The origin of spin current in YIG/nonmagnetic metal multilayers at ferromagnetic resonance**Project supported by the National Basic Research Program of China (Grant Nos. 2015CB921502 and 2013CB922303), the National Natural Science Foundation of China (Grant Nos. 11474184, 116627805, and 11504203), and the 111 Project (Grant No. B13029). Zhang Yin and Wang Xiangrong were supported by the Hong Kong RGC Grants (Grant

Abstract

Spin pumping in yttrium-iron-garnet (YIG)/nonmagnetic-metal (NM) layer systems under ferromagnetic resonance (FMR) conditions is a popular method of generating spin current in the NM layer. A good understanding of the spin current source is essential in extracting spin Hall angle of the NM and in potential spintronics applications. It is widely believed that spin current is pumped from precessing YIG magnetization into NM layer. Here, by combining microwave absorption and DC-voltage measurements on thin YIG/Pt and YIG/NM1/NM2 (NM1 = Cu or Al, NM2 = Pt or Ta), we unambiguously showed that spin current in NM, instead of from the precessing YIG magnetization, came from the magnetized NM surface (in contact with thin YIG), either due to the magnetic proximity effect (MPE) or from the inevitable diffused Fe ions from YIG to NM. This conclusion is reached through analyzing the FMR microwave absorption peaks with the DC-voltage peak from the inverse spin Hall effect (ISHE). The voltage signal is attributed to the magnetized NM surface, hardly observed in the conventional FMR experiments, and was greatly amplified when the electrical detection circuit was switched on.