Twist in the bias-dependence of spin-torques in magnetic tunnel junctions

TL;DR

This paper introduces a new experimental method to accurately analyze the bias dependence of spin-torques in magnetic tunnel junctions, revealing complex behaviors especially in asymmetric structures with large voltage biases.

Contribution

A novel method for extracting bias-dependent spin-torques over large voltage ranges, and insights into the complex bias dependence in asymmetric junctions.

Findings

Symmetric CoFeB junctions match theoretical predictions.

Asymmetric NiFe-containing junctions show twisted bias dependence.

Complex spin-torque behaviors emerge at large bias voltages.

Abstract

The spin-torque in magnetic tunnel junctions possesses two components that both depend on the applied voltage. Here, we develop a new method for the accurate extraction of this bias-dependence from experiments over large voltage ranges. We study several junctions with different magnetic layer structures of the top electrode. Our results obtained on junctions with symmetric CoFeB electrodes agree well with theoretical calculations. The bias-dependences of asymmetric samples, with top electrodes containing NiFe, however, are twisted compared to the quadratic form generally assumed. Our measurements reveal the complexity of spin-torque mechanisms at large bias.