Enhanced and Tunable Spin-Orbit Coupling in Tetragonally Strained Fe-Co-B Films

TL;DR

This study demonstrates that tetragonal strain and boron doping in Fe-Co-B films can be used to enhance and tune spin-orbit coupling, with potential applications in spintronics.

Contribution

The paper introduces a method to control spin-orbit coupling in ferromagnetic films through strain engineering and boron doping.

Findings

Orbital magnetic moment increases with c/a ratio.

Strain stabilization is achieved with boron doping.

Enhanced SOC observed at larger tetragonal strain.

Abstract

We have synthesized 20 nm thick films of tetragonally strained interstitial Fe-Co-B alloys epitaxially grown on Au55Cu45 buffer layer. The strained axis is perpendicular to the film plane and the corresponding lattice constant c is enlarged with respect to the in-plane lattice parameter a. By adding the interstitial boron with different concentrations 0, 4, and 10 at.% we were able to stabilize the tetragonal strain in 20 nm Fe-Co films with different c/a ratios of 1.013, 1.034 and 1.02, respectively. Using ferromagnetic resonance (FMR) and x-ray magnetic circular dichroism (XMCD) we found that the orbital magnetic moment increases with increasing the c/a ratio, pointing towards the enhancement of spin-orbit coupling (SOC) at larger strain. Our results show that careful doping of ferromagnetic films allows to control the SOC by stabilizing anisotropic strain states. These findings are applicable in material design for spintronics applications. We also discuss the influence of B doping on the Fe-Co film microstructure, its magnetic properties and magnetic relaxation.