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September 26, 2007

 

 

X-Ray Imaging Current-Driven Magnetic Domain-Wall Motion in Nanowires

The quest to increase both computer data-storage density and the speed at which one can read and write the information remains unconsummated. One novel concept is based on the use of a local electric current to push magnetic domain walls along a thin nanowire. A German, Korean, Berkeley Lab team has used the x-ray microscope XM-1 at the ALS to demonstrate that magnetic domain walls in curved permalloy nanowires can be moved at high speed by injecting nanosecond pulses of spin-polarized currents into the wires, but the motion is largely stochastic. This result will have an impact on the current development of magnetic storage devices in which data is moved electronically rather than mechanically as in computer disk drives.

 

Spin torque

A spin torque exerted by a spin-polarized current pulse on the noncollinear spin configuration in a domain wall pushes the domain wall along the ferromagnetic wire.

Diskless Computer Data Storage

Between ever more capable operating systems, more sophisticated applications, and huge collections of data files, the computer industry and its customers exhibit an insatiable appetite for larger and larger data-storage capacities with faster and faster data writing and reading. The march of miniaturization has served to meet this requirement well (as in desktops with trillion-byte, or terabyte, hard drives), but more novel technologies are being investigated. One of these, called racetrack memory, involves no rotating disk at all. Instead, it is based on wires made of a magnetic material only a few nanometers wide consisting of a sequence of magnetized regions (domains) whose boundary walls represent the binary bits of information (0 and 1). A short pulse of electric current pushes the domains through the wire to a fixed readout sensor. Invented at IBM, racetrack memory so far has neither high performance nor a well-understood mechanism. To improve understanding, Meier et al. have used an x-ray microscope both to visualize the motion of the domains driven through the wire and to measure their speed. Their findings demonstrate both the value of x-ray microscopy to study domain motion in wires and provide hints on how to improve performance.

The physical effect in current-driven domain-wall motion is based on a spin torque that is exerted by spin-polarized conduction electrons on the magnetic moments in the wire, so that a domain wall moves as the torque rotates the moments. In contrast to today's magnetic hard drives where a disk mechanically spins under a read head that reads the data stored on the disk at fixed positions, the current would move the domain walls, which represent the bits, electronically to a locally fixed read-out sensor. This idea, called racetrack memory and patented in 2004 by Stuart Parkin (IBM Almaden Research Center), would combine the advantages of both solid-state and magnetic memory devices. However, there are still several open questions blocking the jump from physics to technology (including how to boost the readout speed) whose answers depend on a better understanding of the physics involved.

To investigate the fundamental processes of spin-torque-driven motion of domain walls in curved ferromagnetic permalloy (Ni80Fe20) wires, a widely used material in disk drives, the collaboration injected pulses of nanosecond duration and of high current density to drive the motion of a single domain wall along the wire. By making polarized x-ray images with XM-1 before and after the current pulse was injected, they tracked the location of the domain wall with 25-nanometer spatial resolution. The results showed that the magnetic domain walls moved at a speed of 110 meters per second, which is very fast on the nanometer scale, 100 times faster than reported before, and is in accord with a theory of spin-torque transfer. It is believed that the nanosecond pulses reduced the chances that a wall would be pinned by imperfections in the crystalline structure during its brief motion, thereby explaining the high speed.

 

Domain-wall schematic

A magnetic domain wall (DW) is created between contact pads in a permalloy (Ni80Fe20) ring 20 nm thick and 1000 nm wide by applying and releasing an external magnetic field (Hext). A fast electronic pulser then launches short 1-ns current pulses with a current density < 1012 A/m2 into the ferromagnetic wire.

 

Microscopy images

Magnetic soft x-ray microscopy allows imaging domain walls in the nanoring segment with a spatial resolution down to 25 nm before (left) and after (center) the injection of a short 1-ns current pulse. Measuring the distance that the DW has moved (right), one can deduce a DW speed vDW = 110m/s in agreement with theoretical estimates. Repeated attempts to move the DW show a stochastic movement, which is analogous to Barkhausen jumps in the field-driven case.

Although this is encouraging news for technological development, repetitive pulse experiments also showed that many of the pulses gave smaller speeds or no movement at all, so that the current-driven motion followed a statistical distribution comparable to Barkhausen jumps in the case where domain motion is driven by an applied magnetic field. Since domain-wall pinning associated with disorder plays a significant role in field-driven Barkhausen avalanches, one can assume a similar mechanism in case of current-driven domain-wall motion. The random nature of the domain-wall jumps means that reliable reading and writing await the ability to minimize the effect of inhibiting defects by better control of materials (perhaps by changing the wire geometry).

In the meantime, high-resolution soft x-ray microscopy with a 10-nm spatial resolution in combination with ultrafast time resolution in the femtosecond regime and sufficient intensity for snapshot imaging not only will be a powerful analytical tool for characterizing the dynamics in real time with nanoscale spatial resolution, but also provides an accurate experimental tool for testing theoretical models of current-induced phenomena in magnetic materials at the nanoscale.

Research conducted by G. Meier, M. Bolte, R. Eiselt, and B. Krüger (University of Hamburg, Germany); D.-H. Kim (Chungbuk National University, South Korea); and P. Fischer (Center for X-Ray Optics, Berkeley Lab).

Research funding: U.S. Department of Energy, Office of Basic Energy Sciences (BES), and the German Science Foundation (DFG). Operation of the ALS is supported by BES.

Publication about this research: G. Meier, M. Bolte, R. Eiselt, B. Krüger, D.-H. Kim, and P. Fischer, "Direct imaging of stochastic domain-wall motion driven by nanosecond current pulses," Phys. Rev. Lett. 98, 187202 (2007).

 

ALSNews Vol. 280, September 26, 2007

 

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