Influence of the magnetic material on tunneling magnetoresistance and spin-transfer torque in tunnel junctions: Ab initio studies

TL;DR

This study uses ab initio methods to analyze how Co concentration and disorder affect tunneling magnetoresistance and spin-transfer torque in FeCo/MgO/FeCo tunnel junctions, revealing new insights into their dependence on material composition and bias voltage.

Contribution

It provides a detailed ab initio analysis of the effects of Co concentration and disorder on tunneling magnetoresistance and spin-transfer torque, explaining discrepancies with previous calculations and experiments.

Findings

Tunneling magnetoresistance decreases with Co concentration, aligning with recent experiments.

Small disorder causes a significant drop in tunneling magnetoresistance.

Spin-transfer torque exhibits linear and quadratic voltage dependencies, with high bias deviations at high Co concentrations.

Abstract

The dependence of tunneling magnetoresistance and spin-transfer torque in FeCo/MgO/FeCo tunnel junctions on the Co concentration and the bias voltage are investigated ab initio. We find that the tunneling magnetoresistance decreases with the Co concentration in contradiction with previous calculations but in agreement with recent experiments. This dependence is explained from bulk properties of the alloys. By using a realistic description of the disorder in the alloys we can show that even small amounts of disorder lead to a drastic drop in the tunneling magnetoresistance. This provides a quantitative explanation of the difference between calculated and measured values. The spin-transfer torque shows a linear voltage dependence for the in-plane component and a quadratic for the out-of-plane component for all concentrations at small bias voltages. In particular, the linear slope of the in-plane torque is independent of the concentration. For high bias voltages the in-plane torque shows a strong nonlinear deviation from the linear slope for high Co concentrations. This is explained from the same effects which govern the tunneling magnetoresistance.