Microwave excitations associated with a wavy angular dependence of the spin transfer torque : model and experiments

TL;DR

This paper experimentally investigates the unique 'wavy' angular dependence of spin transfer torque in magnetic spin valves, confirming diffusive models and revealing large-angle out-of-plane precession modes through combined experiments and simulations.

Contribution

It provides the first comprehensive experimental validation of the diffusive model predictions for WAD-STT and characterizes the associated magnetization dynamics.

Findings

Macrospin simulations match experimental phase diagrams and frequency dependencies.

Experimental evidence of large-angle out-of-plane precession modes.

Validation of diffusive models of spin transfer torque.

Abstract

The spin transfer torque (STT) can lead to steady precession of magnetization without any external applied field in magnetic spin valve where the magnetic layer have very different spin diffusion length. This effect is associated with an unusual angular dependence of the STT, called "wavy" (WAD-STT), predicted in the frame of diffusive models of spin transfer. In this article, we present a complete experimental characterization of the magnetization dynamics in the presence of a WAD-STT. The results are compared to the prediction of the magnetization dynamics obtained by single domain magnetic simulations (macrospin approximation). The macrospin simulations well reproduced the main static and dynamical experimental features (phase diagram, R(I) curves, dependence of frequency with current and field) and suggest that the dynamical excitations observed experimentally are associated with a large angle out-of-plane precession mode. The present work validates the diffusive models of the spin transfer and underlines the role of the spin accumulation and the spin relaxation effects on the STT.