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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 9 FEATURE 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 head*6, 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 infor- mation. *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 11 FEATURE intense magnetic field in either the upward or downward 3.

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