Influence of intermixing at the Ta/CoFeB interface on spin Hall angle in Ta/CoFeB/MgO heterostructures

TL;DR

This study explores how intermixing at the Ta/CoFeB interface affects the spin Hall angle in Ta/CoFeB/MgO heterostructures, revealing a temperature-independent bulk spin Hall angle and a temperature-dependent interfacial spin Hall angle.

Contribution

It introduces an extended spin diffusion model that separately evaluates the temperature dependence of spin Hall angles in the bulk Ta and at the Ta/CoFeB interface.

Findings

Bulk Ta spin Hall angle is approximately -0.2 and temperature-independent.

Intermixed Ta/CoFeB interface exhibits a strongly temperature-dependent spin Hall angle.

Crystallographic structure varies with Ta layer thickness, affecting interfacial properties.

Abstract

We investigate the spin Hall effect in perpendicularly magnetized Ta/Co40Fe40B20/MgO trilayers with Ta underlayers thicker than the spin diffusion length. The crystallographic structures of the Ta layer and Ta/CoFeB interface are examined in detail using X-ray diffraction and transmission electron microscopy. The thinnest Ta underlayer is amorphous, whereas for thicker Ta layers a disoriented tetragonal beta-phase appears. Effective spin-orbit torques are calculated based on harmonic Hall voltage measurements performed in a temperature range between 15 and 300 K. To account for the temperature dependence of damping-like and field-like torques, we extend the spin diffusion model by including an additional contribution from the Ta/CoFeB interface. Based on this approach, the temperature dependence of the spin Hall angle in the Ta underlayer and at Ta/CoFeB interface are determined separately. The results indicate an almost temperature-independent spin Hall angle of theta_SH-N = -0.2 in Ta and a strongly temperature-dependent theta_SH-I for the intermixed Ta/CoFeB interface.