) 3 0, or Easy Hard H K eff 2 Easy and hard magnetization curves of an elec- D K D D

D 30 40 50 60 80 90 100

E k 0 (emu/cm

d d –100 –50 0 50 s The derivative of the total energy is given by E 1.0 500 0.5 0.0 M/M Saturation magnetization M Curie temperature Curie temperature 1000 1500 –0.5 –1.0 troplated FeCoNi alloy film. (After [49. 2. sin Fig. 49.4 field applied perpendicular tohard the axis, easy the energy axis, density along is given the by resulting in the two solutions: 1. cos b) 8 6 4 2 0 r ). 0. μ 3 c 0). : H –6 D 10 0 D (49.4)

10 e D H 10 sin

alloys, including initial permeability, crystalline anisotropy and magne- e 20 10 0 –10 –20 –30 –40 –50 x  cos 1 (curve labeled MH  soft.Hysteresis e Fe x C 1–x MH

49.4 Fe of Ni in Fe r x x C μ cos Ni / tween stability and insta-

2 . However, the energy must s sin ı λ sin : Fraction eff  eff K

0 implies that the magnetization 2 K 2 M 2 ]. The magnetization curve exhibits 1 D D  8 0 or 180 K cos

. d D D eff = Magnetization, Curie temperature and crystalline anisotropy of Ni c Magnetic properties of Ni

) E K H 3 ) b 2 ( a ( cm erg D 3 0.5 0.6 0.7 0.8 0.9 1.0 D 0 1 2 40 30 20 10 E 10  e K –10

–20 –30 –40 –50 2 ), which was measured on a thin film of FeCoNi (to d d The above analysis also applies to magnetically H 50 a) is responsible for the use oftal magnetic memory materials devices as since digi- themagnetization up could or represent positive a state binary ofor 1 the and negative the state down couldstates represent can a be stable binary even 0 in and zero applied these field ( hysteresis and in the FeCoNismall alloy and the the coercive material field is is magnetically tostriction. hard films, for which thediscuss coercive magnetic field recording is media large. and We will herefield the is coercive measured in units of kOe. With the magnetic be a minimum forchanges sign stability at a and boundary thebility be second derivative The critical magnetic field is the coercive field ( for the magnetization istal given energy, d by minimizing the to- Fig. 49.3 lies at either A curve of thefield magnetization for versus this applied case magnetic is shown in Fig. easy be discussed) [49. The magnetic field that just satisfies this condition is The solution sin Novel Materials and Selected Applications

Part E

Part E | 49.1 1188 Magnetic Information-Storage Materials 49.1 Magnetic Recording Technology 1189

The solution to this case is also shown in Fig. 49.4 The maximum coercivity (Hc,max) of the recording (curve labeled hard) and is seen not to exhibit hys- medium that can be recorded by the write head is given teresis. This is the hard-axis magnetization curve and by (49.4) (in Gaussian units) [49.5] the magnetic field where the magnetization saturates  g à is known as the anisotropy field Hk. The hard-axis  H D 0:20  4M tan 1 2 ; (49.5) magnetization curve is used extensively for magnetic c, max s d C ı recording heads.

where 4Ms (Gauss) is the saturation flux density of 49.1.2 The Write Head the material used in the recording or write head, g is the gap length of the write head, d is the HMS (see The write head is formed from thin films of soft fer- Fig. 49.2 caption) and ı is the thickness of the recording romagnetic alloys patterned in the form of a magnetic film (shown as t in Fig. 49.2). yoke. A schematic of an eight-turn thin-film head is To increase the linear density (transitions along shown [49.9]asFig.49.5.Headsindiskdrivestypi- a recorded track), it is necessary to increase the coercive cally have fewer than five turns to reduce inductance. field of the recording media. This is the case since the The cross-hatched regions are the soft ferromagnetic length of a transition between the two states of the mag- material, and the leads that carry the write current are netization is given by (49.5) (in Gaussian units) [49.5, shown on the right-hand side of the top view. Note 11, 12] that the widths of the copper alloy turns are increased  à near the back region of the head to reduce resistance. d.1  S/ The heads are fabricated using electroplating and the l D a D  Q substrate (not shown) is a hard ceramic that can be ma- " à  Ã Ã# = chined to form the slider [49.5]. Electrical current in  2 1 2 d.1  S / 2Mrı 2d the form of pulses is coupled to the yoke to gener- C  C ; Q H Q ate a magnetic field at the gap. The coil is insulated c from the metallic magnetic yoke by layers of baked (49.6) polymer photoresist. The alloy that has been used most frequently in the past for the magnetic films in the where write head is Ni80Fe20 (Permalloy), which can be de- M =H posited in thin films using electroplating. Electroplating S D 1   r c  . = / is discussed in Comstock [49.5] and in more detail by dM dH .HDHc/ Andricacos and Romankiw [49.10]. quantifies the slope of the hysteresis curve of the record- ing medium at the coercive field and 10 μm0 .dHx=dx/ (A) Q D I Hc=d

Q is the normalized slope of the horizontal component of the write-head magnetic field at the value H D Hc. : A typical value of Q is 0 7. 49.1 | E Part 3 Mr (emu=cm ) is the remanent magnetization of the recording medium with thickness ı. (C) From (49.4)and(49.5) it is seen that for reduced transition length (and hence higher linear density) it is necessary to increase the coercive field .Hc)ofthe recording medium but to do so requires a concomitant increase in the magnetization of the write head (49.4). (E) Materials with magnetization larger than Permalloy  (a) (4 Ms D 10 kG (Ms D 1 T)) and which are magneti- cally soft (large permeability) are listed in Table 49.1. Fig. 49.5 Thin-film head with eight turns showing the The most common material used in disk drives is poles and yoke structure. A is Permalloy, C is copper and electroplated Ni45Fe50, with a saturation magnetization E is baked photoresist. (After [49.9]) of 4Ms D 16 kG (Ms D 1:6 T) [49.13]. ) ] 20 ] 15 ). ] ] 17] ] Fe 13 5 20 16 80 10 showed Ni [49. [49. 45 C [49. 14] 3 et al. [49. et al. [49. Co [49. et al. [49. 35 ]) References Bozorth Comstock Robertson Liu Osaka Liu Ding 6 FeCoNi saturation 2 kG) but with in- : 2 values in the range of > ( 8 Oe). The composition s 39:78) s M > magnetization contours. (After [49. Fig. 49.6 M  / :95 ](  / 6 2 Oe in the same region of the m) = :87) :5(7 < 79:6 159:2) 397:9 0 . . . 90 c  1 2 5 c H :1 :25 (19:9) :3(23 :8 (302:4) H (Oe)(A 0 0 0   3  80 70 cm) 70 Cobalt (%) 60 5kG and   ( 20 85 48 –  38 – 21: However, experiments in a larger cell in conditions 50 12000 14000 10000 closer to those inwith a higher manufacturing Fe plating content systembic and and acid an used organic to additive reduce ascor- the formation of Fe composition diagram (approximately Fe even larger values of 4 21 possibility of achieving 4 creased coercivity [49. 000 of ferromagnetic alloyszero can values for be thePermalloy selected composition magnetostriction of constant to nickel-iron and alloys result (Ni the in 40 3000 19 30 r  1500 8000 1700 1000 – 2450 – 20 49.1). 20 kG ) 1

 10 s 10 21 M (kG)  (Table  s B 10 10 16 21:5 19 20 18 (T 10000 8000 13 12000 Iron (%)

14000 22 16000 17000 90

000

18000 Ni

20000

22 31

24000 80 8000 were accumulated in bulk 30 16000 14000 12000 70 Fe (G) 56 49.6 60 ] care was taken to avoid any im- 49.6. :6%) sputtered 48 8 B–H =1500 Oe) 50 = H 40 πM (for Intrinsic induction, 4 (C) electroplated electroplated ]asFig. 31 6 Properties of materials used in magnetic recording heads 30 30 (Permalloy) Fe Fe 90 80 70 60 50 40 30 20 10 Nickel (%) 13 20 55 2 T). The composition region that results in the 22 20 Ni N Fe Fe D Ternary alloys of Ni, Fe and Co also can be electro- ThedataforFig. 56 48 80 45 s 10 Co FeAl(2% Al)N (N/Fe 6 FeTiN sputtered Material Ni Sendust (FeSiAl) Ni Co with carbon impurity with pulsed plating M plated with saturation magnetization of 4 Table 49.1 ( largest value of saturation magnetizationpermeability but is: also large Co purities in the plating bath and the results showed the A useful guide tothe the ternary selection FeCoNi alloys ofshown [49. is the the composition composition of diagram materials. In a studyfor write of heads electroplating [49. FeCoNi alloys Novel Materials and Selected Applications

Part E

Part E | 49.1 1190 Magnetic Information-Storage Materials 49.1 Magnetic Recording Technology 1191 has a zero magnetostriction constant (Fig. 49.3). The Permalloy composition. This decrease in the permeabil- magnetostriction constant is frequently chosen to be ity of nickel-iron alloys with composition different from small and negative so that the magnetoelastic energy that for Permalloy is the direct result of the increase in defines an easy axis at right angles to the direction of crystalline anisotropy and magnetostriction, as shown the stress ( D 90ı in (49.2)). in Fig. 49.3. The lower permeability of the FeCoNi In the case of thin-film write heads the stress is alloys can be obviated in a design of the write head along the axis of the poles (upward in the patterned in which the high-magnetization films are used only yoke in Fig. 49.4) and with negative magnetostriction in the gap region in a design referred to as a stitched constant the easy axis is across the pole width, which head. The values of coercivity of the recording medium means that the direction of flux flow is along the hard .Hc) that can be recorded with recording heads with the axis with the desired linear relation between the mag- saturation magnetization of Permalloy (4Ms D 10 kG netization and the magnetic field (Fig. 49.4). In the case (Ms D 1 T)) and materials with saturation magnetiza- of the ternary alloys of FeCoNi an alloy composition tion 2:5 times that for Permalloy (4Ms D 25 kG (Ms D that results in zero magnetostriction is Fe13Co31Ni56,in 2:5 T)) are shown versus the magnetic spacing (d)in which the nickel content is too high to be a useful write- Fig. 49.7. head material [49.8]. Alloys of FeAl (2% Al) [49.16] In 2006, magnetic spacing values were 15 nm (car- and FeTi ( 2% Ti) [49.17] sputtered using a mixed bon overcoat for head: 3 nm, pole tip recession tol- N and Ar working gas offer desirable magnetic proper- erance: 3 nm, flying height: 5 nm, and media carbon ties, as shown in Table 49.1, but have not replaced the overcoat: 4 nm). electroplated alloys because of increased costs of sput- In today’s technology magnetic spacing values are tering. 13 nm (carbon overcoat for head: 3 nm, pole tip reces- A material of interest for even higher values of sion tolerance: 3 nm, flying height: 3 nm, and media 00 saturation magnetization is the ˛ phase of Fe16N2, carbon overcoat: 4 nm). which has the potential for a saturation magnetization of The trend in coercivity (Hc)andflux(Mrı)versus 29 kG; however, this phase is metastable [49.7]. The ac- areal density in commercial disk media is shown in tual values of saturation magnetization with this phase Fig. 49.8 [49.7]. in thin-film form was 20 kG and required annealing at Areal density is determined by bit size where size 200 ıC [49.18]. is length in LMR whereas it corresponds to diameter in To record on recording media with increased coer- PMR. civity is not the only issue with the magnetic materials Areal density plays a major role in magnetic record- used in write heads. It is also important that the write ing similarly to minimal feature length F in electronic heads have high efficiency. Efficiency () in this case is circuits (Table 49.15); additionally, most magnetic defined as the ratio spacings scale with respect to it (Sect. 49.1).

Hgg  D ; (49.7) NwI Maximum coercive field (Oe) 8000 where Hg is the value of the magnetic field in the 2.5 Ms of permalloy gap of the write head and I is the amplitude of the 7000 1 Ms of permalloy write current pulse. High efficiency is important to al- 6000 low write-current amplitudes that are easily supplied 5000 from integrated circuits. For high efficiency it is nec- 49.1 | E Part essary that a large percentage of the magnetomotive 4000 force (N I) results in a magnetic field across the gap. w 3000 However, the permeability of the yoke influences the efficiency 2000 g 1000 Ag  D ; (49.8) g C lc 0 Ag rAc 0 0.02 0.04 0.06 0.08 0.1 0.12 d (μm) where Ag and Ac are the area of the gap and core respec- tively; lc is the length of the core and r is the relative Fig. 49.7 Maximum coercivity of recording media ver- permeability of the core or yoke material. As shown sus magnetic spacing .d/ for write heads fabricated from in Fig. 49.3, the relative permeability (r) of nickel- Permalloy and from a magnetic material with a saturation iron alloys drops for compositions different from the magnetization equal to 2:5 Ms of Permalloy 18 Å 35 40 super-  n / 30 Cr = ]. .Fe 20 S (Fe 30Å/Cr 9Å) (Fe 30Å/Cr 12Å) H [49. S (Fe 30Å/Cr 18Å) H ).  S 49.9 H m = A Œ 4 ]) 6 Magnetic field (kG) 20 2 K and the magnetic fields applied were : 2 K. The thickness of the layers in Angstroms : The magnetoresistance of three –40 –30 –20 –10 0 10 20 30 40 50 8nm)isshowninFig. : H=0 1  –50 The number subscripted to the parenthesis charac- The current used to measure the magnetoresistance, The initial discovery of GMR was done with a tem- The large change in resistance with magnetic field R/R 9 : 1.0 0.9 0.8 0.7 0.6 0.5 Fe Cr Fe terizing the magnetic andnumber transition-metal of films such is films,lattices. which the With are no referred appliedwere to magnetic antiferromagnetically as coupled; field, that super- is, the thetizations two magne- in films the twooppositely directed. films The are application of equalmagnetic a in field large rotated magnitude in-plane the and magnetization offilms the to coupled a parallel configuration, reducing(to the be resistance discussed). the change in resistance withapplied in applied the magnetic plane of field, the is rent films. This is orientation of referred cur- towill discuss as an alternative current-in-plane orientation (CIP). ofcurrent-perpendicular-to-plane the (CPP) Later current, case. we the perature of 4 (0 Fig. 49.9 A curve of resistance normalizednetic to field that applied with no versusferromagnetic films mag- magnetic of Fe field with thickness forcoupled of 30 arrays Å by (3 of nm) Cr with thickness varying from 9 lattices at 4 is shown. (After [49. was in contrast to that observednetoresistance in the (AMR) anisotropic mag- effect,resistance which observed is when the thegle change magnetization ferromagnetic in in film a isa sin- rotated hard axis. from This an latter technology, easy for toward which the resis- measured in kOe (10 ) ) 2 2 2.4 2.0 1.6 1.2 0.8 0.4 0.0 SAF 100 ]. GMR results ) and media flux c 20 for disk drives. (Af- Product (memu/cm H 2 δ r Areal density (Gbit/in in = M gnetic transition metal. ng time resulting from the 5 in drives the data rates are : 10 5 in drives the data rates are larger ]: δ r s, while with 2 c 19 = M H Trend in coercive field ( ]) 1 7 ) versus areal density in Gbit We discuss synthetic antiferromagnetic media (SAF With the increased linear density associated with in- fact that magnetic momentswith angular are momentum that always cannot associated bestantaneously changed in- magnetic fields both decreaseand slow the down the magnetic switching speed fields Data rates depend on the form factor of the disks ı r Magnetic coercivity (Oe) Magnetic coercivity 5000 4000 3000 2000 1000 media) further below. creased areal density inin disk the drives rotation and speed of withare the increases recorded disks, the (and rate read atThis which back) increase data has in increasedthe data over magnetic rate time. fields leads in theever to gap decreasing a of times. The a requirement switching writenetic that time head of films switch thin in as mag- factors used [49. in write heads is limited by three 1. The fundamental switchi 2. The eddy currents associated with the3. changing The inductance of the coil. and with high-end 3: than 50 Mb (M ter [49. 49.1.3 Spin-Valve Read Head The technology that hasing the evolved flux for emanating from sensing transitionsmedia or in is thin-film read- disk the spinthe giant-magnetoresistive valve. (GMR) This effect discovered technologyFrance in is and based published on in 1988 [49. Fig. 49.8 when thin ferromagnetic films arethinner coupled spacer by film an of even a nonma roughly one-half as large. Novel Materials and Selected Applications

Part E

Part E | 49.1 1192 Magnetic Information-Storage Materials 49.1 Magnetic Recording Technology 1193

a) GMR ratio (%) b) Coercivity (Oe) 10 80

Single Co90 Fe10 layer 8 60 Co90 Fe10 6 Co 40 4 20 2 Single NiFe layer

0 0 10 15 20 25 30 35 40 0820 40 60 0

Cu layer thickness (Å) Co90 Fe10 layer thickness (Å)

Fig. 49.10 (a) Comparison of GMR ratio . = / as a function of copper layer thickness in Co90Fe10 and Co films (see text). (b) Coercivity as a function of Co90Fe10 layer thickness in Ta (50 Å)/[Ni80Fe20/Co90Fe10] (75 Å) films. (Af- ter [49.21]) tance change with Permalloy was of the order of 23%, though the cobalt alloy would have a large coercivity was being used in read heads during the time of research in the bulk, the coercivity for thin films, together with on the GMR effect. the underlying Ni80Fe20 film, is low for thicknesses less By the early 1990s it was found to be possible to than about 60 Å. observe GMR in other film materials with increased The advantage of the cobalt-iron film is the in- spacer film thickness at 300 K and with magnetic fields creased GMR ratio over Ni80Fe10. In order to increase of tens of Oersteds. For example, the GMR ratio for two the GMR ratio even more it has been found advanta- coupled films of an alloy of cobalt and iron (Co90Fe10 geous to deposit an interfacial nanolayer of cobalt metal for which the magnetostriction constant is zero) with on one or both sides of the free film [49.23]. We will a variable thickness of the spacing layer of copper is learn in a discussion of the origin of GMR that cobalt shown in Fig. 49.10 [49.21]. has the potential for large GMR ratios. It is seen that the GMR ratio is considerably reduced A schematic of the spin-valve read head is shown in from that for the Fe/Cr superlattices at 4:2K(Fig.49.8) Fig. 49.11 [49.22]. The magnetization of the free film but the ratio is larger than that for the AMR effect. is shown rotated to an angle 1 and the pinned layer The discovery of the GMR effect led to the inven- is shown with a fixed angle of 2. The pinning effect tion of the spin-valve read head, in which one of two ferromagnetic films coupled by a thin transition-metal yA Exchange film is maintained constant in direction (the pinned layer film) and the orientation of the other film (the free film) A Spacer is allowed to rotate (in the plane of the magnetic film), x resulting in a decrease in resistance [49.22]. We will I

discuss the technology of pinning later. By free, it is 49.1 | E Part meant that the magnetization in the film is easily ro- Lead M2 Lead tated, for example by the magnetic field arising from M1 transitions in the recording medium.

It is surprising that cobalt is found to be useful in θ1 θ2 this application since cobalt is magnetically hard (pure cobalt metal has a high coercivity and is used in record- ing surfaces) and one of the requirements for the GMR effect is that the free film has high permeability and Free layer Pinned layer high efficiency (49.6). The reason for this result is shown in Fig. 49.10, Fig. 49.11 Schematic of spin-valve read element. The magnetiza- where the coercivity of a free film consisting of two tion of the free layer is shown at an angle 1 to the easy axis (and films: Ni80Fe20 (60 Å (6 nm)) and Co90Fe10 is plotted direction of the current) and the magnetization of the pinned layer ı versus the thickness of the Co90Fe10 film [49.21]. Even is fixed at 90 to this axis. (After [49.22]) ı is || rec r H in dis- ]). (49.11) (49.12) M ua the free the satu- 25 H 50 ; s sv t / à the half-gap M [49. s 2 t ; t g M ; g C à C 2 / g 2 a 0 max  G ]) . R C R g ı d  12 C . rec Mallinson r , 2 2  5 Geometry of the ferromag- s M 1 à ) and the magnetization of the sv θ t the head square resistance b ı 2  tM ( max ?    ) the head resistivity and thick- R H C  R θ the head width and r H r 2 R  a 2tan t r I ; r  and  C JW W

k d (Ksin Easy axis D D H  4 ] with no external resistance can be ap- max à s , with R r t (peak)   = is read efficiency, defined as the fraction of is the total spacing between the shields and 0 max I r R R

50  G  GMR D V  PW Pinning with an Antiferromagnetic Film The read-head peak output voltage between  R . The read pulse is characterized by the amplitude but b) ration magnetization of the free layer, between the shields, layer thickness (modified from tance units) (discussed in [49. ( the magnetization of the recording layer, ratio is a typicalfor GMR measure performance of (see resistance further below). variation used For a spin valvefilm to function, must the have pinned its ferromagnetic magnetization perpendicular to the ness respectively), the flux sensed by the head to the total flux, where where is the thickness of the recording layer. shields [49.12 proximated by t also by the width at one-half amplitude (PW 2

(49.9) (49.10) ] about . , the external magnetic fields Antiferro- magnetic film Interface Ferro- magnetic film ı 24 ua ]) H 90 5  and the thick- , again with G ı 90 is not uniform over 1 ;

/ is the width of the read 1 r 90C is the density of the current  W  J 2 sin being limited to 0 1 

from ; 1 . 1

, in spin valves, peak output voltage to the easy axis. (After [49. 2 k 

r H in y, disk Schematic of the coupling of an antiferromagnetic to a ferromagnetic film. H ) ). In the absence of any other magnetic JW is the anisotropy field of the free film a ) through the rotation of the magnetization  ( D m. Spin-valve read heads are shielded on each k is the efficiency of the read head, which ac- 1 is the change in resistivity of the stack of films D H

 = y, disk 49.1.1 V H  sin The device senses magnetic field in the plane of the  ua a) H per unit of track width must be larger than [49. film at an angle free film resulting fromdia transitions in ( the recorded me- where fixed. The factor of 2 inthe the actual denominator rotation is of the result of where is due to thelying coupling antiferromagnetic of the film pinnednonmagnetic as spacing film layer to we separates the an will twonetic under- ferromag- films. discuss. The The maximumvalve output with voltage no for external thethe resistance spin flux due (and to no shielding reduction – to in be discussed) is Fig. 49.12 counts for the fact that the angle netic film showing the induced easy-axis field in the free film flowing through the films, the height of the free film, (see analysis ofSect. the hard-axis magnetization curve in fields, the magnetizationThe will hard axis lie is perpendicular along to theage easy the axis. is The easy measured volt- across axis. thecurrent lead and terminals the width using of a thetween sense track the is leads. just Typically, when the areal spacingthan density be- is 100 higher Gbit= with a change of head (always less thanand the width of the written track) 10 mV side of the stack of films toThe realize total a narrow width pulse width. between the shields is ness of the stack of films comprising the spin valve is Novel Materials and Selected Applications

Part E

Part E | 49.1 1194 Magnetic Information-Storage Materials 49.1 Magnetic Recording Technology 1195

Table 49.2 Comparison of alternative antiferromagnetic films for transverse bias of Permalloy films Property NiFe/FeMn NiFe/NiMn NiO/NiFe CoNiO/NiFe NiFe/IrMn NiFe/CrMnPt NiFe/Pd-Pt-Mn [49.26] [49.26] [49.26] [49.27] [49.27] [49.28] [49.29] Corrosion Low High Very high Excellent Moderate Good High resistance Coupling field  77 (6:12)  206 (16:4)  46 (3:66)  45 (3:58)  350 60 (4:77) .Pd0:6Pt0:4/0:5Mn0:5 Hua Oe (kA=m) (27:84) 80 (6:36) Coercivity Hc Oe  6(0:48)  116 (9:23)  35 (2:78)  23 (1:82) 11 (0:88) 8(0:64) 16 (1:27) (kA=m) Blocking temper-  150  450  200 150 250 320 300 ature (ıC) Annealing Not required Required Not required Not required Not required Required Required recorded track, as shown in Fig. 49.11 (the vertical magnetic field from the transitions in the disk is in the y- M/Ms direction). This orientation of the magnetization in the 1.0 pinned film is achieved by coupling the ferromagnetic film to an antiferromagnetic film and the coupling re- sults from the layer of spins in the antiferromagnetic 0.5 film adjacent to the top layer of spins in the ferro- magnetic film and is characterized by a coupling or Hua – Hk exchange field of value Hua. The geometry for the cou- 0.0 Hua + Hk pling is shown in Fig. 49.12 and the ferromagnetic film geometry with a magnetic field along the easy axis (Hjj) and the hard axis (H?)asFig.49.12. –0.5 The coupling results in shifting the easy axis hys- teresis curve .Mk=Ms versus Hjj) of the ferromagnetic film by an amount Hua, as shown in Fig. 49.13 [49.5]. –1.0 0 50 100 150 200 250 300 350 The magnetization of the pinned film is held constant H (Oe) at Ms over the range of magnetic fields from large negative fields to Hua CHk. Note that the magnetization Fig. 49.13 Magnetization curve with the magnetic field could be held at CMs by reversing the direction of the along the easy axis .Mk=Ms versus Hk) for coupled an- lower layer of spins in the antiferromagnetic film. The tiferromagnetic and ferromagnetic films. (After [49.5]) coupling field depends on the temperature and vanishes at the blocking temperature. The direction of the cou- antiferromagnetic films are typically formed on top of pling is established during an annealing step by heating the stack of films. Another advantageous film is the al- the structure above the blocking temperature, where the loy of Pd30Pt20Mn50 [49.29]. film is paramagnetic, in the presence of a magnetic field This ternary alloy has a favorable coupling strength ı and then cooling the film below the blocking tempera- Hua but also a high blocking temperature (300 C) ture. and favorable corrosion resistance. Pd30Pt20Mn50 does ı

The choice of antiferromagnetic films to be used require annealing the stack at 220240 C. The advan- 49.1 | E Part in this application is extensive as shown in Ta- tage of using an antiferromagnetic film for the pinning ble 49.2 [49.26–29]. The parameters listed in Table 49.2 film instead of a permanent magnetic film is that the are the strength of the coupling field (Hua), the blocking antiferromagnetic film does not itself respond to any temperature and the susceptibility of the antiferromag- external magnetic field since the film has a zero net netic and ferromagnetic films to corrosion. Corrosion is magnetization. of concern in recording heads because of the mechan- ical lapping process used to form the surface on the Synthetic Antiferromagnets bottom of the slider on which the air-bearing contour The magnetic coupling between magnetic films closely is etched [49.4]. spaced by certain nonmagnetic metal films oscillates One antiferromagnetic film with particularly favor- between antiferromagnetic and ferromagnetic coupling. able corrosion propensity is NiO. This film is typically Using this fact, it is possible to design film structures used in a bottom spin valve in which the antiferro- that have a desired magnitude and sign of the mag- magnetic film is closest to the substrate. The other netic coupling [49.30]. A schematic of a structure of 2 ). 20 10 20 cm Fe Fe Fe 80 90 80 Ni = 10 Fe 90 (60 Å)/Co 20 2575= memu Fe C 49.15, which we used 80 (55 Å)/FeMn (100 Å)/Ta ]). The permanent mag- 10 12 Fe 28 Å, where the first four films 90 ]. The advantages of the synthetic 20 23 Fe 80 ]. In this case the antiferromagnetic film to be zero. 21 II F An overall spin valve consists of a sequence of thin larger. layer to the freefilms layer in is the reduced synthetic since antiferromagnetmagnetically the coupled are and two antiferro- hence their Co magnetostatic fields at the free film cancel. The synthetic antiferromagnet contributes to ensur- The purpose of the hard bias is to reduce the pres- The stack of films used in a spin valve has been dis- Origin of GMR 250750 A). By varying the thickness of the Co C a magnitude varying between films such as: MnFe 2125 Å/Co Å/Co 30 30 Å/Ru Å/Ni 7 Å/Co 30 Å/Cu (MnFe/Co/Ru/Co) form a syntheticfilm since antiferromagnetic the final Co filmto of a these direction four opposite films to isCo that pinned for film the then first acts Cogether film. with as This the a final three pinned films film (Cu, Co, in and a Ni spin valve to- layer it is possiblethe to layer adjust the total magnetic field at free film,(55 was: Å)/Cu (variable)/Co Ta (50 Å)/Ni ( antiferromagnet for pinning thewith pinned film just compared an antiferromagneticNiMn or film IrMn) are: (e.g., FeMn, NiO, 1. The pinning coupling field is an order-of-magnitude 2. The magnetostatic coupling field from the pinned ing that the onlyfrom magnetic the field transitions at in therequirement the for free recording the medium. film proper One isis operation last that of that the there spinture is valve a abutted permanent magnetic on filmbias the referred film two to (discussed as ends in the of [49. hard the struc- The thickness of rutheniumfilms listed is in optimum thetween for sequence the of antiferromagnetic Cowhich coupling films. increases The be- the magnetoresistive last coefficient,have as Co discussed [49. we film is a nanolayer, netic films are similar to theused cobalt for alloy disk magnetic recording films surfaces (to be discussed). ence of magnetic domains in theBarkhausen free jumps film of and to the reduce domainof walls the in the external presence magneticthe fields recording from medium. the transitions in cussed. The films usedto in illustrate Fig. the GMR effect in a Co was FeMn (typically about 50%, 50%)are and used the to Ta protect films the rest of the stack. The physical origin offerential the scattering GMR of effect electrons lies with in spins the in dif- the same (70 Å) [49. I 2 D F 20 20 a cm Co Co 50 50 II I 410 nm) : F Ru Ni F Ni Ru Co 03 ) for this 0 12 of the films J D II c or= memu F 0) and can have ) to the direction I 2 > F and cm 12 I = J F 49.14. The units of the film ( emu 20 Ru spacer layer thickness (Å) is used to pin the magneti- 3 Antiferromagnetic Ferromagnetic 250 nm and . Ruthenium has the hexag- :  20 Co 0 20 Magnetic coupling constant per 80 ) Co D b Co ( 80 a 428 nm) while cobalt also has the 80 : 0 ) 2 D c Geometry of ferromagnetic films coupled and Ni versus the thickness of the Ru layer. (Af- ) , is shown in Fig. 0) or ferromagnetic ( ) 5 1015202535 a 20 ( 20 a ( < ]) Fe (memu/cm Co 12 ). Ruthenium is selected for the coupling film 12 30 80 J J II 80 – 0 F 49.14. P Here the strong antiferromagnetic coupling of Co The experimental coupling constant ( 0 12 J 75 50 25 J 271 nm and 125 100 –25 –50 b) a) : 0 through Ru to soft Ni ter [49. onal close-packed (hcp) crystal structure with ( opposite to the Coond film layer of and Ru thethe can thickness coupling be of between adjusted the the to sec- change two the soft-magnetic sign films of ( zation in the adjacent Ni shown in (10 A inThe SI units) coupling (coupling constantnetic ( per can unit be of either film antiferromag- area). and films that exhibitsFig. this variable coupling is shown in Fig. 49.14 since it has aCo, strong Ni and oscillating coupling between unit of film area between layers configuration of layers, whereis Ni the soft-magnetic film coupling constant are 10 with thin layers of Ru. hcp structure with ( and therefore the twotaxially. metals are expected to grow epi- Novel Materials and Selected Applications

Part E

Part E | 49.1 1196 Magnetic Information-Storage Materials 49.1 Magnetic Recording Technology 1197

is the velocity of electrons at the Fermi energy,  is the Ni Ni mean scattering time and D.E ) is the density of states (paramagnetic) (ferromagnetic) F at the Fermi energy [49.32]. If the mean scattering time E E () is held constant then it is seen that the conductivity 4s 4s increases with the density of states at the Fermi energy; 3d 3d 0.3 0.3 0.3 0.3 however, the scattering of the 4s electrons with a given – + – + spin is large when there are a large number of scatter- 4.7 – 4.7 + 4.4 – 5.0 + ing centers and empty states with the same spin in the 3d band, i. e., the density of states at the Fermi energy D.EF/ in the 3d band is large. In ferromagnetic transition metals this is the larger 1 . / dn dn dn dn effect [49.33] leading to D / D EF . dE dE dE dE The scattering is due to spin-spin scattering, and Co Fe spin-orbit scattering is believed to be small [49.34, E E 35]. The magnetic moment per atom can be deduced 4s 4s as the difference in the number of electrons/atom with 0.35 0.35 0.3 0.3 spin-up (the majority) versus those with spin-down. For – + – + 3d 3.3 – 3d example, the magnetic moment per atom for iron is 2.6 – 4:82:6 D 2:2 Bohr magnetons (B) per atom. From 4.8 + 5.0 + Fig. 49.15 we conclude that the differential scattering will be large in cobalt since for this element the dif- ference in the density of states at the Fermi level is large (approximately 810 to 1 [49.31]). A schematic of the density of states versus energy for the three dn dn dn dn dE dE dE dE films comprising the spin-valve structure from the two extreme cases of the orientation of the ferromagnetic Fig. 49.15 Schematic of density-of-states curves for the films (ferromagnetic configuration and antiferromag- ferromagnetic transition elements (Ni, Co and Fe). Both netic configuration) is shown [49.31, 36]inFig.49.16. the 4s and 3d bands are shown. The magnetization of the All films have same Fermi energy. Mathon [49.31]de- elements is due to the difference between the spins in the scribed the GMR effect by discussing the differential spin-up and spin-down bands and the bands are full of elec- scattering of holes in the 3d bands, but by concentrat- trons to the Fermi level (the shaded region). (After [49.31]) ing on the scattering of electrons from the 4s to the 3d band we can make the same conclusions [49.34, direction as the local magnetization (spin-up electrons) 36]. or opposite to this direction (spin-down electrons) at It is assumed that the mean free path for the elec- the interfaces and in the bulk part of the thin films trons is large compared with the thickness of any layer comprising the GMR stack of films. The ferromag- in the superlattice. Consider the ferromagnetic con- netic transition elements (Fe, Co and Ni) have different figuration case (top of Fig. 49.16): here the majority 3d band structures, as shown in Fig. 49.15 [49.31] electrons have spin-up and the scattering of 4s electrons and discussed by [49.5]. This figure illustrates the en- is low in both ferromagnetic films since both have a low

ergy of electrons in the two half-bands for spin-up and density of states at the Fermi level. The nonmagnetic 49.1 | E Part spin-down electrons versus the density of states D.E/ film has a larger density of states and will scatter elec- (number of states per unit energy per unit volume) in trons regardless of the orientation of the ferromagnetic the 3d band for the ferromagnetic transition metals. The films, which is why this film must be thin. Now consider 4s bands are also shown. Electrons in the 4s bands make the antiferromagnetic configuration case in Fig. 49.16: only a small contribution to the net spin, however, due from the density of states at the Fermi level in the three to the larger mobility (inversely proportional to the cur- films it is seen that the scattering is large in the right- vature of the bands), they are largely responsible for the hand ferromagnetic film for the electrons leaving the current. left-hand ferromagnetic film with spin-up (top density- The bands are filled to the Fermi energy at 0 K, and of-states curve), resulting in large resistivity for this states above, but close to, the Fermi level are occupied class of electrons. Similarly, the scattering of spin-down at room temperature (illustrated by the shaded region electrons is large for the left-hand ferromagnetic film. in Fig. 49.15). The conductivity of a metal is given by The GMR ratio (%) resistivity for the spin-up  2v 2 . /= v D e F D EF 3, where e is the electron charge, F and spin-down electrons for the three films (ferromag- s ] 0 L M :

5 for

8 31 20 Å 0 D R (49.13) [49. D and = ˛ M H M t max

R 49.16 for  decreases. Fig- ; R is lower. = Á R R R = M = t  NM R t R  ]) C  is rapid with increasing 2 31 ˇ ˇ/ R Thickness of magnetic layer (Å) Thickness of magnetic layer = Á  R M for assumed values of s t NM L M t  .˛

M t C D versus the thickness of the magnetic ˛ shows the variation of

R 4 = ]. R ;ˇ D 0, which is appropriate for a Co/Cu super- 49.18  31 . 51015202530 : s shows the variation of H M 1 the thickness of the nonmagnetic film (spacer)

NM the thickness of the magnetic film. 0 max t 10 Å versus D R R D NM M t 03 ˇ t D  49.19 The change in resistance of the spin-valve structure ˛ Figure The decrease in This combination of films is not used in spin valves When magnetic film thickness is beyond approx- 0 80 60 40 20 GMR ratio (%) 100 120 NM and orientation of the ferromagneticspin-up films and is spin-down high electrons. for both can be analyzed in terms ofthe the ferromagnetic different and resistivities for antiferromagneticthe orientation two ferromagnetic of films shown in Fig. where with are respectively the highspin orientations and of low the resistivities magneticis (M) for the films, nonmagnetic two whereas (NM) spacer resistivity. Fig. 49.18 t versus and layer fora a Co/Cu spin nonmagnetic valve. (After [49. layer thickness of 10 Å for thickness of the NM layer. since the cobalt film ischoice not is magnetically Permalloy with soft; a the variety usual sition of metals nonmagnetic tran- and the resulting imately 10 Å, the change in lattice [49. ure E E and ↓ ) p a ( ]) part of the 31 . The resistiv- ) b ( configuration bottom Antiferromagnetic ↓ p (a) Nonmagnetic Ferromagnetic ]) ], where it is shown that the ferro- 31 S S 31   configuration Ferromagnetic [49. H M L M Density-of-states curves for the spin-valve Distributions of local resistivities for the spin-   FM NM FM NM FM NM FM NM ↑ ↑ p p Ferromagnetic (E) (E) (E) (E) ↑ ↓ ↑ ↓ 49.17 up D D D D Spin Spin down b) a) up and spin-down electrons inferromagnetic the spin ferromagnetic configurations. and (After anti- [49. ities of the layers are shown on the figures. (After [49. Fig. 49.17 netic –netic FM, – FM) nonmagneticnetic in orientation – the casesFig. ferromagnetic NM is and and shown antiferromag- as ferromag- a schematic in Fig. 49.16 magnetic orientation ofa the (relatively) low ferromagnetic resistivitytrons films path for has while the the spin-up elec- resistivity for the antiferromagnetic the antiferromagnetic spin configuration structure in the ferromagnetic spin configuration Novel Materials and Selected Applications

Part E

Part E | 49.1 1198 Magnetic Information-Storage Materials 49.1 Magnetic Recording Technology 1199

results in less exchange coupling between grains and GMR ratio (%) reduced noise – as we will discuss. Chemical seg- 160 regation of Co100xCrx alloys forms a nonmagnetic 140 phase (Cr-rich) and a magnetic phase (Co-rich) and 120 the nonmagnetic phase forms at the grain boundaries. 100 The microstructure is controlled by undercoats that promote grain growth with the axes of the grains in 80 the plane of the film. Platinum results in higher crys- 60 talline anisotropy and boron promotes the formation 40 of an amorphous alloy of CoCrB, which grows at 20 the grain boundaries, further reducing exchange cou- 0 pling. 35302520151050 Figure 49.20 shows the microstructure for a mag- Thickness of spacing layer (Å) netic recording surface, illustrating the separation of the Fig. 49.19 R=R versus the thickness of the nonmagnetic grains by the amorphous CoCr phase and, in the lower layer for a magnetic layer thickness of 20 Å for a Co/Cu image, the c-axis stacking of the hcp grains [49.37]. spin valve. (After [49.31]) In five-element alloys (CoCrPtBTa), Ta assists the seg- regation of Cr at the grain boundaries [49.38]. Some 49.1.4 Longitudinal Recording Media (LMR) recording media use as many as three magnetic layers with the saturation magnetization increasing towards Magnetic recording media used in disk drives are the read head, so that the average saturation magneti- all sputter-deposited thin films of cobalt alloys for zation is reduced while the magnetization nearest to the high coercivity and media flux (Mrı). Coercivities of read head is increased. 40005000 Oe are used (2006) and are limited by the The layer of films comprising a recording layer at ability of the write head to record on the media. The al- the areal densities for a 130 Gbit=in2 case were: NiP- loys that have evolved are four- or five-element alloys plated aluminum substrate with mechanical texturing in of CoPtCrX, where X is Ta or B or both. A typi- the data-recording zone/Cr seed layer/a CrMoX (X D cal atomic concentration of the four-element media is grain-boundary segregation-enhancement element) un- Co64Cr24Pt8B4. It has been found necessary to form derlayer/a CoCrT intermediate layer/two ferromagnetic a microstructure in the magnetic film that consists of layers (one with high Cr/low B concentration and the a phase of small magnetic grains isolated by a non- second with low Cr/high B concentration) and a double- magnetic phase. The isolation of the magnetic grains layer amorphous carbon overcoat [49.39].

a) b)

10 nm 10 nm atE|49.1 | E Part

Fig. 49.20a,b Plan-view electron microscope images ((a) is lower resolution and (b) is higher resolution) of the CoPtCrB magnetic alloy layer used in the 35 Gbit=in2 case made by IBM. The amorphous grain boundaries are seen as white and the c-axis stacking of the hcp grains can be seen within the grains in the higher-resolution image. (Af- ter [49.37]) B . = Z 49.3 50 is the 49.20 (49.15) N Z the ratio of :32 :1 :24 :4 N 4200 0 9 0 0 80 GB/90 mm disk = 0 S ; ! 2 3toavoidseveredis- / ]) and the ratio PW N 2 Z 5 Z  8 for the type of detection ]. A serious problem with :  7  2 2 Z  . :46 :41 :7 5 is the standard deviation of : [49. 2700 0 10:8 0 0 20 GB/90 mm disk  Z ]) 

7 value of the grain diameter, has been 10 49.21 a exp and = ) Z 2  B ]. Both the mean and standard deviation 2 7 cm 1 Comparison of 20 and 80 GB/platter disk char- = p Z [49. is the log  Z D (memu decade) (Oe) 49.3 We will discuss the decay rates shown in Table The distribution of the grain sizes in recording me- Grain-size distributions are measured on foils by y Therefore, to maintain constant The ratio = ı r c H M Average grain size (nm) Distribution sigma Amplitude decay rate (% this trend is themagnetic fact volume that in the the film is ratio decreasingaverage of grain faster than size, magnetic the which to results in non- per the number bit decreasing of grains faster than theprice average to grain size. be The paid forthe stability the of increasingly the small recorded grain datain over size time the is as next we section. discuss for the 80 GB/platter20 GB/platter are media. reduced from those of the in the next section. The trendgrains in per the mean bit grain size foris and shown recording in films Fig. versus areal density shows the microstructure of aemphasizing typical recording the surface, segregation ofmagnetic the constituents grains of by the the alloy. non- dia for 20 GB/platter (counting bothand sides the of the higher platter) densityble 80 GB/platter are shown in Ta- tunneling electron microscopy (TEM). Figure the track widthstant to and, with the the decreased grainmust track decrease size width, in proportion. the Increased must grain areal density size required be has decreased kept mean grain con- sizetant, tighter but, distributions equally of grain impor- size. Theare distributions closely matched byform log-normal distributions of the acteristics. (After [49. has been in the region 2 where Table 49.3 mean value of channels used in disk drives (partial responselikelihood maximum – PRML). tortion of the transitionpercolation across (discussed the in [49. track, referred to as is N = c was was 0 over S W r / (49.14) 0 W S . 49.20 is the width has been pub- 50 ; / r c N W W . is the transition pa- à a is the length between ycrystalline media. To 50 B B PW 27 dB and this has required  system, the value of 2 2 à in a B = 12]). Transitions also tend to follow  , 5 31 : 0 ]. ] D 41 40 is the width of read head, PW 2 r à 0 W N S m and the required mean of the grain size was  The substrate used for the samples in Fig. Forareliablelowerrorratethevalueof20log The amorphous carbon is required to protect the Media Noise Transitions are not straight across the recorded is the mean of the distribution, c 505 : the cross-track correlationproximated width, as which the grain has ormedia. been cluster The size ap- grain of size the has a recording W log-normal distribution and rameter discussed above, and bits. lished [49. the media noise voltage amplitude where 0 of the read pulse at 50% of the peak amplitude, is required to be abovean 26 increasingly smaller grainread head size has as decreased, thesities. resulting width in For higher of a track the 10 den- Gbit the grain boundaries inachieve low the media pol noise, itgrain is size required so toaveraged that have a the over small random thewidth). nature width An of analysis of of the the the signal-to-noiseas grains ratio, the read defined is base-to-peak head signal (the voltage amplitude track glass. Glass substratesaluminum have with several electroless advantagesstrates, Ni(P) over including (for increased hardness)reduced flutter, sub- stiffness, increased hardness which and reducedsity leads propen- to to corrosion. However,tinue aluminum to substrates be used con- inreduced nonportable cost. applications because of magnetic film and to giveon a which mechanically the rigid slider surface can fly at less than 25 nm. In digital recording systems the dominantwhich noise appears voltage, simultaneously with the desirednal read sig- at the terminalsrecording of media, the assuming that read theread head, resistance head is from that is the from notis the excessive. caused Noise by in randomrecorded the transitions. variations in read the channel location of the track, as we assumed inparameter, our but discussion instead of form the zig-zag transition reduce patterns the in order large to demagnetizinging field energy and associated demagnetiz- (discussed with in the [49. longitudinal transition 12 nm [49. Novel Materials and Selected Applications

Part E

Part E | 49.1 1200 Magnetic Information-Storage Materials 49.1 Magnetic Recording Technology 1201

Grain size (nm) Grains per bit a) b) Hd Ø – + MS 800 – + z 24 – + Θ Grain size – + 22 700 – + Grains per bit – + W 20 – + H 600 – + 18 – + – + 16 500 – + 14 B 12 400 c) Energy (eV) 10 300 <Ø> = 21 8 2.5 6 200 4 2.0 100 2 1.5 0 0 E+ 10 100 B 2 1.0 Areal density (Gbit/in ) – EB 0.5 Fig. 49.21 Grain size and grain=bit for sputtered cobalt alloys as a function of areal density in Gbit=in2.(Af- 0.0 ter [49.7]) –0.5

To investigate how far the areal density can be –100 –50 0 50 100 150 200 250 300 extended with LMR, use has been made of the signal- Magnetization angle Θ (deg) to-noise ratio calculation to see if an areal density of 200 Gbit=in2 is feasible. To ensure that the signal- Fig. 49.22 Calculated energy (c) of an isolated single- to-noise ratio does not degrade when extending the domain particle (b) at the center of an 80 nm-long dibit (a) areal density from 60 to 200 Gbit=in2, it is required (double transition). The properties of the particle are listed that: (a) the transition parameter be reduced from in Table 49.4. The ratio of the barrier height to thermal en- 12:86:2 nm, to achieve which the coercive field (Hc) ergy is 43 (see text) and the height 43kBT is shown shaded. must be increased from 40005000 Oe, and (b) the There is margin between the top of the shaded region and ı C product of the magnetization and thickness (Mr )bere- EB and the particle is stable for the time tx of 10 y. (Af- duced from 0:320:2memu=cm3. In addition, the HMS ter [49.42]) must be reduced from 3015 nm [49.7]. the superparamagnetic effect using an isolated small Superparamagnetic Effect single-domain asymmetrical particle is summarized in To maintain the growth in areal density as shown in Fig. 49.22 [49.42]. Here, the magnetic particle is as- Fig. 49.1, it is necessary to reduce the physical di- sumedtobeinthecenteroftwotransitionsandthe mensions of the recording system components and to energy contour results from an analysis using only maintain the signal-to-noise ratio at levels required for shape and crystalline anisotropy (combined into Ku) satisfactory error rates. This includes reducing the mean and magnetostatic energy terms grain size of the recording media. However, at some 2 E. ; / D KuV sin C HMs cos.  / : (49.16) grain size the thermally driven fluctuations of the ori- 49.1 | E Part entation of the magnetization in the isolated grains It is assumed in Fig. 49.22 that the angle  has a mean result in increased probability of the magnetization be- value  D 21ı. The energy contour shown in Fig. 49.22 ing switched from the desired orientation, as initially results from the application of (49.15) with the parame- recorded, to the opposite direction. This instability of ters listed in Table 49.4. The energy required to switch the magnetization is the superparamagnetic effect. The the particle from the initially magnetized direction is magnetic energy in an isolated grain is K V,where C u shown as EB . In the absence of an external or demag- K is the uniaxial anisotropy consisting of crystalline, C u netizing field, the energy barrier is EB D KuV. magnetostriction and shape contributions whereas V D With a demagnetizing field (Hd) the switching field ..D=2/2ı/ is the volume of the particle or grain with is reduced since the field is in the direction of the diameter D. switched magnetization Thermal energy supplied from the environment  à 1 to the particle is kBT,wherekB is the Boltz- C jHdj 2  : mann constant (0:8619 10 4 eV=K). An analysis of EB D KuV 1  (49.17) H0 ). 0 the 0 0 / 49.22 03 eV, V 38 (49.20) : . (13 nm) Time (s) Time 9eVand T= : T)=0.3 B 7nm/4nm] 0 B : ) are SFM or /k b /k b  E 49.22 E less than about solid diamond ( σ open). circle The ı/ r 100 in) for three magnetic M . = 33 is defined as ; from the top shows amplitude loss of T= R B Á at 350 K (the assumed op- 1 Á /k b 0 is reduced to t t T E  B

49.23 n/n]( 4nm/4nm] / / C : B k 0 t t . . E A A log 10

]. The three media are: a single-layer in (kflux changes per inch) at room 1 s, the decay rate per decade for the 43 . Figure 100 2 = D decreases and the particle could become D ), [12 nm/1 ]) cm 29 eV and the shaded region in Fig. 0 : / t Signal loss versus time at room temperature C B 1 T 43 E ; d  01 H T . B . With R The thermal energy For example, if the thickness of the recording me- k / t 25= memu –21.0 –22.0 –23.0 –24.0 –25.0 –21.5 –22.5 –23.5 –24.5 –20.5 . Signal (300 kfci, dB) Signal (300 : recorded data for threesity different media of at 300 a kfc linear den- temperature [49. of CoCrPtB (lowest curveand with two a recording thickness surfaces ofsection. we 12 The nm); will signal discuss decay in rate the next unstable. dia is reduced from that assumed in Fig. to 6 nm, the value of where the signal amplitude ofA a square-wave pattern is Fig. 49.23 and 300 kflux changes per inch (kfc AFC media with layer thicknesses of: [12 nm/0 (open square 0 43 erating temperature of a typical disk drive) is 0 the thermal energyA would dramatic easily loss switchrecording of media the with data media particle. flux over time is observed with (After [49. recording surfaces: The first two ( lower curve is for a 12 nm single layer ( has this energy level.tween Since the there is thermallyand some driven the margin top contribution of be- the to barrier,10 the year switching particle period). is However, stable for (over smaller the volumes value of C B D of E 0 / 1 H m) x = (49.19) (49.18) t < A , usually 05 and x : 3 m)) is ap- and is in- 0 10 < orresponding   B :9mA) 0 . E (3 3 : x m) ) 2 95jD = (375 3 : cm ! 3 = m cm / = = J is particle anisotropy ln 0 cm H erg T ı 5 49.4, the depth of . j = 6 2 B u / ; 10 k C B 2 K 10 =  E Ã  attempt frequency ; j D ı :5

x :39 memu :5 0 ! f 0 375 emu 15 nm 13 nm . 780 Oe (62 kA 21 Value 80 nm 1 (1 95 and ]. The energy barrier depends x / : double transition c ln t 0 j is an exp H 42 T . 0 0 Â B f D  k C B x j ln E x : T is the demagnetizing field between tran- and  ln B Hz [49. d 0 43 . When the magnetization of the particle

k 9eV. f 9 d H is mean field angle with respect to grain or =  Dj D H 1 Energy of single-domain particle in a mag- 10  exp / x / t D D 0 ])  H H  T . The magnetization of the particle will switch 0 0 . . s B 42  s (10 years) and the percentage loss of data am- H k C B C D B 8 M E E = As an example, if the required storage time is The time constant for the thermally driven switch-  ı (grain/particle diameter) (grain/particle volume) u (dibit length) r s 10 (media thickness) d u K ı V H  Parameter B K M M D  is switched, the new energy barrier is plitude is 5%, then sitions and particle axis. A dibit is a 2 to a doubleter [49. change in magnetization orientation. (Af- energy, consisting ofcontributions, shape and crystalline anisotropy creased in depth sincein the the demagnetizing same field directionset is as of now the parameters magnetization. listed in Using Table the on the thermal energy and the fraction the retained magnetization and the storage time when ing of the magneticnetic particle field, which in in the the presencedemagnetizing case field of under acting consideration a on is the mag- the magneticbe particle, found can using an Arrhenius–Néel model where (with a demagnetizing field of 780 Oe (62 kA= With a random two-dimensional (2-D) system proximately 1: Table 49.4 taken as netic field of two transitions. Novel Materials and Selected Applications

Part E

Part E | 49.1 1202 Magnetic Information-Storage Materials 49.1 Magnetic Recording Technology 1203 media shown as the lower curve in Fig. 49.23 is found a) + : + to be  7 19%. Mrt1 The decay rate for the other media will be discussed + – in the next section. The decay rates for the two record- Mrt2 – ing films shown in Table 49.3 are  0:7%=decade and 0:4%=decade for the 20 GB/platter and 80 GB/platter b) M (memu/cm2) media respectively. In a conventional disk recording system this superparamagnetic effect was judged to 0.4 limit the areal density to somewhere in the range 36100 Gbit=in2 (reviewed in [49.42]). Formally the 0.2 superparamagnetic limit to areal density in LMR is set 0.0 at 100 Gbit=in2. The superparamagnetic effect and the requirement –0.2 for low media noise result in recording-system design- Mrt1 = 0.31 –0.4 Ru ers being caught in two inconsistent requirements: first, Mrt2 = 0.09 a small grain size for the recording media is required –10 –5 0 5 10 for adequate signal-to-noise ratio, so that the number H (kOe) of grains per bit remains constant (49.13), and second, a large grain size is required for small data loss over Fig. 49.24 (a) Schematic of the antiferromagnetically time because of the superparamagnetic effect ((49.16) coupled (SFM or AFC) media showing the two ferromag- and (49.17)). The requirement to reduce the thickness netic layers coupled by a thin layer of ruthenium (pixie of the recording media can be deduced from the need dust) and with a single magnetic transition for which the for low values of Mrı for short transition lengths (49.5). magnetizations in the two layers are oppositely directed. The approach of increasing the anisotropy of the grains (b) Magnetic hysteresis curve for the SFM or AFC media to increase KuV to resolve the superparamagnetic effect with remanent thickness products for the two ferromag- is limited by the ability of write heads to record on the netic layers shown in the inset and measured at 350 K. media with the resulting increased media coercivity. The filled triangles are the major hysteresis curve and the Table 49.3 shows that the decay rates observed with filled circles are the remanence hysteresis curve. The ar- the smaller grain size/higher density media is actually rows show the directions of the magnetization in the two smaller than that for the larger grain size/lower density films. (After [49.43]) media. A possible explanation is that the KuV values for the smaller grains are higher than for the larger teresis curve is shown in Fig. 49.24 [49.43]. The sharp grains [49.7]. In the next section we discuss a novel reduction in the remanent magnetization at  80 Oe is approach that has extended the progression in areal den- due to the switching of the thinner lower layer .Mrt2/ sity. and the lowered value of the remanence is advantageous for reduced transition noise and transition length. Antiferromagnetically Coupled (AFC) The curve with solid circles is a remanence hys- or Synthetic Ferrimagnetic (SFM) Media teresis curve measured by subjecting the sample to The invention of a novel recording media decreas- a negative reversing field and then increasing the field ing the impact of the superparamagnetic effect was in a positive direction to zero and plotting the remanent

accomplished simultaneously by teams of researchers magnetization. This measurement is an attempt to simu- 49.1 | E Part at the Fujitsu and IBM Almaden Research Laborato- late what happens when a recording medium leaves the ries [49.44, 45]. vicinity of the recording head. The recording media consists of two ferromagnetic By analyzing the read pulse from the transition in layers coupled by a thin layer of ruthenium (antiferro- the upper layer combined with the read pulse from magnetically coupled – AFC or synthetic ferrimagnetic the transition with reversed sign from the lower layer media – SFM), which results in an antiferromagnetic (the layers are antiferromagnetically coupled), it can be coupling of the two films, similar to that used in the shown that no degradation of the resolution (PW50)of spin valve for a synthetic antiferromagnet. In order for the system results from having two as opposed to one the coupling between the two ferromagnetic layers to recording layer [49.45]. The advantage of the AFC or be antiferromagnetic, the thickness of the ruthenium SFM configuration is that the medium is in fact thicker layer, referred to by IBM as pixie dust, is required to than would be required for the given recording density be approximately 0:6 nm [49.45]. A schematic of the using a single layer and hence is less susceptible to the antiferromagnetic coupled media and the resulting hys- superparamagnetic effect (49.17). 2 cm = ]. The less than 42 ]. n.The 4nm. R : 45 [49. 2memu : also shows that 49.25 values constrained be- (head field versus cur- and values of ). The dibit is read with c ]. Areal densities larger in and the bit density was p t / = H T I 45 . The resulting curves of B . p t k = head B with increases as the thickness de- ]. Analysis comparing SAF and E H 2 values lower than 0 7 C t have been used with LMR using r ]. ]. cm 2 M 48 48 in = ]. (1 ns) are shown in Fig. 49 in [49. = 25= memu as a function of CR H / decade with = Dynamic Coercivity AFM media started out being used in IBM portable The magnetic recording layers were alloys of 49.3 ThedatashowninTable = CR CR H Another manifestation of thermalproperties is effects a change on in magnetic coercivityWith with long switching time. periods ofcle time will the switch probability iswith that increased short a by parti- periods thermal thising excitation probability to and is a decreased,experimental change study lead- in of thestand this coercivity effect in of was which thea done a particle. current on stationary pulse An a of writemagnetically test variable saturated head duration recording is over medium.pulse a driven The previously records current by twoeach transitions transition (a is dibit). essentiallyand The the the gap width of length of thevaries of write with head the the pulse reversed width magnetization ( region disk-drive applications [49. than 100 Gbit the middle curve andnetic the layer. lower The AFM curve or is SFM mediadecay a clearly single reduces rate. the mag- The improvementwith a is larger even value for more significant have been reportedmagnetization measurements by with a IBM reversing magnetic field researchers to mimic using the demagnetizing static field [49. low 4300 Oe. If recording headsadequate are able overwrite to record on with than media 4300 with Oe, coercivityferred then [49. greater the single-layer media is pre- rent) it is possible. to deduce the remanent coercivity a magnetoresistive sensortransitions moved across resulting the inthe recorded amplitude a of dibitpulse the pulse. width dibit and signal By varyingfield as the measuring a and magnitude function of measuring the of reverse the creases. experimental slope H AFM media [49. CoCrPtB with an average grain size of 9 1% write head was trimmedthe final on track the width air-bearing usingtrack density a surface was focused 149 to ktracks ion beam and the 720 kbits it is possiblerecording to layers achieve [49. low decay rates with single single-layer media has shown thatdia the single is layer me- preferredabout 0: unless the media flux drops below , 2 ex ]) < > t J r ex,2 1 val- 47 H V M ex,2 ex u 6 :5 J H  K ]. 1 : 1 :24 t Co-Cr-X 0   46 r M the amplitude a listing of . (After [49. ) [49. :6 E layers 3 3 1 :59 m Co-Cr 0   49.5 = 49.23 is with cobalt, but this E layers :5 (6 mJ 4 ex 3 :73 J Co 0 10  ]. ]. The signal decay rate is cm to both sides of the ruthenium 47 43 . In the case of tight coupling 2 49.10). Because of the antiferro- (dB) V [49. u m 06= erg K : 2 ) product while close to E layers t C 2 ı is 0 ex,2 r 1 and we show in Table is 38 [49. E layers J M ]. The increase in the antiferromagnetic ex- ) V ]. Comparison of materials for M is reduced to zero, it is required that E layers 2 u ex,2 T are considered without 47 43 K D J H B cm 45% per decade for the middle curve (open cir- : m = k < 6 = ex,2 B 14% per decade for the top curve (open squares)  eff (erg : E layers For the magnetization of the lower layer to assume The largest increase in The effective volume is bounded by H The coupling between the two magnetic films is an- . The antiferromagnetic exchange constant can be E V 4 ex c2 u Degradation in (S/N) Material for (all 1 nm thick) J Degradation in overwrite (dB) with respect to SFM media with respect to SFM media  the opposite direction tothe field that of the top layer, before loss with twoand SFM 12/1.4/4 media: (openthe 12/0.7/4 circles), thickness (open of where the squares) of the three numbers layers in are nm and the value reduces the pulserecording width medium over ( magnetic that coupling, of an a exchange magnetic single-layer field ues and degradation in overwrite andNote signal that to media negative noise. dB valuesdicate of improved overwrite. overwrite Both degradation degradations in- inand overwrite S/N K H Table 49.5 for several acts on the lower magnetic film and a typical value forconstant the antiferromagnetic exchange increased by adding thinloys layers known of as cobalt or cobalt al- and alignmentbe of approached. grain The effectivethickness axes, remanent magnetization- ( the upper limit can material suffers from significantwrite degradation and on poor over- cobalt alloys. signal-to-noise To ratio illustratedia that compared configuration actually the with results AFM inloss or reduced over amplitude SFM time, me- we show in Fig. change constant is rapidthe with increasing thickness of layer [49. tiferromagnetic for the top curve and ferromagnetic for and cles) [49. Novel Materials and Selected Applications

Part E

Part E | 49.1 1204 Magnetic Information-Storage Materials 49.1 Magnetic Recording Technology 1205

HCR/HCR (1ns)

1.0 1/C 1000 0.8 200 0.6 92 72 0.4 51 I: δ = 13.2 nm II: δ = 10.2 nm 0.2 III: δ = 7.5 nm (32) SUL IV: δ = 5.5 nm 0.0 Fig. 49.26 Schematic of a perpendicular recording sys- –10 –7 –4 –1 2 10 10 10 10 10 tem. (After [49.5]) Field pulse length tp (s)

Fig. 49.25 Dynamic coercivity HCR.t/ normalized by its a single-layer media, but this approach has not been de- value at t D 1ns for a series of Co64Pt14Cr22 films with veloped. variable thickness. (After [49.42]) In contrast to LMR, the SUL is required for a low- reluctance path for the magnetic flux generated by the The inverse slope parameter is given by coil on the write element. One advantage of perpendic- ular recording is that the magnetic field for recording is 1 KuV larger than that for LMR since the magnetic field from  ; C kBT the write pole is in the gap of the write head, in con- trast to LMR where the field leaks from the write gap. which is the superparamagnetic stability parameter A schematic of the total geometry for LMR and PMR discussed above and it is observed that, even with is shown in Fig. 49.27 [49.1]. Reading for PMR is ac- a stability factor larger than 51, for which the super- complished with a GMR sensor as with LMR evolving paramagnetic region is avoided, the coercivity increases later into tunnel magnetoresistance (TMR). significantly for short switching times. This is the ac- Historically, when areal density was around tual coercivity that must be exceeded by the write-head 100 Mbit=in2 (at the beginning of the 1990s) the read magnetic fields to switch a recording film. sensor was based on simple magnetoresistance. In the mid-1990s it evolved into a spin-valve GMR sen- 49.1.5 Perpendicular Magnetic Recording sor for areal densities larger than 1 Gbit=in2. In 2006 TMR valves were used for areal densities larger than The technology that supplanted LMR is perpendicular 100 Gbit=in2 and in 2011, CPP-GMR read heads started magnetic recording (PMR). Perpendicular recording of- to be used for areal densities close to 1 Tbit=in2. Typi- fers a way out of the conundrum caused by the need for cally a TMR head is about 70 nm wide whereas a CPP- thinner recording media for improvements in recording GMR head is about 30 nm in width. density and the need for larger particles to reduce the The major difference in the write head is that it uses

impact of superparamagnetism. a single pole to generate the perpendicular magnetic 49.1 | E Part While smaller particles are needed to increase areal field with a wider return pole (to reduce reluctance). density in LMR, particle size in PMR has kept its value The write head for perpendicular recording is imaged around 79 nm since it was introduced for the first time in the SUL and this results in an increase in the strength in 2006. of the magnetic field by approximately a factor of two. In perpendicular recording the magnetization in the In addition, due to imaging effects, the effective number recording media is held perpendicular to the surface of turns on the write head is doubled for perpendicular of the recording media by a perpendicular anisotropy recording. It is important for perpendicular recording large enough to overcome the large demagnetizing field that the magnetization of the SUL magnetic material be with this orientation of the magnetization. A schematic larger than that for the pole of the write head [49.50]. of a perpendicular recording system including a probe This is the case to prevent the SUL from saturat- write element and a magnetically soft underlayer (SUL) ing before the pole tip saturates, resulting in a decrease is shown in Fig. 49.26. It is also possible to have a per- in the gradient of the head field, which would result in pendicular recording system with a ring write head and an increased transition length (to be discussed). A large ]. 53 [49. Soft underlayer (SUL) Recording medium Recording medium 49.28 49.29, where it is seen Write current Write , which reduces the transition ]. Results of the simulation of / ]) x 1 53 Write current Write d = y 49.27. P2 H NS d ]. A schematic of a write head with . 52 , Magnetization 51 Shield 2 SN SN P2 P1 The particular geometry and saturation magnetiza- Shield 2 head that results intion an of the improvement writing inof flux the the and head concentra- field an increase in the gradient length [49. a trailing-edge shield is shown in Fig. The shield connects to the largerhead return structure shown for in the Fig. tion of the main pole andshield the front were yoke or used trailing-edge model in a to two-dimensional calculaterecorded finite transition the element [49. head fields and shape of the transition shape are shown in Fig. that the recorded transition isthe considerably sharper head with with the front yoke compared with the head Monopole inductive write element Return pole / T P1 B k “Ring” inductive write element “Ring” inductive = NSNS V u Read voltage K . by virtue of the u K Read voltage Shield 1 Read element GMR sensor Shield 1 with the thicker media. Reading of the mag- NSSNNSSN Schematic of longitudinal and perpendicular recording systems. (After [49. / V . Track width Track Track width Track A significant development for perpendicular write Perpendicular recording Perpendicular Longitudinal recording Longitudinal is increased both by an increase in increased coercive field butvolume also by the increase in the netic transitions with perpendicularwith recording a is spin-valve done headbeing that as with with perpendicular recording LMR thea response step – is instead the of a difference pulse (to be discussed). heads is the addition of a trailing-edge shield to the Fig. 49.27 magnetic field and gradientwrite of head magnetic can record field media fromthan with the a for larger coercive LMR, field length. which Perpendicular-recording results media inthicker can than a for be reduced longitudinaldensity. media transition made The for media the thermal same stability areal factor Novel Materials and Selected Applications

Part E

Part E | 49.1 1206 Magnetic Information-Storage Materials 49.1 Magnetic Recording Technology 1207

3 Calculation area Main pole Magnetization (emu/cm ) 20μm x 20μm BS = 2.4 T 300 Front yoke head Hc of media 200 6000 Oe 8000 Oe Regular head 100 50nm H of media Front yoke BS = 2.4 T c Spacing 20nm 6000 Oe 8000 Oe Recording layer 12nm 0 Seed layer 20nm μ = 1.0 –100 Under layer BS = 1.25 T

–200 Fig. 49.28 Schematic of a perpendicular recording head including the main pole and the front yoke or trailing-edge –300 shield. (After [49.53]) –180 –170 –160 –150 Down track position (nm) without the shield, particularly with the higher coerciv- ity media (Hc D 8000 Oe). Fig. 49.29 Magnetic transitions in perpendicular record- The perpendicular magnetic field has the amplitude ing with and without the front yoke or trailing-edge shield. reduced to one-half at a distance of 0:8d0 with the shield (After [49.53]) and 1:3d0 without the shield (assuming that the spacing of the shield is separated from the main pole by a dis- Normalized signal amplitude tance d0) [49.52]. The shield shunts some of the write flux, reducing the amplitude of the write field, so it is Longitudinal 1.0 important to keep the height of the shield short. Another Perpendicular advantage of the trailing-edge shield is that a horizontal component of the field is added. This field reduces the field for switching since, if the angle of the field is , the switching field is

0.5 Hk ; Hswitch D h i 3 2 2 2 .cos /3 C .sin /3 where Hk is the anisotropy field of the perpendicular medium. The denominator of this equation is always larger than one and hence the switching field is reduced 0 –200 –100 0 100 200 for angles larger than zero degrees. Note that this Offtrack position (nm) 49.1 | E Part equation is the switching astroid equation used to ex- plain the operation of the MRAM. Fig. 49.30 Cross-track profile of the signal with perpen- A significant advantage of PMR as compared with dicular and longitudinal recording. (After [49.52]) LMR is the narrower off-track read profile as shown in Fig. 49.30 [49.52]. This improvement in read pro- disk, being largest at the inside and outside diameters. file leads directly to increased track density. There are With head skew it is possible for the write element of limitations to the achievable track density in disk drives the head to be placed over the guard band or adjacent where there is significant skew of the head with re- track of a selected track, resulting in unwanted recording spect to the track center line. Head skew results from in these areas. This problem is reduced by using trape- the fact that disk drives use rotary actuators and the an- zoidal poles [49.54]. Shields can be added to the side of gle of the written transition with respect to the track the head to reduce this erasure of adjacent track informa- center line varies as the actuator is rotated across the tion; however, the processing of such shields is complex. D s M (49.24) (49.25) , 3 : cm 0 = 49.7) with the D ; s erg 49.24) by recog- 6 :  M 2 s :  10 / . The coercive field ext 2 s M j 2 s Á factored out is  H  e s  M u M s

4

K M ; H  C 45  s : M 2 4 s  4

1 j M u ; cos is not the same as the coer-   M ext  s 84 kOe and K D u  :  Ã H e u j : cos 2 u s 6 K M u   K 84 kOe e u H 2 : ; K  2 K M :  K H 6 0 . 4 j j 2 2 0 2

H  e D D D D  C D H / / / / 68 kOe s C : j D D ı j ı / ı ı j

49.21) with sin c M  0 0   e

e ext 13: is as follows ı H  H H H 0 180 180 cos D D 3 j 4  2 cos s D 2 Â D D D is measured from . . cm M D D D 0 ext  

e e Dj D c  .  . s D H H s H H ? 4 ?   H e e .  e ext c ext M M M H H  M H H The coercive field is found from ( H M H The result for a Co-Cr-Pt alloy (Table nizing that the magnetic field 280= emu whereas This solution applies whennot the saturated. magnetization Note that curve is thus in The solution to ( Thus cive field where and where is then following parameters: ], 2 / 3 . 55 2 s ]. It is cm M (49.21) (49.22) (49.23) = 2 s [49. ; 56 ])  M 5 2  :46 :49 :77 :53 emu 0, resulting 2 . 0 0 0 3 cos > ) and the sta- s 49.6 ¤ u ı M M K 3

ext ext H cm ])

H = ) or 180 :95 :45 :98 6 55 cos 2  u 49.31. The energy of 0 1 2 u ı K (erg 10 10:5 . The equilibrium angle :

sin 2 0 K : ext 2  0 u 2 0 H D ) K θ sin 3 C 0 results in 2

D u u C > solution is no longer stable is

49.21) with cm K K D

0( = ı 0 49.8. A review of work on per- cos C 49.22). The critical magnetic field ext D s sin s cos 2

s H D sin 2 M (emu 270 280 350 750

2 s M 2 2 s M

and M ext M ext cos  H  2 s 4 H k 7 2 49.7 C 10 17 28 H (kOe) Magnetic properties of selected materials for M C Magnetization and perpendicular anisotropy  D 2 D 2 E D

2 E d d d d Media for Perpendicular Recording E Alloys under consideration for perpendicular Co-Cr Co-Cr-Pt Co/Pd Fe/Pt Material and Tables The magnetization and anisotropyrecording in film a is perpendicular shownthe magnetization in in the presence Fig. of theanisotropy demagnetizing, and magnetostatic energies (Gaussianis units) below which the observed from this tablesatisfy that the these condition ( candidate materials recording include those listed in Table pendicular recording media was published [49. for a perpendicular recording media. (After [49. given by the solution to ( is given by the solution to Table 49.6 Fig. 49.31 The solution is sin For stability where the demagnetizing factor fortized a normal to thin the film surface magne- is 4 perpendicular recording. (After [49. bility condition for Novel Materials and Selected Applications

Part E

Part E | 49.1 1208 Magnetic Information-Storage Materials 49.1 Magnetic Recording Technology 1209 3 = ] 1 / u 70 K = 58 ] 59 ] 60 ] 61 ] 62 ] 63 ] 64 ] 65 ] 66 ] 67 ] 68 – T : 8 : 2 B 2 2 References [49. [49. [49. [49. [49. [49. [49. [49. [49. [49. [49.   (nm) , domain wall : 0 : 8 : 0 : 3 : 6 : 1 : 7 : 7 u . 60 k p 8 4 5 3 3 5 3 2 D 10 : 4 K D = p A D p  25 dB 31 dB D : 96 D D w 0 ı m) c : 89 : 06 : 21 : 20 : 34 : 61 : 71 : 23 : 71   D ( 0 0 0 0 0 0 0 0 0 cm) 57 = w : 7 : 5 (erg 5 8 18 17 32 28 16 27 42  , domain wall width: cm, minimal stable grain size: s Comments Good, poor thermal stability Good SNR, poor thermal stability Good thermal stability, SNR Good thermal stability, poor SNR SNR improved – Good SNR, poor thermal stability Good thermal stability, poor SNR Good thermal stability, good SNR Good thermal stability, good SNR Good thermal stability. SNR = M = erg u 6 3 K 30  2 10 : 7 : 8 D w 70 75 39 45 77 46 22  1 1  1to k n ı (Å) 222 148 D   H (kOe) – – – – – – – –  H A ]. All these materials are considered as uniaxial and capable of sustaining 50 50 30 57 c 760 750 840 650 585 T (K) – 1404 – 1000 Magnetic layer thickness (nm) 50 25 – 10  – 10  – – 10  – – 30 400 : 4 30 k 6 k 13 : 7 36 33 69 73 H (kOe) 6 13 – 15  – > 12 – 18 : 6 – – H (kOe) 116 123 240  , exchange coupling constant: 2 s ) c M H = = ) r w 3  M 4 cm :  = 1 : 25 : 09 : 05 : 84 : 35 : 87 : 83 : 86 : 08 : 11 : 23 ˛ (4 1 2 1 0 1 2 1 1 1 1 1 D s 298 800 560 910 c M (emu 1400 1100 1100 1140 1270 ) D 3 ) cm 3 450 600 = cm r = M (emu 250  500 460 200  160 800 400 520 430 400 395 10 erg atE|49.1 | E Part 20  7 : 20 : 45 : 0 : 8 : 6 : 9 : 7 : 6 u 0 0 2 1 6 4 1 4 K (10 11  : 4 : 0 5 3 5 4 10 5 5   5 c   : 5  : 2  : 5 : 5  : 02 H (kOe) 2 2 5 2 3 2 2 3 > 4 4 B 2 5 Nd Pt , single particle domain size: 3 14 u Material CoPtCr Co Co FePd FePt CoPt MnAl Fe SmCo AK /Ti/CoZrNb p 7 4 2 14 : Media for perpendicular recording Properties of hard magnetic materials that might be used for PMR [49. /CoPt  Pt SiO 13  12 w 15 n    Pt Cr 3 18 10 nm grain sizes over 10 year storage time. Physical parameters include anisotropy field: Pd PdSi 10 years) are displayed phases = = 70 : 0 Ä D Co Co p Media CoCrNbPt/Ti CoCrPt/Ti/CoZr Co Œ Œ FePt CoCrPtB/Ti- CoCrTa/laminated SUL CoCrPtO/Ti-Ru/laminated SUL CoPtCr Coupled/granular/continuous CoCrPt/CoPt Coupled/granular/continuous CoCr Alloy type Co-alloys L1 Rare-Earth Transition met- als energy: Table 49.7 (  Table 49.8 D , . / / for u d H . . 0 N (49.28) (49.27) , the con- recording c s / H P M for a perpen- a 5 : . d and the spacing for longitudinal H / for perpendicular c ; .ı/ / H à c . s : ( ; H . /ı à ı 4 d  Q x ]. Here, S H  d  d 49.34 /  71 n M  1 s C . S H ] Q – M α  ) and perpendicular 72 C /  d s x head 1 2 1 α / ; d . : a L H s 49.27) with the parameters listed in c a d /  s π–N 4πM . H  t C  c n ]) (4 s S for perpendicular recording can be esti- c M Q s c H H t H /  55 H M  C – – 4 a d D d and a reduced demagnetizing factor M )and( is shown in Fig. Hysteresis curve with demagnetizing field 1 1 . QH . 4 a is the total spacing of the write pole tip to the D x head D D D s =˛/ d 49.9 x s 1 H M a ˛ d a d d A comparison between the transition parameters M is the demagnetizing field. Using  d . (After [49. top of the SUL, consisting of the physical spacing a perpendicular recording film˛ with loop slope parameter .4 for longitudinal dicular transition [49. where the thickness of the recording media equal to or greater than approximately 1 This equation is solved by assumingsition an shape arctangent tran- and denoting the slope of the head field by Fig. 49.33 dition that ensuresparameter adequate overwrite. Themated transition using the same write-slopelongitudinal criteria recording [49. discussed for from (49.5 Table recording is the dark line and between the SUL and the magnetic recording film H ), ]. to n and and 55 H M . (Oe) ˛  (49.26) 30000 H [49. )forthe 49.32 ext is limited by H while the nu- 49.33 c s H M  c versus H 54 : M 0 characterizes the slope of D : 01 D H d 1 ]. : .˛/ N c  55 H / :  e s D j H H M result from variations in the direction  e 2  à 2. For high areal density, the reduced  H ˛ = 4 j s H M ext d d M C H n .  n Hysteresis curve for Co-Cr-Pt perpendicular H –20000 –10000 0000 20000  H D s 4 . This reduction in demagnetizing field results D D M s ? / D 0 =˛ eff M 0 M ˛ The magnetization curve for the fourth quadrant is For high-density recording, large values for H The effective anisotropy energy, which assists in The magnetization curve is shown in Fig. perp are required; however, the value of K M 1.5 1.0 0.5 –30000 c –1.0 –1.5 –0.5 M  cleation field is increased to Finite values of Fig. 49.32 of the perpendicular magnetization,perpendicular anisotropy and dispersion variations in in the exchange the coupling between grains and clusterseffective in demagnetizing the field media. The is reduced from 4 overcoming the thermal energy andnetic the limit, superparamag- is given in terms of the nucleation field recording media using the analysis in the text recording medium is as illustrated in Fig. the magnetization curve at at which field theby magnetization just starts to reverse, Here the slope parameter the ability of the pole head to generate magnetic fields demagnetizing factor is H 4 from the formation oftern a of rectangular domains checkerboard with pat- high-densityhigh linear track recording densities and [49. exhibits the desiredsheared large due to the coercivity large demagnetizing but field.the In general, the magnetic hysteresis curve curve is ( Novel Materials and Selected Applications

Part E

Part E | 49.1 1210 Magnetic Information-Storage Materials 49.1 Magnetic Recording Technology 1211

Table 49.9 Parameters for evaluating transition parame- –4 ters for longitudinal and perpendicular recording aP and aL (10 cm) 0.09 Parameter aL aP 0.08 d (nm) 26 26 t (nm) – 50 0.07 u (nm) – 0 0.06 S 0:95 0:98 3 0.05 Mr (emu=cm ) 200 200 ı (nm) 14 – 0.04 Q 0:6 0:6 0.03 H (Oe) 3500 8000 c,max 0.02 recording is dashed). It is seen that the transition 0.01 parameter for perpendicular recording can be made 0 smaller than that for longitudinal recording if the 0 4000 6000 12000 H (Oe) coercive field for perpendicular recording is larger c than 6500 Oe. Note that the thickness of the recording Fig. 49.34 Comparison of transition parameters versus the media for perpendicular recording is larger than that coercive field .Hc/ for perpendicular (solid line) and lon- for longitudinal recording. The magnetic squareness gitudinal (dashed line) recording. (After [49.71]) .Mr=Ms/ for perpendicular recording media is larger than that for longitudinal recording. Gao and Bertram have shown a significant ad- the results for media parameters for areal densities of vantage for perpendicular recording if the uniaxial 1001000 Gbit=in2. Several key media parameters are anisotropy is not aligned perpendicular to the surface, the grains per bit and the coercivity .Hc/. An esti- but oriented at 45ı to the perpendicular [49.73]. mate of the signal-to-noise ratio (SNR) using Poisson The geometry for this tilted perpendicular recording statistics of additive noise for the magnetic grains is . / is shown in Fig. 49.35. The crystallites comprising the SNR D10 log10 N ,whereN is the number of mag- media are still aligned perpendicular but the crystalline netic grains per bit [49.74]. There are two possibilities anisotropy is assumed to be at 45ı. No material system to realize recording media with a large perpendicular was disclosed that exhibits this tilted anisotropy. With anisotropy and that can approach Table 49.10 parame- tilted perpendicular recording the crystalline anisotropy ters: granular media typified by CoCr alloys and contin- field .Hk/ can be made significantly larger than if the uous multilayer media typified by [Co-Pd]n multilayers. anisotropy is perpendicular. An example is 15 000 Oe CoCr alloys exhibit good SNR but insufficient Hc for perpendicular recording and 28 000 Oe for tilted for ultrahigh recording densities, while the multilayers perpendicular recording [49.73]. Some of the advan- do not exhibit a satisfying SNR. tages of tilted perpendicular recording are: reduced Co/PdSi multilayers where the Pd layer is switching field and reduced average grain diameter, doped with silicon have shown an improvement in resulting in increased signal-to-noise ratio since the SNR [49.62]. The 50+ Gbit=in2 technology uses a gran- number of grains per bit is increased. The requirements ular CoCr alloy. Tables 49.7 and 49.8 contain several for media for ultrahigh-density perpendicular recording metal alloy used in perpendicular recording. In general, Pt plays a significant role in determining the magnetic have been analyzed [49.74]. Table 49.10 summarizes 49.1 | E Part

a) 320nm b)

Anisotropy Anisotropy direction direction 60nm 120nm 300nm Fig. 49.35 Typical geometry Medium Medium of tilted perpendicular recording. The medium SUL SUL anisotropy directions and not the medium grains are tilted Cross track direction Down track direction in the cross-track direction. (After [49.73]) ]. and 50 m  12 (49.29) :3 Pt 0 17:7 (70) 550 pole tip 18 Co M C (5 nm) Pt Pt buffer CoCr Ti (18 nm) Ti NiAl (40 nm) Glass substrate ]) m  67 :1 0 16:3 (90) 175 ]. A guide for select- 50 is / ; are the area and the circum- SUL T SUL . pole tip 49.20) [49. M pole tip M ], where it is seen that the thickness C 50 and pole tip pole tip [49. A C Effect of the thickness of the FeAlN soft Schematic representation for coupled granu- 20 kG) ])  pole tip D layer layer A 50 49.11 s are the saturation magnetizations of the pole tip of nm) SUL M = Granular T  SUL Continuous Thickness of FeAlN SUL (4 Maximum magnetic field (kOe) at recording layer at saturation current (mA) (in parentheses) Maximum head field gradient (Oe M magnetically soft for lowexhibit reluctance magnetic but domains, also which mustto can not the contribute read noise signal. Thement SUL that must is have larger than aelement that magnetic for mo- so the that polesaturates. tip the of This the SUL write condition saturatescess assures reluctance after that directly the there undermust pole is the also tip pole no be tip.path ex- thick through which The enough the flux SUL that travels allows themagnetic the maximum field under reluctance the gap. of Decreasingof the the reluctance the flux path alsonetic field increases from the the pole, gradient which of reducestransition the the length parameter mag- of ( the where Table 49.11 Fig. 49.36 ing the SUL thickness underlayer (SUL) onter [49. recording head parameters. (Af- lar/continuous (CGC) media. (After [49. the write head and soft underlayer respectively [49. The effects of changingthe the recording parameters thickness of ofTable a the probe SUL head are on shown in ference of the pole tip respectively, whereas ]. 78 ]) 1000 5 15 32 4 1 24 6 875 14:3 74 700 12:5 500 6 16 40 5 1 22 4 5 dB [49. ]. ], for example 78 76 550 11:0 250 7 17 50 7 1 20 3 , (CGC) medium that 75 200 kOe) and can re- , , a reduced pulse width / :5  ]. n 66 400 8 100 8 18 63 9 1 18 2 ]. The last two listings are mal stability but the high- H 68 . 77 , . 2 s u 66 , M K ) D 55 2 in ]. The continuous layer causes an ex- = (nm) ı 67 (nm) (kOe) N ]. An issue with oxide media is the nonuni- d ) Requirements for perpendicular recording c 3 [49. 77 ) H ]. and good thermal stability) and granular me- 3 cm multilayers coupled through a Pt buffer layer to c = 78 cm n H = coupled granular/continuous erg 49.36 6 The initial results for CGC media were encouraging A cross-section of the CGC media is shown in Magnetic materials used for the soft underlayer Oxide media can have their coercivity improved and Another alternative for perpendicular recording  Coercivity S Media thickness Perpendicular anisotropy (10 Saturation magnetization (emu Areal density (Gbit Flying height Signal-to-noise ratio (RMS signal/RMS noise) Grains per bit Average grain diameter (nm) CoPt from the standpoint of ther density SNR was 2 dBalone. less than Further for the optimization granularŒ of media the CGC media using Fig. sult in considerableity [49. improvement in thermal stabil- change coupling that stabilizesthis the exchange granular field medium; is large ( (SUL) for a perpendicular recording media must be combines the best properties(high of the continuous media formity of thedisk coercivity surface and due to oxygengas the reactive content high sputtering gas [49. in pressure the and oxygen- for a CoCrPtO or CoCrPt-SiO a CoCrPt granular layer hasSNR shown over the an granular improvement media in alone by 3 The improvement in SNRfact is that believed magnetic to recording beuous layer due transitions are to in in the the thethe form magnetic contin- interaction of between narrow the domain twosult walls layers and in may re- breaking uptransitions the in magnetic the granular clusters medium that [49. form the a negative nucleation field and reduced signalmedia [49. decay with respect to CoCrPtB anisotropy and Cr, Ta, Nbcoupling, resulting and in B reduced reduce noise. the exchange media is oxide media [49. Table 49.10 media for ultrahigh recording densities. (After [49. dia (good SNR) [49. Novel Materials and Selected Applications Part E

1212 Part E | 49.1 Magnetic Information-Storage Materials 49.1 Magnetic Recording Technology 1213

Table 49.12 Magnetic materials for soft underlayers Table 49.13 Comparison of perpendicular recording pa- (SUL). Permalloy has an insufficient magnetic moment for rameters for two areal densities using CoCrPt. BER is bit SUL with a field gradient that decreases at maximum field. error rate whereas CR is code rate, a measure of coding ef- Sendust also has an insufficient magnetic moment for SUL. ficiency given by m=n where m is the number of initial bits FeAlN is sensitive to process conditions and believed to be and n > m the number of code bits less useful than FeCoB that must be annealed to reduce Areal density 52:5Gbit=in2  100 Gbit=in2 stress. CoZrNb has a lower magnetostriction than FeCoB. Type CoCrPt CoCrPt IrMn antiferromagnetic film is used along with CoZrNb to M (emu=cm3) 250 330 reduce spike noise due to domain walls s Hc (kOe) 2:6 4:94 3 6 Material Bs (kG) Relative References Ku (erg=cm ) Hk D 10 kOe 1:82 10  1 (T 10 ) permeability S 0:98 0:92  Ni80Fe20 10 1500 2000 [49.50] D (nm) – 11:9 (Permalloy) Underlayer FeTaC (16 kG) – Sendust 10 8000 [49.5] 400 nm (FeSiAl) Head FeAlN 20 2000 [49.55] Turns – 1 FeCoB 2024 200240 [49.83, 84] Flying height (nm) 6:4 CoZrNb 14 [49.84] Magnetic spacing (nm) 20 – NiFe Mo [49.85] 17 4 Pole (Ni-Fe/Si) [49.86] n Material FeNi (16 kG) – IrMn/CoZrNb 14 [49.87] Width (nm) 250 105 Thickness (nm) 400 105 and hence reluctance are key to head performance. Can- Write width (nm)  250 180 didate materials for the SUL are listed in Table 49.12. Shield-shield spacing 80 70 A major issue with the soft underlayer is the motion (nm) of domain walls driven by the magnetic field from Read width (nm) 200 100 the write head, which can lead to spike noise. The Component performance = last entry in Table 49.12 is for an antiferromagnetic Sensitivity (mV m) 3 25 pinning layer to prevent the motion of such domain PW50 70 walls. Channel performance Channel type Simulator Software D = D = Perpendicular Recording Developments (CR 32 33) (CR 16 17) Bit density (kb=in) 590 650 Perpendicular recording is presently used in commer- Track density 88:9 143 cial disk drives. The first was the Toshiba 40 GB, = : (ktracks in) 1 8 in disk drive with an areal density of approximately Areal density 52:5(user) 84 (user) = 2 75 Gbit in , while later the capacity was increased to (user=channel) 93 (channel) 2 80 GB and the areal density to 133 Gbit=in [49.79]. (Gbit=in2 ) In 2006, Seagate provided 2:5 in notebook drives On-track BER 105 6:7 105 with a capacity of up to 160 GB and an areal density of 132 Gbit=in2. An earlier technology with an areal Note also that the superparamagnetic limit of PMR 2 2 density of 52:5Gbit=in was reported in 2001 [49.80] is formally set at 1 Tbit=in as it was set previously at 49.1 | E Part and one of approximately 100 Gbit=in2 in 2003 [49.81]. 100 Gbit=in2 for LMR. A cross-section of the write and Hitachi Global Storage Technologies reported later in read elements for the perpendicular recording head used 2005 an areal density of 240 Gbit=in2 using perpen- in the 52:5Gbit=in2 case is shown in Fig. 49.37 [49.80]. dicular magnetic recording and a tunneling current- The pole of the write head is identified as the main perpendicular-to-the-plane (CPP) GMR read head (see pole and the thick return pole and upper shield of the Mueller [49.82]). GMR sensor are identified as the auxiliary pole/upper We show in Table 49.13 the parameters used for the shield. The read signal versus density (roll-off curve) two areal density configurations. for the 52:5Gbit=in2 case is shown in Fig. 49.38 and In both cases a pole write head and a spin-valve read results from the increasing interference of positive and head were used. The parameters for the  100 Gbit=in2 negative step pulses in a long string of transitions. At case are close to those required for the ultrahigh densi- low densities a step response is observed in perpendicu- ties (> 100 Gbit=in2) as described in Table 49.9, except lar recording since the GMR sensor detects the vertical that the coercivity is about one half that specified. component of the stray magnetization that reverses at , ] c to in 90 H :1 :4 :1 :6 2 2 4 8 26:3 18:9 32:6 5 8 26:7 10:4 42:1 1 49.12 in in barrier = = 2 :8 :5 :8 :3 in 3 8 26:7 27:1 25:8 9 8 25:9 10:1 35:0 1 ]. The lin- = et al. [49. 1Gbit 89 : :6 :5 :4 2 8 33:8 30:0 31:6 12:2 8 35:0 15:6 64:0 3 :8 :7 :7 1 8 57:1 38:8 31:4 19:4 8 54:2 24:1 64:3 7 Marchon 49.14). ] through optimized selection in and the track density was = 88 ] who reported 240 Gbit : 2 82 , crossing the 100 Gbit 2 in = in in November 2002 [49. [49. = Gains & Clusters Grain size (nm) Distribution (%) Cluster size (nm) Distribution (%) Grains per cluster Grain size (nm) Distribution (%) Cluster size (nm) Distribution (%) Grains per cluster 2 ]. In addition, CGC combines high in in. In 2004 Seagate reported an areal den- Progress of areal density as we change ma- = = 88 GRL Mueller C Scaling Laws Areal density can be further increased in PMR by Material type MCL GRL only Areal density controlsparameters scaling of laws LMR impacting and several PMR. derived the following rules valid from 0 beyond 1000 Gbit 2005. the use of compositecalled layers. magnetic A cap layer lowerreversal (MCL) anisotropy and provides is layer continuous deposited over aular higher recording anisotropy layer gran- (GRL). This resultscoupled-composite in (ECC) an exchange- with a continuous-granular- composite (CGC) that provides lateralmedia exchange in [49. the before good thermal stability andular good nature, SNR as due to previouslycombination the discussed. gran- The paves (ECC/CGC) thehigher anisotropy way GRL for materials. Clustertribution exploitation size originating and of dis- from even lateralcan magnetic be interaction optimizedGRL separately and through MCL the materialsof selection resulting the of in number of steadyproving grains decrease areal density per (Table cluster, consequently im- recording, or with equalizer circuits that directlythe handle step response.ized The with channel simulator performanceand channels as approximates is real- the listed requirementstem. of in a Table Seagate practical sys- of technology 100 reported Gbit an areal density Table 49.14 143 ktracks sity of 170 Gbit ear density was 700 kb terial type inof PMR composite layers [49. (thicknessmagnetic and material cap composition), layer(GRL). (MCL) While and grain size granular staysnumber recording approximately of grains constant, layer per the cluster decreasesing steadily in from progress- type 1areal density to 4, decreasing bit size and increasing 2. 50 ) = T A 1000 kb/in (49.30) 2to p–p = A μV  ]) Bottom shield 80 = 620 Linear density ( LF E 100 ; à t in. (After [49. = Auxiliary pole/Upper shield 50 T ln 3  20 kb/in 200 kb/in tanh Normalized voltage amplitude for a sequence Cross-sectional view of a perpendicular head A is one half of the signal amplitude and ]) D A MainPole Coil 80 / t 0 . h GMR sensor 0.01 0.10 1.00 Normalized output where is the time for the signal to rise from Fig. 49.38 the transition and atpears higher as densities the alow-density response sinusoid. response ap- is Thewidth due decay of to the in the preamplifier. thetransitions The insufficient can read be band- signal signal described by at for the isolated response the function Fig. 49.37 with a GMR sensor including theThe bottom and upper upper shields. shieldter also [49. serves as the auxiliary pole. (Af- of read signals from asus perpendicular linear recording density system in ver- kb The step signalsating can the be signal to detected produce either pulses, as by with differenti- longitudinal Novel Materials and Selected Applications

Part E

Part E | 49.1 1214 Magnetic Information-Storage Materials 49.2 Magnetic Random-Access Memory 1215

 Bit size (in nm)  167:01 x0:3715, x in Gbit=in2 Additionally, we have the approximate relation  Track width (in nm)  3890:7 x0:6313, x in Gbit=in2 [49.90] between HMS and bit size, expected to be  Bit aspect ratio (BAR)  23:296 x0:2598, x in valid even when the bit size is smaller (Table 49.14) Gbit=in2 than 10 nm: HMS  (bit size)=2 as long as 10 <  Head media separation (HMS) (in nm) bit size (nm) < 1000.  71:5 x0:317, x in Gbit=in2.

49.2 Magnetic Random-Access Memory

The application of the giant-magnetoresistive effect to and word or digit lines and the stack of films comprising recording heads that we have discussed assumed that a basic MTJ [49.93, 94]. the sense current is in the plane of the recording film. Figure 49.40 shows a more detailed stack of This is not the only possibility and devices with films for an MTJ, including synthetic antiferromag- the current perpendicular to the plane (CPP) have been proposed both for magnetic-recording read heads but also for fast magnetic random-access memory D1 D2 D3 (MRAM). B1 The performance of MRAM in comparison with other memory technologies is displayed in Table 49.15. SRAM, DRAM and FLASH are electronic memories, B2 and FeRAM is ferroelectric memory similar to DRAM called 1C-1T technology (since a single bit requires a capacitor (C) and a transistor (T)) with the capacitor using a ferroelectric instead of an ordinary dielectric, also called 2C-2T technology because a single bit re- quires twice as much capacitors and transistors with W1 W2 W3 respect to CMOS-DRAM technology. The magnetic tunnel junction (MTJ) belongs to CPP Fig. 49.39 Schematic of MRAM cross-point architecture technology and is important for both magnetic record- with bits between orthogonal conductors and each cell de- ing and MRAM developments. It involves a pinned and fined by one MTJ and one transistor. The top lines in a free ferromagnetic film spaced by a thin insulating contact with the top electrode of the bits provides hard-axis film, usually Al2O3 or MgO. fields, while the bottom lines are isolated and provide easy- Figure 49.39 shows the schematic of an MTJ used axis fields. Turning on the transistor provides a current path for the MRAM application, including orthogonal bit so that the corresponding bit can be sensed. (After [49.93])

Table 49.15 Read and write times are given for the year 2007 and forecast for the year 2022. Cell surface is given in terms of parameter F the minimal feature length for the corresponding technology (current value F D 14 nm progressing toward 10 nm in 2017). FLASH memory has a poor endurance (program/erase cycle) of about only 105 because erasing uses

Fowler–Nordheim tunneling that might leave electrons behind in the dielectric. FeRAM is ferroelectric RAM currently 49.2 | E Part used in smartphones and electronic tablets because of its very low energy consumption, whereas STT-RAM is based on spin-transfer torque [49.91] as discussed in the text. Multilevel cell means that 1 to 3 bit might be stored in a single memory element. (After [49.92]) Memory type SRAM DRAM NAND-FLASH FeRAM STT-MRAM Read (2007) 0:3ns 110 ns 50 ns 45 ns < 20 ns Read (2022) 70 ps 0:210 ns 10 ns < 20 ns < 0:5ns Write (2007) 0:3ns 0:7ns 1ns 10 ns < 20 ns Write (2022) 70 ps 0:2ns 1ns 1ns < 0:5ns Retention time Requires power 64 ms > 10 years > 10 years > 10 years Cell size 140F2 612F2 5F2 22F2 68F2 Read voltage (V) 1:1 2 15 0:93:3 3 Write voltage (V) 1:1 2:5 2 0:93:3 3 Comments Multilevel cell Destructive readout Multilevel cell possible e 1.0 ). The m arrows 0.80.60.40.20–0.2–0.4 M  = 2 u : Stack 1 K Easy axis h ) ]) a  ( 6 94 : versus magnetic R Hysteresis curves of = ) R b Fig. 49.40 of films comprising a typical MTJ. The show the direction of the magnetizations in the two coupled films. (  field for 0 bits on a 6(After in [49. wafer. 100 Field (Oe) –0.6–0.8 h Switching astroid plotted with magnetic fields h –1.0 0 The switching field can be significantly reduced if For reading a bit, the sense transistor is turned on by 1.0 0.8 0.6 0.4 0.2 Hard axis –0.2 –0.4 –0.6 –0.8 –1.0 0 magnetic fields are appliedhard-axis directions and along an optimum both is to theequal. have the The fields easy- bit line and isnetic used to field supply the and hard-axiswriting mag- the a bit word the sense line transistorpulses is the turned are off and easy-axis applied current field. alonggenerating a For both magnetic orthogonal field at conductors, amplitude the to free switch film the sufficient magnetization in of the free film. the word line and pulses of current are driven along the Fig. 49.41 normalized to the induced anisotropy field (2 ferromagnetic film willeasy- switch and hard-axis for fields on any the outside combination of the of astroid 0.6 x 1.2 10 in for k Fe 15 Å, R H 90 = 49.15),  : R u s  K :10 m) versus M –60 –20 20 60 3 2 2 49.41 : O 2 1 D k  H 49.39). 6 MR (%) 0 –100 ; 50 40 30 20 10 b) also shows 2 3 / k H . 49.41 ]. D 5 2 3 films for the free layer enhances / 10 hard Fe H Word line Word . sense 90 ]. Figure I C ]. 21 2 3 ferromagnet: 20 Å. As for the spin valve, / 94 I [49. 10 easy antiferromagnet free layer pinned layer 45 Fe R H 49.41. 10 10 10 . Switching of the state of the magnetization will oc- A typical MTJ has the following thickness values The switching is described by an astroid given by Switching of a soft-magnetic film with a magnetic The change in resistance (TMR) with orientation = tunneling layer 90 3 Fe Fe Fe Fe R 55 O 90 90 90 2 cur whenever the combinationfields is of on easy- the outside and of the hard-axis astroid. the use of Co netic films, antiferromagnetic pinning films and thefilm. free for the keyferromagnet: films: 20 Å, antiferromagnet: ruthenium: 300 Å, 8 Å, Co Al Co  the following equation and shown in Fig. Fig. Note that themalized easy and with hard respect magnetic to fields are the nor- anisotropy field a typical MTJ (for a bit area of 0: field applied along theial anisotropy direction (the of easy axis) ana and induced simultaneously direction along uniax- orthogonal tohas the been easy discussed [49. axis (the hard axis) of the free layercantly relative larger to than the for pinned GMR layer technology is (Table signifi- but a thin insulatingfilms layer is between required.evolved the has The a ferromagnetic single memory MOS transistor coupled architectureand to the the that MTJ word line. has Thethe current in magnetic the field word along lineand supplies the is insulated easy from axis the MTJ of (Fig. the free film magnetic field withover a the 6 in multiple wafer:48% the curves [49. maximum being change is taken greater than Novel Materials and Selected Applications Mn Al Ta electrode Ta Co Seed layer electrode Ta Co Ru Co Bit line

a)

Part E

Part E | 49.2 1216 Magnetic Information-Storage Materials 49.2 Magnetic Random-Access Memory 1217

bit line (with the amplitude of the magnetic field insuf- of thickness a with barrier height V0,isgivenby ficient to switch the free film) and the current is either 2˛a ; high when the R is low, corresponding to say a 1 or the T D T0e current amplitude is low when R is high, correspond- where ing to a 0. Without the sense transistor there would be . / . / 16E V0  E 2 2m V0  E many sneak paths for the current through other MTJs, T0 D and ˛ D : V2 „2 reducing the available current for reading the selected 0 bit. This result applies when ˛a > 1. A barrier height MTJ memory cell technology shows consider- V0 D 10 eV applies for CoO. able promise for replacing semiconductor SRAM and The exponential dependence of the transmission DRAM memories, primarily because of the nonvolatil- coefficient on the thickness of the barrier shows the ity of storage in MRAM technology. requirement for a thin barrier. A typical value of the The speed for reading data is comparable to that thickness of an Al2O3 layer is 10 Å and the resistance of SRAM and the density is comparable to that for changes by a factor of 100 million in changing the DRAM. The physics of the MTJ is different from thickness from 010Å. that for GMR [49.95]. Ferromagnetic films have two The reason that the MTJ is more attractive than types of electrons: those polarized parallel to the mag- a spin valve for the memory application is that the MTJ netization (") and those polarized antiparallel to the can be made with a resistance of many ohms, while magnetization (#). A polarization ratio .P/ for a fer- the spin valve is nearly a short circuit. The use of the romagnetic material can be defined as synthetic antiferromagnet for the MTJ also helps to avoid the superparamagnetic limit for this device, since D".E /  D#.E / P D F F ; the films comprising the synthetic antiferromagnet are D".EF/ C D#.EF/ thicker than the single antiferromagnetic film that was used to pin one of the ferromagnetic electrodes. where D".EF/ and D#.EF/ are the densities of states A significant problem with the MTJ is the differ- for electrons at the Fermi energy for the spin-up and ent magnetic states that can be found depending on the spin-down electrons respectively. size and shape of the bit. A review of an analysis of The magnetoresistance ratio of two films coupled the states for a rectangular-shaped bit has been pub- by an insulator is [49.96] lished [49.97] and we will quote some of their results to illustrate the problem. The analysis uses finite element  R  R analysis to solve the equation of motion for the magne- R a p 2P1P2 ; D D (49.31) tization .M/ with Gilbert damping in the presence of an R Rp 1  P1P2 effective magnetic field .Heff/ where Ra and Rp are the resistances of the coupled films M ˛ d . / . P /: in the antiparallel and parallel configurations respec- D M  Heff  M  M (49.32) dt M tively, and the spin polarizations are P1 and P2 for the two ferromagnetic films. Values of the spin polariza- The effective magnetic field Heff D•G=•M can be tion ratio are: PCo  0:35, PFe  0:44 and PPermalloy  derived from the magnetic free-enthalpy density (or 0:3 [49.95]. With cobalt used as one electrode and Gibbs potential) G per unit volume given by the sum Permalloy for the other electrode, the predicted value of terms pertaining to exchange, anisotropy, demagne- for R=R is 24%. Conduction through the insulating tization and external field 49.2 | E Part film is by quantum mechanical tunneling, described by : a transmission coefficient .T/ G D Gexchange C Ganisotropy C Gdemag C Gexternal An example of this analysis is a rectangular film of j .x/j2   T D transmitted ; thickness 20 nm, length 1 m, width 1 m with mag- 2 j incident.x/j netic properties shown in Table 49.13. The exchange 2 2 stiffness constant is A D S a JexNv=2, where S is the where .x/ is the wavefunction for the electron plane spin of the atom, a the lattice constant and Nv is the wave and j .x/j2 is the probability of finding an elec- number of nearest-neighbor atoms per unit volume. tron at the position x. The critical length determines the maximum size The transmission coefficient is a measure of the of the mesh used in the finite element analysis. Fig- barrier resistance. The transmission coefficient for elec- ure 49.42 shows three of the six states of the magne- trons with energy E tunneling through an insulating film tization in the sample. is ) b ]are ( 101 area (RA) prod- films) is shown is in S-state, 3  ) ]. STT-MRAM are a O ( 2 91 ]. The insulating barrier ]. 49.15). undergoing oscillatory mo- 102 99 [49. 49.17. Figure and c) 49.43 ]. Generally, spin currents [49. 49.16 100 endurance (Table 5 materials with largerating switching temperature fields [49. at the oper- Longitudinal bias to put the ferromagnetic TMR Another important development in MRAM is the (again usually fabricated from Al 49.2.1 Tunneling Magnetoresistive Heads Tunneling magnetoresistance (TMR)a can be magnetic usedTMR recording heads in use the read-head CPP orientationand, of the application. as sense with current the These MRAM,a the thin tunneling insulating is done barrier. through Asurface schematic in at the the air-bearing showninFig. simplest form of such a TMR head is 2. The write3. threshold is reduced. Thermal instability limits are increased by using on edge and theper. current flow is in the plane of theelectrode pa- films inand a bottom shields single-domain are shown.TMR state The effect GMR is and large ratio but with the the the resistance top uct, which ismay the also resistivity be timesShot large, the noise resulting film is in thickness, current (DC) excessive intrinsic bias shot current to required noise. foration tunneling GMR and is head with the oper- dominant noise the source in direct- the TMR head. either spin-polarized currenttype) (SPC) carriers made orlocalized of SWC (spin-wave free spins current) (s- (d-type) tion carried by or STT(called originating also from double hybridand localized exchange) (s-d) carriers between (d-type) exchange [49. free (s-type) extremely fast (2 ns readingpletely electrically and controlled writing with no speeds), movingwith com- parts and unlimited endurancecycles). In (number contrast, electronic of FLASH memory has program/erase typ- ically 10 use of(STT) a [49. spin current called spin-transfer torque ]. s l 99 49.17 ) ]) 20 Fe 80 :7 Critical length (nm) 5 J) 18 b)  and it is seen that the (10 88.733 G 169.897 169.890 49.16 ]) m) = 97 J 12  (exchange stiffness) A (10 13 49.39) during the write operation. The ) Parameters for Permalloy (Ni ]. Changing the shape of the bit may also Total energies for different spin configura- 3 m 98 = 1 is in shifted vortex state. (After97 [49. K (J 500 ) Configurations for spins in a Permalloy sample described in Tables c 49.16). (After [49. ( The first two are high-remanence states referred to One development in MRAM architecture is to pass An issue with the MRAM is the possibility of ex- perature, selection errors are reduced. s :0 Structure S-state C-state Shifted vortex state M (T) 1 as S-state and C-stateing conditions and are can a be single-domainshown state. is formed The a if last low-remanence state the stateThe with start- last an state shifted is vortex. two formed domains when along the the initial long conditionsenergies are axis for of the the three film. states The total are shown in Table using parameters of Table assist in minimizing the vortexanalyzing structures. these Software problems for isSimulator the developed LLG by M.R. Micromagnetics Scheinfein. the write current through theelement MTJ and in thus order lower to the heat the writing threshold [49. lowest energy is the low remanence state. citing the differentthen states result in in the awith storage different low BAR switching cell bit astroid. that shapesin Thin have can been films a predicted to reducedtures result [49. propensity to excite the vortex struc- This is accomplished bysistor turning on (Fig. the selectionadvantages of tran- this modification toture the are: MRAM architec- 1. Since the selection at write is controlled by the tem- Table 49.17 Table 49.16 tions in theTable Permalloy film (described in the text and Novel Materials and Selected Applications

a)

Part E Fig. 49.42a–c in C-state and

Part E | 49.2 1218 Magnetic Information-Storage Materials 49.2 Magnetic Random-Access Memory 1219

TMR ratio (%) Top shield 25

Insulator Insulator 20 Longitudinal TMR Longitudinal Bias Bias 15 Al2O3

Metal Gap 10 Insulator Insulator

Bottom shield Smooth-top 5 Conv-bottom Smooth-bottom 0 706050403020100 Fig. 49.43 Schematic drawing of the air-bearing surface RA (Ωμm2) view of a tunneling magnetoresistive (TMR) head. (Af- ter [49.102]) Fig. 49.44 Correlation between TMR ratio and resistance area product (RA) in three structures: top-andbottom-type Figure 49.44 is a plot of the TMR ratio in % MTJ with smoother interfaces and bottom-type MTJ with versus the RA product for three different TMR struc- conventional stacking that has relatively rough interfaces. tures all made from the basic structure: buffer/PtMn/ (After [49.102]) CoFe/Ru/CoFe/AlOx/CoFe/NiFe/cap [49.102]. PtMn is an antiferromagnet, CoFe/Ru/CoFe is a synthetic anti- Table 49.18 TMR ratio increases strongly when AlOx is ferromagnet, AlOx is the insulating barrier (prepared by replaced by MgO. Materials such as CoFeB/MgO/CoFeB in situ oxidation) and CoFe/NiFe is the second elec- and Co/MgO/Co reach about 400% at room temperature trode. The data points for TMR versus RA for this Insulator Junction TMR (%) Reference structure are labeled conv-bottom (conventional bot- AlOx 170 Zhu et al. tom) and the data points for the smooth-top structure is [49.105] for the PtMn layer on top instead of the bottom, while CoFeB/AlOx /CoFeB 70 Wang et al. the smooth-bottom data points are for a head made [49.106] with smoother interfaces. The TMR % remains con- MgO 5400 Ikeda et al. stant at  20% until a knee, below which the TMR % [49.104] drops due to pinholes and incomplete insulation cover- Fe/MgO (100)/Fe 220 Parkin et al. age. [49.107] Typically, room temperature TMR is about 170% when the insulating barrier is AlOx and rises to al- The stack of films is ion-milled and the stripe most 400% when the insulator is MgO (Table 49.18). height is defined by lapping. Magnetic recording re- At low temperature (T D 4:2 K) it can reach a value of sults showed the expected large signal (peak-to-peak 1800% in half-metallic compounds such as La2=3Sr1=3 Vpp D 42 mV=m) with a biasing current of 1 mA with MnO3 (LSMO)/SrTiO3 /LSMO, and 600% in Heusler only 23:4 dB SNR deemed insufficient to demonstrate

alloys such as Co2MnSi/AlOx/Co2MnSi [49.103]. Per- the required low error rate [49.102]. 49.2 | E Part haps the main issue is thermal stability, and that is A significant problem with the TMR head is that why a system such as a perpendicular MTJ consisting there is a large resistance in parallel with the large of Ta/CoFeB/MgO/CoFeB/Ta possessing high thermal capacitance resulting from the tunneling barrier. This stability, a size as small as 40 nm diameter and a low combination results in a low-pass filter and care is switching current of 49 A [49.104] is quite interest- required to limit the capacitance. In the example dis- ing in spite of a TMR ratio of only 120%. The thermal cussed here the cutoff linear density is near 580 kfc=in. stability factor EB=kBT of the free layer should be Because of the significantly larger value of R for larger than 40 to retain information (as previously seen), CPP as opposed to the CIP technology, CPP has be- where EB is the energy barrier. Because the volume come the dominant technology for read heads when V of the free layer decreases with junction size, the areal density became much larger than 100 Gbit=in2, anisotropy energy density MsHk=2 needs to be large whereas TMR was used when areal density was just enough to ensure high thermal stability. around 100 Gbit=in2 (see Mueller [49.82]). 2 in = 3nm ].  100 nm) Magnetic 110 < tor InSb was structure (FCT (Wiley, New York 0 Vs. = 2 IV semiconduc 49.8) with L1 ]. and ]. An advantage of such sensors is 109 108 [49. 49.7 . The coercivity of these materials, being 2 Solin in Recording: The First1998) 100 Years = Magnetic quantum dot arrays that will be used in However, a major limitation was the ability to in diameter yielding100 areal Tbit densities about as large as phase), which can provide grains as small as 2 49.3 C.D. Mee, E.D. Daniel, M.H. Clark: level is reached,will information be competing storageis with estimated by human to hard brain beabytes. on capacity, disks the which order of several hundred ter- much larger than any magneticated by field typical that recording can heads, beby can heating gener- them be above reduced Curie to temperature zero [49. HDMR are individuallyscale addressable integration and sincepossess adapted submicron properties to ordered such as dotercivity moment, and arrays susceptibility, remanence co- thatwith are respect significantly to enhanced theirfrom bulk counterparts. the This fact originates theytures are closer with to augmented quantum magneticcated atomic properties struc- on when the fabri- nanometer scale. When the 100 Tbit maintain the high mobility in thin films ( required for read-headdressed applications by and this formingerostructure [49. was a ad- thin-film quantum well het- (heat assisted magnetic recording) and(heated later dot by magnetic HDMR recording)patterned a recording combination and of HAMR. bit- ogy, In large magnetic the anisotropy materials latter are technol- as used, such FePt (Tables the lack of any magneticnetic noise materials since involved in there the are headof no structure. mag- A EMR review asported magnetic by field nanosensors has been re- chosen for the initial workmobility on of EMS electrons because of of 60 the 000 cm high mobility semiconductor, are deflectedmagnetic by field the into external in a the trajectory, semiconductor and atthe the low magnetic locations voltage field. is The drop used III– to sense ] , 338 110 ,509 42 13 ]. The concept is 108 be made to function as is the velocity of the charge v ; is the magnetic flux density acting on the B B  v (2003) (2002) is the charge, q q D Perpendicular recording as an alternative to lon- F Electrons have the highest mobility as compared to The electrons are driven by an external source References 49.2 R.L. Comstock: J. Mater. Sci. Electron. 49.1 E. Grochowski, R.D. Halem: IBM Systems J. It was therent purpose status of of this the chapterused technology of to in magnetic review disk recording thenetic as drives. cur- materials The used emphasisof was in the on the technical the applicationsity problems mag- and steady that on increase. mayevolved from some MRAM magnetic limit recording applications, technology, was areal also whichreviewed. den- has It wasnetic found materials is that essential forrecording a the and advance the wide of MRAM magnetic rangemagnetization technology, including soft-magnetic high- of materials for mag- antiferromagnetic write alloys heads, with hightures blocking and low tempera- propensity toin corrosion for giant-magnetoresistive pinning films sensors,alloys and with ferromagnetic largeA values significant offound limitation giant to be magnetoresistance. to the superparamagneticin magnetic effect and multilayer advances recording ferromagnetic filmsof was to the reduce effect, the but impact were also discussed. to allow high-density recording, gitudinal recording wasby reviewed the and ASTC itsortium) is (Advanced that projected Storage it will Technology be Con- replaced by HAMR [49. 49.4 Summary An interesting development in read-headthe technology is discovery thatmobility narrow semiconductors band can detectors gap of and magnetic hence field high [49. 49.3 Extraordinary Magnetoresistance (EMR) based on the Lorentzby force a magnetic exerted field on charge carriers carrier and where charge. holes and are the carriers used. similar to the sense current in GMR sensors in a high- Novel Materials and Selected Applications

Part E

Part E | 49 1220 Magnetic Information-Storage Materials References 1221

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