A team of researchers from the NIST
Center for Nanoscale Science and
Technology, the University of Muenster, and West Virginia University
have demonstrated control of magnetic thermal fluctuations using current.
The work, reported in Physical Review Letters,
represents an important step towards manipulating the noise properties of
magnetic nanosensors and memory devices.
The magnetic fluctuations of a 2 µm-diameter disk of a Ni-Fe
alloy (permalloy) were measured using microfocus Brillouin light scattering
while a current passed through a supporting Pt strip. The current generated a spin
current, which was injected into the permalloy disk through its back surface.
As electrons flow along the Pt strip, they scatter differently, depending on
each electron’s spin: those with “up” spin scatter slightly toward the top
surface, while those with “down” spin scatter slightly toward the bottom
surface. This “spin Hall effect” drives a spin current, but not a charge
current, into the bottom of the magnetic disk.
The measurements show that the thermal fluctuations of the
disk’s magnetization are suppressed if the injected spins are parallel to the
magnet’s spins, and that the fluctuations are strongly amplified if the
injected spins and the magnet’s spins are antiparallel. By changing the current
down the Pt strip, the fluctuations were controllably reduced to 0.5 times or
amplified to 25 times their thermal level. The measured population of the
disk’s magnetic excitations differs from a thermal distribution, showing that
the effect is not simply cooling or heating.
These intriguing results provide insight into the complexity
of spin current phenomena and suggest a route for controllably manipulating
fluctuations in future magnetic nanodevices.