Spin currents control thermal fluctuations

Teams from the The National Institute of Standards and Technology (NIST) Center for Nanoscale Science and Technology, the University of Muenster and West Virginia University have published their findings in Physical Review Letters. The magnetic fluctuations of a 2µm diameter disc of a Ni-Fe alloy (permalloy) were measured by the researchers 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 disc 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. According to the researchers, this 'spin hall effect' drives a spin current, but not a charge current, into the bottom of the magnetic disc. The measurements show that the thermal fluctuations of the disc's magnetisation 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 researchers state that the measured population of the disc's magnetic excitations differs from a thermal distribution, showing that the effect is not simply cooling or heating, These results provide insight into the complexity of spin current phenomena and suggest a route for controllably manipulating fluctuations in future magnetic nanodevices.