Direct detection of pure AC spin-current by x-ray pump-probe measurements

Despite recent progress in spin-current research, the detection of spin current has mostly remained indirect. By synchronizing a microwave waveform with synchrotron x-ray pulses, we use the ferromagnetic resonance of the Py (Ni_81 Fe_19) layer in a Py/Cu/Cu_75 Mn_25/Cu/Co multilayer to pump a pure AC spin current into the Cu_75 Mn_25 and Co layers, and then directly probe the spin current within the Cu_75 Mn_25 layer and the spin dynamics of the Co layer by x-ray magnetic circular dichroism. This element-resolved pump-probe measurement unambiguously identifies the AC spin current in the Cu_75 Mn_25 layer.