Spin Hall Effect Induced Spin Transfer Through an Insulator

TL;DR

This paper predicts a quantum tunneling mechanism for spin transfer across insulators driven by spin Hall effect-induced spin accumulation, enabling noninvasive electrical probing of ferromagnetic insulators.

Contribution

It introduces a novel tunneling mechanism for spin transfer through insulators, linking spin Hall effect to quantum spin tunneling in multilayer structures.

Findings

Spin transfer can occur via quantum tunneling without charge transfer.

The spin transfer decay depends on the layer composition and thickness.

Potential for new tunneling spectroscopy to probe magnon states.

Abstract

When charge current passes through a normal metal that exhibits spin Hall effect, spin accumulates at the edge of the sample in the transverse direction. We predict that this spin accumulation, or spin voltage, enables quantum tunneling of spin through an insulator or vacuum to reach a ferromagnet without transferring charge. In a normal metal/insulator/ferromagnetic insulator trilayer (such as Pt/oxide/YIG), the quantum tunneling explains the spin-transfer torque and spin pumping that exponentially decay with the thickness of the insulator. In a normal metal/insulator/ferromagnetic metal trilayer (such as Pt/oxide/Co), the spin transfer in general does not decay monotonically with the thickness of the insulator. Combining with the spin Hall magnetoresistance, this tunneling mechanism points to the possibility of a new type of tunneling spectroscopy that can probe the magnon density of states of a ferromagnetic insulator in an all-electrical and noninvasive manner.