Ab initio studies of the spin-transfer torque in tunnel junctions

TL;DR

This paper uses ab initio calculations to analyze the spin-transfer torque in Fe/MgO/Fe tunnel junctions, revealing its interfacial nature and potential for out-of-plane components, with implications for spintronic device design.

Contribution

The study provides the first ab initio comparison of spin-transfer torque in tunnel versus metallic junctions, highlighting the interfacial origin and conditions for out-of-plane torque components.

Findings

Spin-transfer torque is predominantly interfacial in Fe/MgO/Fe tunnel junctions.

Half metallic behavior of Fe influences the torque characteristics.

Ideal samples can exhibit out-of-plane, bias-linear torque components.

Abstract

We calculate the spin-transfer torque in Fe/MgO/Fe tunnel junctions and compare the results to those for all-metallic junctions. We show that the spin-transfer torque is interfacial in the ferromagnetic layer to a greater degree than in all-metallic junctions. This result originates in the half metallic behavior of Fe for the $\Delta_1$ states at the Brillouin zone center; in contrast to all-metallic structures, dephasing does not play an important role. We further show that it is possible to get a component of the torque that is out of the plane of the magnetizations and that is linear in the bias. However, observation of such a torque requires highly ideal samples. In samples with typical interfacial roughness, the torque is similar to that in all-metallic multilayers, although for different reasons.