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49. Magnetic Information-Storage Materials

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49. Magnetic Information-Storage Materials 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 Part E | 49 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 1186 Part E Novel Materials and Selected Applications The discussion covers longitudinal magnetic recor- Areal density (Mbit/in2) 106 ding (LMR), perpendicular magnetic recording (PMR) IBM disk drive products Travelstar 40GN and the transition process between them allowing an 5 10 Industry lab demos 1st AFC media increase in areal densities in hard disks. In PMR the Microdrive II Deskstar remanent magnetization is perpendicular to the disk 104 120GXP 1st GMR head Ultrastar surface reducing the impact of the superparamagnetic 103 73LZX limit. Technology underlying MRAM (magnetic ran- 60% 100% dom-access memory) is also discussed since it provides 102 1st MR head CGR CGR nonvolatility to RAM, blurring the borderline separat- 10 1st thin film head ~17 Million × increase ing permanent storage and RAM. 25% CGR Consequently, the operating system and application 1 programs will always be present in RAM since the com- 10–1 puter’s very first boot, in sharp contrast with respect to the electronic RAM whose contents totally disappear 10–2 IBM RAMAC (first hard disk drive) every time the computer is switched off or when it is 10–3 jammed and not responding. 1960 1970 1980 1990 2000 2010 Production year MRAM is a possible replacement for the famil- iar semiconductor memories used in microcomputers – Fig. 49.1 Historical variation of areal density from an IBM dynamic and static random-access memory (DRAM perspective. (After [49.1]) modules are placed on the computer motherboard close to central processing unit (CPU) whereas static random- rection, advanced servo-control systems for accurate access memory (SRAM) is used inside CPU cache control of magnetic recording heads on data tracks, memories). and improvements in the mechanical structures com- MRAM technology combines a magnetic storage prising a disk drive, including advances in motors technology together with metal-oxide semiconductor used to drive the disks. However, this paper discusses (MOS) devices to result in fast and high-density data only the fundamental technology associated with dig- memory devices. The technology on which the mag- ital magnetic recording, including the devices used to netic part of MRAM is based is an extension of the record and read back the recorded data and the media technology used in magnetic recording devices – the on which the data is recorded. The discussion is also magnetic tunneling junction (MTJ). MRAM technol- restricted to the materials and not to any of the mechan- ogy will also be discussed in this chapter. Parts of this ical structures associated with the recording heads or work have been previously published in the Journal of disks. Materials Science: Materials in Electronics [49.2]. 49.1 Magnetic Recording Technology The technology of magnetic recording was one hun- store digital data, in which case the current supplied to dred years old [49.3] in 1999. The fundamental concept the write head is in the form of pulses encoded to rep- Part E | 49.1 of magnetic recording is to use a magnetic structure resent the digital data [49.4, 5](1or0). (the write head) driven by current that represents the In the case of disk drives the write and read heads data to be recorded to generate a magnetic field that are separate thin-film structures deposited on the back can change the state of the magnetization in a closely of a mechanical slider, which uses a hydrodynamic air spaced magnetic recording medium, which in the ear- bearing to fly over the surface of the disk [49.5]. Fig- liest realization was magnetic wire, and today is either ure 49.2 is a schematic of a digital LMR system. The the familiar magnetic tape or a magnetic layer on a rigid recording (write) and read elements are shown together disk substrate. The data are recovered by the genera- with the magnetic recording surface, which in disk drive tion of an output voltage in the read head by sensing technology is a thin metallic film of a cobalt alloy (to the magnetization in the recording medium, e.g., by be discussed). The digital data are recorded in the mag- Faraday’s law (V D Nd=dt), where N is the number netic film as transitions between the two possible states of turns on the read head and is the magnetic flux of the magnetization (pointing to the left or right) and coupled to the read head from the media. The magnetic with the width approximately equal to the width of the recording system to be discussed here is that used to write head and the width of a data track. The transi- 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.
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