Non-trivial spatial dependence of the spin torques in L1$_0$ FePt-based tunnelling junctions

TL;DR

This study uses ab-initio calculations to explore how spin-transfer torque behaves in FePt-based magnetic tunnel junctions, revealing non-trivial spatial dependence and potential for device applications.

Contribution

It provides the first detailed ab-initio analysis of spin-transfer torque in FePt/MgO/Fe junctions, highlighting the impact of atomic-scale magnetic texture.

Findings

Torque in FePt layers is less attenuated than in pure Fe.

Torque sign alternates at atomic planes in FePt.

Intercalating Fe reduces torque transfer and magnetoresistance.

Abstract

We present an {\it ab-initio} study of the spin-transfer torque in a Fe/MgO/FePt/Fe magnetic tunnel junctions. We consider a FePt film with a thickness up to six unit cells, either in direct contact with the MgO spacer or with an intercalated ultra-thin Fe seed layer. We find that in the FePt layer the torque is not attenuated as strongly as in the case of pure Fe. Moreover, in FePt the torque alternates sign at the Fe and Pt atomic planes throughout the stack for all FePt thicknesses considered. Finally, when Fe is intercalated between MgO and L$1_0$-FePt, the torque is sharply attenuated and it is transferred to FePt only for a Fe seed layer that is less than two-atomic-planes thick. We attribute these features to the different spatial profiles of the exchange and correlation field and the induced non-equilibrium spin accumulation. The calculated tunnelling magneto-resistance of the Fe/MgO/FePt/Fe junctions studied is enhanced with respect to the one of Fe/MgO/Fe, while it is reduced with Fe intercalation. Our work shows that L$1_0$-FePt junctions can be promising candidates for current-operated magnetic devices and that the magnetic texture at the atomic scale has an important effect on the spin transfer torque.