Direct observation of current-induced nonlinear spin torque in Pt-Py bilayers

TL;DR

This paper reports the experimental observation of nonlinear spin torque in Pt-Py bilayers, revealing mechanisms like nonlinear spin polarization and magnon interactions, with implications for advanced spintronic devices and magnetic metamaterials.

Contribution

It provides the first experimental evidence of nonlinear spin torque mechanisms in metallic bilayers, highlighting the roles of spin polarization and magnon dynamics.

Findings

Nonlinear spin torque observed under high dc current in Pt-Py bilayers.

Magnon generation and annihilation influence effective magnetization.

Potential for developing nonlinear spintronic devices and metamaterials.

Abstract

We experimentally observe nonlinear spin torque in metallic bilayers of platinum and permalloy by means of spin-torque ferromagnetic-resonance (ST-FMR) under massive dc current injection. The observed nonlinear spin torque exerted to permalloy magnetization is attributed primarily to nonlinear spin polarization. Additional origin of the nonlinear spin torque is magnon generation (annihilation) followed by shrinkage (expansion) of effective magnetization, which is reveled by STFMR and unidirectional spin Hall magnetoresistance measurements. The present study paves a way to spin-Hall effect based nonlinear spintronic devices as well as time-varying nonlinear magnetic metamaterials with tailor-made permeability.