Magnetic Nanowire Memory Utilizing Motion of Magnetic Domains for Developing a High-Speed Recording Device

FEATURE Magnetic Nanowire Memory Utilizing Motion of Magnetic Domains for Developing a High-speed Recording Device

Mayumi KAWANA, Mitsunobu OKUDA and Yasuyoshi MIYAMOTO

We have proposed magnetic memories utilizing paral- tems will impede the development of a compact and high- lel nanowires with no mechanical moving parts, in order to speed storage system capable of recording 3D video. achieve ultra-high data transfer rates required for three-di- At NHK Science & Technology Research Laboratories mensional video. In an experiment, we used a structure con- (STRL), we have been researching and developing the sisting of a magnetic nanowire and a magnetic head used magnetic nanowire memory having features applicable in a commercial hard disk drive, in which a pair of a write to high-speed video recording with the aim of solving the head and a read head was placed in line. A magnetic nanow- above problem. In this paper, we first introduce the opera- ire memory element was constructed by fixing the magnetic tion principle of the magnetic nanowire memory. We then head onto a fabricated nanowire. We conducted a perfor- report on a successful demonstration of this recording/re- mance test of recording, bit-shifting, and detecting data in production operation principle by constructing a prototype the memory element and succeeded in demonstrating the element having the basic structure of this memory using a fundamental principle of the magnetic nanowire memory. hard disk drive (HDD)-type magnetic head placed onto the magnetic nanowire and by forming, driving, and detecting 1. Introduction magnetic domains*1 corresponding to the recorded binary Future storage technologies for recording the video data information. of 3D television and other data-intensive applications will require an extremely high recording speed in addition to 2. Features and operation principle of magnetic large data capacity. The data transfer rate of 8K ultrahigh- nanowire memory definition television (UHDTV) signals is approximately 2.1 Magnetic nanowire memory technology for ultrahigh 144 Gbps for uncompressed full-featured 8K, but even Sol- -speed recording id-State Drives (SSDs), which use semiconductor memory Magnetic memory, as typified by the HDD records bi- and are currently the fastest commercially available record- nary information using the north-pole (N-pole) and south- ing devices, have a fundamental data transfer rate of only pole (S-pole) orientations of a magnet. It has been reported several Gbps. As a result, SSDs are incapable of recording that magnetic memory is capable of recording data in an uncompressed full-featured 8K video signals unless multi- extremely short time, essentially within several tens of ple devices are used in parallel. Looking to the future, there picoseconds*2, which is 10 to 20 times faster than current is a require for storage technology that can record at consid- semiconductor memory in terms of pure recording time1). erably higher speeds to accommodate the uncompressed re- As shown in Fig. 1, semiconductor memory records infor- cording of 3D video generated by the Integral Photography *1 A region representing the smallest unit in which the direction (IP) method, which will require even higher data transfer of magnetization (magnet orientation) is aligned. rates. In short, failure to radically upgrade recording sys- *2 One picosecond is equal to 10-12 of a second.

Memory cell N ‘0’ ‘1’ S

‘1’ e e e Electron physically moves

‘0’ Grounded Electrode e Electrons only change their spin S direction without position movement N Time required for recording → Enables ultrahigh-speed recording Time required for recording 50ns ‒ 10µs 1ns ‒ 50ns (a) Semiconductor memory (b) Magnetic memory

Figure 1: Comparison of semiconductor-memory and magnetic-memory operation principles

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mation by the presence or absence of electron charge in a magnetic nanowire is formed by extracting one of the sev- unit memory cell*3, so time is needed to physically move eral million data tracks concentrically formed on a hard disk electron charge between grounded electrode and the mem- medium and stretching it to give it a linear shape. This mag- ory cell. In contrast, magnetic memory records information netic nanowire is a magnetic material, so it can be used to re- by changing the orientation of a magnet to N-pole or S-pole, cord binary information in terms of the N-pole/S-pole orien- that is, by changing the spin directions of the electrons mak- tation of a magnet. A certain amount of pulse current can be ing up the magnet without the position movement, which applied in the lengthwise direction of a magnetic nanowire means that recording can be completed extremely rapidly. on which magnetic domains have been formed as shown in However, the magnetic memory used in HDDs requires the Fig. 2 (a). This action makes it possible to move a magnetic recording media to be rotated using a motor and for infor- domain by a distance corresponding to the amount of the cur- mation to be recorded and detected by controlling the posi- rent in one direction (specifically, in the direction of movement tion of a magnetic head mechanically. This limits the speed of the injected electrons) while preserving the shape and size of of recording and detecting information, which is governed the magnetic domain as shown in Fig. 2 (b) 2)-4). This is the phe- by the operating speed of the mechanical moving parts and nomenon of current-driven domain wall motion. Since the makes it difficult to achieve any further gains in data trans- discovery of this phenomenon, research has been focused fer speeds. Magnetic memory without any mechanical mov- on finding a physical explanation for this magnetic-domain ing parts therefore has the potential to be used as memory and domain wall movement5). However, it has recently for video recording having the high-speed recording perfor- been theoretically predicted that a magnetic domain can be mance intrinsic to magnetic materials. moved approximately 20-70 times faster (500-2,000 m/s, 2.2 Magnetic nanowire and phenomenon of current- over sound speed) than the relative speed between an rotat- driven domain wall motion ing HDD medium and a fixed magnetic recording *6head , A new nonmechanical principle of accessing magnetic so research is also proceeding on engineering applications information is needed to take advantage of the high-speed based on this capability. recording performance of magnetic memory. In this regard, 2.3 Operation principle of magnetic nanowire memory the phenomenon of magnetic domain wall*4 driving (known At NHK STRL, we have proposed a magnetic nanowire as current-driven domain wall motion) has been observed in memory with no mechanical moving parts using the phe- recent years by applying a current to a quasi one-dimension- nomenon of current-driven domain wall motion described al structure called a “magnetic nanowire”, achieved by fab- in the previous section. A conceptual diagram of this mag- ricating a magnetic wire with a width of about one hundred netic nanowire memory is shown in Fig. 3. This magnetic nanometers*5 2)-4). Using this phenomenon, attempts have nanowire memory consists of unit recording elements ar- been made to access magnetic information electrically. We ranged in parallel, where each unit consists of a write head describe this phenomenon schematically in Fig. 2. Here, a and read head installed opposite each other at both ends of a magnetic nanowire. Each magnetic nanowire adopts a per- *3 In semiconductor memory, the circuit configuration needed to store one bit of information, i.e., the smallest unit of information. *6 A “magnetic recording head” is the general term for a pair *4 An area in which the magnetization direction between mag- of a write head that applies a magnetic field to a magnetic netic domains undergoes a transition to the opposite direc- recording medium such as a hard disk or magnetic tape, tion while rotating. and a read head that detects magnetic flux leaking from the *5 One nanometer is equal to 10-9 of a meter. medium.

(a) Before applying pulse current Magnetic domains Pulse power supply N S N Magnetic nanowire

S N S Domain walls

High-speed magnetic domain movement (b) After applying pulse current Injected Electrons e-

・Atoms making up the magnetic nanowire do not move ・Only the magnetized information of the magnet moves

Figure 2: Schematic of current-driven domain wall motion in magnetic nanowire

10 FEATURE pendicularly magnetized*7 thin film in which the direction film thickness. Each nanowire is also magnetized before- of magnetization is easily oriented in the direction of the hand in one direction (in this case, upward). Here, a unit recording element is defined as one magnetic *7 The state in which the N-pole/S-pole of a very small magnet nanowire and a pair of write-head and read-head. The re- within a magnetic material aligns in the perpendicular direc- cording and detecting procedure of each unit recording ele- tion with respect to the substrate. All commercially available ment is shown in Fig. 4. hard disks record information using such perpendicular magnetization. To write data, the write head generates a sufficiently

Uncompressed high-definition video signal input Uncompressed high-definition video signal output

Preprocessing part for recording Post-processing part for reproduction

- Parallel driving e Write heads Read heads

Stored data area

Pulse power supplies for Magnetic-domain drive Magnetic nanowires arranged in parallel

Drive direction of recorded magnetic domains (data) ⇒

Figure 3: Conceptual diagram of magnetic nanowire memory

(1) Write procedure Write head (a) Initial state

Input ‘1’

(b) Write Magnetic field generation

Downward magnetic-domain formation (c) Drive (bit shift) e-

Pulse current Repeat Current driving of magnetic domain

(d) Data storage

Recorded data Read head (2) Read procedure (a) Initial state Recorded data

- (b) Cue e

Continuous pulse current Detection Output ‘1’ (c) Read

(d) Drive (bit shift) Detection Output ‘0’

Repeat Pulse current Current driving of magnetic domain

Figure 4: Write/read procedure of magnetic nanowire memory

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intense magnetic field in either the upward or downward 3. Experiment on formation (recording), driving, direction depending on the binary information (0 or 1) to and detection (reproduction) in magnetic nanow- be recorded. For example, when generating a downward ire memory magnetic field, a very small area on the magnetic nanowire 3.1 Fabrication of a prototype unit recording element of directly below the write head is magnetized in the down- magnetic nanowire memory ward direction so that one bit of information is recorded We fabricated a prototype unit recording element of mag- as a magnetic domain. Next, a pulse current is applied in netic nanowire memory using a magnetic nanowire and an the lengthwise direction of the magnetic nanowire to drive HDD-type magnetic recording head. We used this element this magnetic domain to the right by a length of one bit (bit to conduct an experiment on magnetic-domain formation, shift) at a high speed, making use of the phenomenon of driving, and detection to assess the operation principle of current-driven domain wall motion. This action generates the magnetic nanowire memory. The magnetic nanowire space for recording the next bit of information directly be- used in the experiment had a perpendicularly magnetized low the write head, so repeating the above writing and driv- multilayer-film structure formed by alternately depositing a ing process enables the storage of information as a sequence 0.3-nm-thick cobalt thin layers and 1.2-nm-thick palladium of magnetic domains along the length direction of the mag- thin layers (Fig. 5 (a)). This composite film was formed into netic nanowire. the shape of a thin wire using electron beam lithography*8 To read data, a pulse current is applied continuously to and the lift-off method*9 to obtain a magnetic nanowire with the magnetic nanowire to move the sequence of recorded a width and length of 150 nm and 20 μm, respectively. Both magnetic domains up to the fixed read head. At this time, optical microscopy and atomic force microscopy (AFM)*10 the read head detects the direction of magnetic flux leaking images for the fabricated magnetic nanowire and the associ- from the magnetic domains now underneath it and outputs ated electrodes are shown in Figs. 5(b) and (c), respectively. a signal corresponding to the direction of magnetization. These electrodes were formed on the upper section at both This process of driving the recorded magnetic domains and ends of the magnetic nanowire to apply current. Addition- detecting it by the read head is repeated to reproduce the ally, as the size of a magnetic domain is only several hun- original binary information. dred nanometers, a 2-μm-long reference marker used for Preparing multiple copies of this unit recording element checking the magnetic-domain position was placed on both and driving them in a synchronized manner enables this sides of the magnetic nanowire to facilitate the observation magnetic nanowire memory to achieve high-speed record- of magnetic-domain movement by an AFM (Fig. 5 (c)). ing. In addition, the first-in-first-out (FIFO) configuration of Next, Figure 6 (a) shows how we applied HDD-type this type of memory, enabling the storing of sequential in- write/read heads to a magnetic nanowire. In this process, formation in order on a magnetic nanowire, has high affin- *8 A method of forming circuit patterns using an electron beam. ity with sequential data typical of video information. More- It can form ultrafine circuits with a width on the order of 10 over, it is thought that the track recording density (recording nm. density in the lengthwise direction) per magnetic nanowire *9 A method of thin-film patterning that performs patterning on a substrate such as silicon using a resist (a material sensitized will reach the level of current hard disks sometime in the by light or an electron beam that acts as a master during future, so large-capacity magnetic nanowire memory is pos- pattern formation), forms a metallic film on top of the pat- sible. Finally, increasing the magnetic-domain speed should terned substrate by sputtering (a typical thin-film fabrication make magnetic nanowire memory applicable to recording method similar to vacuum evaporation), and dissolves the resist to form the desired pattern. devices for 3D video and other data-intensive applications *10 Equipment for measuring the shape of a target surface with that require high-bit-rate recording and reproduction. high precision by proximate scanning with a sharp probe tip to detect the interatomic forces acting between the atoms of the probe tip and those of the target surface.

Cobalt and palladium Electrodes Electrodes 150nm are alternately deposited in 20 cycles Magnetic nanowire 500µm 2µm

1.2nm Palladium Reference markers 20µm 0.3nm Cobalt Magnetic nanowire (a) Structure diagram (b) Optical microscopy image (c) Atomic force microscopy image

Figure 5: Prototype magnetic nanowire

12 FEATURE the position of the magnetic nanowire is finely adjusted so recording medium targeted for operation. that the center of the write-head/read-head set within the 3.2 Magnetic-domain formation (recording), driving, and HDD-type magnetic recording head installed in a head detection (reproduction) experiment using nano-MDS slider*11 is aligned with the center of the magnetic nanowire equipment in the width direction. This is called the on-track state as Targeting the prototype magnetic nanowire described shown in Fig. 6 (b). We then brought the magnetic head above, we performed an experiment on magnetic-domain into fixed contact with the magnetic nanowire to construct formation (recording), driving (bit-shift), and detection (re- a unit recording element of magnetic nanowire memory. In production) using “magnetic domain scope for a wide area the experiment, we used a magnetic head in which the write with nano-order resolution (nano-MDS)” equipment6) 7) *12. head and read head were physically separated by a distance The experimental configuration is shown in Fig. 7. In this of 4.5 μm, so by fixing the magnetic head onto the magnetic *12 Equipment for directly detecting the surface magnetization nanowire as described above, the section of the magnetic state and local magnetic flux of a sample using an HDD- nanowire corresponding to this 4.5 μm interval becomes the type magnetic recording head. It can detect the distribution of magnetic flux leaking from a sample as an absolute value *11 An aerodynamically designed structure for floating the mag- by contact scanning of the magnetic head, and when the netic head in an HDD above the magnetic disk at a height of magnetic head is fixed, it can detect the temporal changes in only 10 nm. A magnetic head device is affixed to this slider. the magnetic flux directly underneath the magnetic head. Write head Read head Centers of magnetic nanowire and write/read Write head Read head heads coincide

Magnetic nanowire Head slider (b) On-track state Contact Electrode Centers are offset Electrode

X-Y linear stage Magnetic nanowire

(a) Application of HDD-type write/read heads to magnetic nanowire (c) Off-track state

Figure 6: Application of HDD-type write/read heads to data recording/reproduction on magnetic nanowire

PC for recording/reproduction control Read-head output

Recording/drive control Reproduction system Variable trigger board for system synchronization Read head preamplifier Recording system

Trigger signal Arbitrary waveform Iwrite generator ΔRchange

Drive system Write head Magnetic head Read head High-speed-pulse power supply

Electrode Magnetic nanowire Trigger signal Idrive

X-Y linear stage

Figure 7: Configuration of magnetic-domain recording/driving/reproduction experiment using nano-MDS equipment

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experiment, the silicon wafer on which the specimen mag- was +12 mT*14 in the red region and -12 mT in the blue netic nanowires were formed was placed on an X-Y linear region. Finally, using these recording conditions, we con- stage and, in the manner described above, a unit recording ducted an experiment targeting the sequence of operations element of magnetic nanowire memory was constructed by consisting of magnetic-domain formation, driving, and de- precisely adjusting the position of the magnetic nanowire tection. and bringing it into contact with the magnetic head. Mag- Next, we applied a current of 100-ms pulse width to the netic-domain formation (recording) was performed by con- write head every 2 s to generate magnetic fields that formed necting an arbitrary waveform generator to the write head (recorded) upward magnetic domains on the magnetic affixed to one end of the magnetic nanowire and generating nanowire. Additionally, to simultaneously drive these mag- a write-current magnetic field by applying current (Iwrite). netic domains, we applied a current of 4.5-ns pulse width 8 2 A pulse current (Idrive) synchronized with this recording with a current density of 1.1 × 10 A/cm every 1 ms in the operation was also applied in the lengthwise direction of lengthwise direction of the nanowire. The output from the the magnetic nanowire to drive multiple magnetic domains read head at this time, that is, the temporal change in the recorded in the above way. In addition, the change in the density of magnetic flux leaking from the magnetic domain direction of the magnetic flux generated by the magnetic directly under the read head, is shown in Fig. 9. The positive domains that were shifted within the magnetic nanowire and negative signs of the read-head output correspond to the by current driving was simultaneously detected. This was magnetization directions of the magnetic domains passing achieved by measuring the change in the magnetoresistance underneath the read head: a positive sign denotes a domain *13 (ΔRchange) output from the read head affixed to the other with upward magnetization and a negative sign a domain end of the magnetic nanowire 4.5 μm from the write head. with downward magnetization. According to these results, In other words, we performed a recording/reproduction an upward magnetic domain is first detected at the read- experiment on multiple magnetic domains by repeatedly head position approximately 8.7 s (the time taken to shift 4.5 forming (recording) magnetic domains by the write head, μm-distance) after beginning the simultaneous application driving them in one direction by a pulse current along the of write current and drive current. There are subsequently magnetic nanowire, and detecting them by the read head as several occasions when the read head detects a signal gener- they pass underneath. ated by an upward magnetic domain that has shifted along Before making measurements, we applied a sufficiently the nanowire. As shown by the enlarged view in the figure, large magnetic field throughout the magnetic nanowire in an upward magnetic domain is detected over a time interval the downward direction to uniformly magnetize the mag- of approximately 100 ms, which agrees with the write-head netic domain of the nanowire in this direction. We then current application time (100 ms). This change in the read- used the write head to apply an upward magnetic field to head output shows that the magnetic domains formed by the magnetic nanowire, thereby forming a magnetic domain the write head have been driven to the position of the read corresponding to a 1-bit recording. The results of measur- head by the current applied to the magnetic nanowire. These ing the magnetization state of this magnetic nanowire using results demonstrate that magnetic domains were formed (re- nano-MDS equipment are shown in Fig. 8. The red region corded), driven, and detected (reproduced) in the prototype in the figure is the recorded magnetic domain in the upward magnetic nanowire fabricated for this experiment. direction, while the blue region indicates the initial mag- However, while an upward magnetic domain should have netic domain in the downward direction. Here, we applied been recorded once every 2 s, such a magnetic domain was a write current of Iwrite = 40 mA to the write head. In addi- only detected once every several seconds as shown in Fig. 9. tion, the length of the magnetic domain formed by the write In other words, there were some locations where the output head was approximately 100 nm and the magnetic flux that signal from the read head was lost. In fact, as the experiment leaked from a magnetic domain on the magnetic nanowire continued, the locations at which the output signal dropped

*13 The phenomenon of a change in magnetoresistance caused *14 Denotes “tesla,” the SI unit of magnetic flux density (Wb/m2). by a change in the intensity of an external magnetic field. 1 T = 10,000 gauss.

Recorded upward magnetic domain (red) 12mT 1µm

－12mT Initialized downward magnetic-domain region (blue) Reference marker

Figure 8: Distribution of magnetic flux density of a magnetic nanowire after forming a 1-bit magnetic domain

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12 Agrees with write-current pulse width 8 8 100ms Delay time “1” 4 4 Time taken to shift 4.5 µm-distance 0 0 “0” -4 -4

Read-head output (mT) -8 -8 16.6 16.7 16.8 16.9 17 17.1 17.2 -12 0 42 6 8 10 12 14 16 18 20

Elapsed time (s) Enlarged view of read-head output

Figure 9: Temporal change in read-head output gradually increased in number. We consider the reason for netic domain. The positional relationship between the write this to be that the X-Y linear stage used for positioning the head and the magnetic nanowire in this experiment is shown magnetic head directly above the magnetic nanowire actu- in Fig. 10. Taking the center of the nanowire as the origin, ally changed position over time due to, for example, ther- we denote the relative distance between this point and the *15 mal drift . That is, the write head, which should have been center of the write head as Dh.p.. fixed directly above the center of the magnetic nanowire in The relationship between the relative distance Dh.p. and the width direction, shifted slightly from this position im- the write-head current Iwrite, i.e., the threshold current need- mediately after the beginning of the experiment. We there- ed to reverse the magnetization, is shown in Fig. 11. To before surmise that, although some magnetic domains were gin with, these results show that magnetic domains can be successfully recorded at first, the effects of thermal drift formed by applying Iwrite = 8 mA up to the position at which then became larger, causing the magnetic head to gradually the write head is about 20 nm from the nanowire center (ori- shift toward the side boundary of the magnetic nanowire. gin). However, they also show that the write current needed This resulted in a situation in which the recording was only to reverse the magnetization rapidly increases as the write sometimes successful. head shifts further away from the origin. Here, in contrast 3.3 Investigating cause of recording loss in magnetic to the magnetic flux density of ±12 mT detected by the read nanowire head in the distribution of the magnetic flux density on the To investigate the cause of the recording loss described in same magnetic nanowire shown in Fig. 8, the read-head out- the previous section, we examined how the offset between put in the recording/reproduction experiment of Fig. 9 is the center of the magnetic nanowire and the center of the about ±4 mT, or only about 1/3 of the original magnetic flux write head affected the formation of a magnetic domain. density. In a separate experiment, we obtained a sensitivity Specifically, we intentionally placed the write head at a po- distribution for the read head with respect to the distance sition offset from the center of the magnetic nanowire in the from the magnetic nanowire, and from this distribution, we width direction and examined the relationship between the estimated that the read head in the recording/driving/repro- offset distance and the write current needed to form a mag- duction experiment of Fig. 9 was 50-60 nm away from the center of the nanowire, which indicates that the center of *15 The gradual drifting of the position due to the effects of a the write/read heads was in an off-track state relative to the temperature change in the positioning system as a result of heat generated during the experiment. 55 50 45 Write head (width: 50 nm) 40 35 30 25 20 15 10 Dh.p.

forming magnetic domain (mA) 5

Magnetic nanowire Threshold write-head current for 0 0 -75 0 75 10 20 30 40 50 60 Dh.p. (nm) Distance from nanowire center (nm) Figure 11: Relationship between relative distance Dh.p. (between Figure 10: Positional relationship between write head and nanowire center and write-head center) and threshold write-head magnetic nanowire in recording loss experiment current Iwrite (no SUL*) * Soft magnetic Underlayer

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magnetic nanowire as shown in Fig. 6 (c). Furthermore, we the edge of the nanowire, is smaller than that at the center of assumed that Iwrite, the current needed to form a magnetic the nanowire, which means that the magnetization is easier domain at this write-head position, was about 42-55 mA to reverse by the diagonal magnetic-field components from by extrapolating from the results of Fig. 11, which led us the write head. The above results indicate that applying an to conclude that the Iwrite of 40 mA used for the write-head SUL enables the threshold write-head current to be kept current in the experiment of Fig. 9 was insufficient to form small, thereby enabling the accurate formation of magnetic a magnetic domain. In short, as a reason for the inability domains in the magnetic nanowire regardless of the thermal to record an upward magnetic domain, we consider that drift in the X-Y linear stage. the write-head current was insufficient to form a magnetic Based on the above results, we again performed an exper- domain due to the positional offset of the write/read heads iment on magnetic-domain formation (recording), driving caused by thermal drift of the X-Y linear stage. However, (bit-shift), and detection (reproduction) using nano-MDS this positional offset caused by thermal drift is difficult to equipment on a magnetic nanowire with an SUL. First, after control. In response to this problem, we investigated the magnetizing the entire magnetic nanowire in the downward application to magnetic nanowires of a structure similar direction, we applied a write current with a 100-ms pulse to an underlayer with high-permeability and soft-magnetic width to the write head every 2 s to form magnetic domains properties*16, or a soft magnetic underlayer (SUL), which is in the upward direction. Furthermore, to drive these mag- used in HDD perpendicular magnetic recording media. In netic domains, we simultaneously applied a 10.7-ns-wide particular, we attempted to expand the allowable range of pulse current with a current density of 1.3 × 108 A/cm2 every the write-head position, that is, the margin in which the on- 1 ms in the lengthwise direction of the nanowire and mea- track state can be preserved, by forming an SUL under the sured the change in the read-head output at this time. The magnetic nanowire and improving the recording efficiency. temporal change in the read-head output for the magnetic 3.4 Magnetic-domain formation (recording), driving, nanowire with the SUL is shown in Fig. 13. It can be seen and detection (reproduction) experiment in magnetic from the figure that the first recorded magnetic domain -ar nanowire with SUL rives at the read head approximately 9 s after the beginning Given the formation of a 30-nm-thick alloy called Su- of magnetic-domain formation and driving. It can also be permalloy*17 underneath the magnetic nanowire as an SUL, seen that magnetic domains continue to pass periodically Figure 12 shows the relationship between the relative dis- underneath the read head at a pitch of approximately 2 s. tance Dh.p. (between the nanowire center and write head) and We consider that these results demonstrate that the sequence the threshold write-head current Iwrite for forming a magnetic of operations consisting of magnetic-domain formation (re- domain. These results show that magnetic domains can be cording), driving, and detection (reproduction) can be performed with a small write current of 8 mA even at a write- formed on a prototype unit recording element of magnetic head position 70 nm from the nanowire center. It can be nanowire memory. seen from the plot that the write-head current is smaller at However, on examining the results for the read-head out- this position than at 40 nm. We consider the reason for this put in Fig. 13, it can be seen that some variation exists in to be that the demagnetizing field*18 at 70 nm, which is at the interval between the peaks and also in the pulse width of the read-head output with respect to the length of the re- *16 A magnetic property by which a material is easily magnetized corded magnetic domain (100 ms). As to the reason for this, in the direction of an applied magnetic field. This property is commonly used in the iron cores of electromagnets. we consider that the use of the SUL results in a magnetic *17 An alloy having a compositional ratio of 79wt.%Ni16Fe5Mo. interaction when the magnetic flux generated by each magnetic domain permeates the SUL as the magnetic domains are driven along the magnetic nanowire by the current. Ac- 35 cordingly, when using a magnetic nanowire with an SUL as memory, some means of controlling the distance of domain 30 wall movement is needed, such as the design of structural 25 magnetic-domain trap sites8) *19 in the nanowire. 20 4. Conclusion 15 We introduced the operation principle of a magnetic 10

5 *18 The oppositely oriented magnetic field produced within a forming magnetic domain (mA) Threshold write-head current for magnetic body by the magnetic poles appearing at either end 0 0 2010 30 40 50 60 70 of the body upon magnetization. It has the effect of weaken- Dh.p. (nm) ing the externally applied magnetic field. *19 A structure preventing the movement of a magnetic domain Figure 12: Relationship between relative distance Dh.p. (between nanowire center and write-head center) and threshold write-head by having morphological and magnetic properties different current Iwrite (with SUL) from its surroundings.

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-20

Read-head output (mT) -40

-60 0 42 6 8 10 12 14 16 18 20 Elapsed time (s)

Figure 13: Temporal change in read-head output with SUL nanowire memory that we have been researching at NHK References STRL with the aim of achieving memory that can record 1) L. Le Guyader, M. Savoini, S. El Moussaoui, M. Buzzi, A. video data with extremely high data transfer rates. We also Tsukamoto, A. Itoh, A. Kirilyuk, T. Rasing, A. V. Kimel and reported on the fabrication of a prototype unit recording F. Nolting: “Nanoscale Sub-100 Picosecond All-optical Mag- element of this memory through the use of an HDD-type netization Switching in GdFeCo Microstructures,” Nat. Com- magnetic recording head and a perpendicularly magnetized mun., Vol. 6, pp. 5839-1 - 5839-6 (2015) magnetic nanowire consisting of alternate layers of cobalt 2) L. Berger: “Exchange Interaction Between Ferromagnetic Do- and palladium films, and we presented the results of an main Wall and Electric Current in Very Thin Metallic Films,” experiment on magnetic-domain recording and reproduc- J. Appl. Phys., Vol. 55, No. 6, pp. 1954-1956 (1984) tion. These results show that magnetic-domain formation 3) A. Yamaguchi, T. Ono, S. Nasu, K. Miyake, K. Mibu and T. (recording) by a current-induced magnetic field, magnetic- Shinjo: “Real-Space Observation of Current-Driven Domain domain driving by a pulse current, and the detection (repro- Wall Motion in Submicron Magnetic Wires,” Phys. Rev. Lett., duction) of the magnetic-domain magnetization direction Vol. 92, No. 7, pp. 077205-1 - 077205-4 (2004) can be performed in a magnetic nanowire with an SUL and 4) T. Koyama, H. Hata, K. J. Kim, T. Moriyama, H. Tanigawa, T. that the operation principle of this memory is valid. Suzuki, Y. Nakatani, D. Chiba and T. Ono: “Current-induced In future research, we aim to increase the speed of mag- Magnetic Domain Wall Motion in a Co/Ni Nanowire with netic-domain driving and reduce the drive current with the Structural Inversion Asymmetry,” Appl. Phys. Express, Vol. 6, overall objective of achieving a recording device using No. 3, pp. 033001-1 - 033001-3 (2013) magnetic nanowire memory. 5) T. Ono: “Spinelectronics-Basic and Forefront-,” CMC Pub- lishing, pp. 117-125 (2004) (in Japanese) This paper is based on the following papers, which 6) M. Okuda, Y. Miyamoto, E. Miyashita, N. Saito, N. Hayashi appeared in ITE Technical Reports and IEEE Transactions and S. Nakagawa: “Detection of Current-driven Magnetic Do- on Magnetics. mains in [Co/Pd] Nanowire by Tunneling Magnetoresistive Sensor,” J. Appl. Phys., Vol. 117, pp. 17D516-1 - 17D516-4 M. Okuda, Y. Miyamoto, M. Kawana, E. Miyashita, and N. (2015) Saito: “New Magnetic Nanowire Memory Utilizing Motion of 7) M. Okuda, Y. Miyamoto, E. Miyashita and N. Hayashi: “Eval- Magnetic Domains: Formation, Accumulation and Detection uation of Magnetic Flux Distribution from Magnetic Domains of Magnetic Domains in Magnetic Nanowire by Magnetic in [Co/Pd] Nanowires by Magnetic Domain Scope Method Recording Head,” ITE Tech. Rep., Vol. 40, No. 6, MMS2016- Using Contact-scanning of Tunneling Magnetoresistive Sen- 45, pp. 27-31 (2016) [in Japanese] sor,” J. Appl. Phys., Vol. 115, pp. 17D113-1 - 17D113-3 (2014) 8) M. Okuda, Y. Miyamoto, M. Kishida and N. Hayashi: “Mag- M. Okuda, Y. Miyamoto, M. Kawana, E. Miyashita, N. netic Properties of Magnetic Nanowires with Ultra-small Trap Saito, and S. Nakagawa: “Operation of [Co/Pd] Nanowire Sites Fabricated by Anodic Oxidation and Nanoindentation Sequential Memory Utilizing Bit-Shift of Current-Driven using Scanning Probe Microscopy,” IEEE Trans. Magn., Vol. Magnetic Domains Recorded and Reproduced by Magnetic 47, No. 10, pp. 2525-2527 (2011) Head,” IEEE Trans. Magn., Vol. 52, No. 7, 3401204, pp. 3401204.1-3401204.4 (2016)