Time-resolved soft X-ray transmission microscopy is applied to image thecurrent-induced resonant dynamics of the magnetic vortex core realized in amicronsized Permalloy disk. The high spatial resolution better than 25 nmenables us to observe the resonant motion of the vortex core. The result alsoprovides the spin polarization of the current to be 0.67 +/-0.16 for Permalloyby fitting the experimental results with an analytical model in the frameworkof the spin-transfer torque.
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