Probing the spin polarization of current by soft X-ray imaging of   current-induced magnetic vortex dynamics
  Time-resolved soft X-ray transmission microscopy is applied to image the current-induced resonant dynamics of the magnetic vortex core realized in a micronsized Permalloy disk. The high spatial resolution better than 25 nm enables us to observe the resonant motion of the vortex core. The result also provides the spin polarization of the current to be 0.67 +/-0.16 for Permalloy by fitting the experimental results with an analytical model in the framework of the spin-transfer torque. 
Shinya Kasai, Peter Fischer, Mi-Young Im, Keisuke Yamada, Yoshinobu   Nakatani, Kensuke Kobayashi, Hiroshi Kohno and Teruo Ono