Diskless Computer Data Storage
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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.
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.
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 |