Team controls thermal fluctuations with spin current
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 electrons 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 disks magnetization are suppressed if the injected spins are parallel to the magnets spins, and that the fluctuations are strongly amplified if the injected spins and the magnets 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 disks 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.