Who pumps spin current into nonmagnetic-metal (NM) layer in YIG/NM multilayers at ferromagnetic resonance?

TL;DR

This study reveals that in YIG/NM multilayers under FMR, the spin current in the NM layer originates from the magnetized NM surface caused by magnetic proximity effect, not directly from the precessing YIG magnetization.

Contribution

The paper demonstrates, using combined microwave absorption and DC-voltage measurements, that spin current in NM layers is due to magnetic proximity effect rather than YIG magnetization precession.

Findings

Spin current in NM layers is from the magnetized NM surface.

Magnetic proximity effect causes the surface magnetization in NM.

Enhanced detection method reveals FMR absorption of the magnetized NM surface.

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 YIG/Pt and YIG/NM1/NM2 (NM1=Cu or Al, NM2=Pt or Ta), we unambiguously showed that spin current in NM came from the magnetized NM surface (in contact with YIG) due to the magnetic proximity effect (MPE), rather than the precessing YIG magnetization. This conclusion is reached through our unique detecting method where the FMR microwave absorption of the magnetized NM surface, hardly observed in the conventional FMR experiments, was greatly amplified when the electrical detection circuit was switched on.