Angular harmonic Hall voltage and magnetoresistance measurements of Pt/FeCoB and Pt-Ti/FeCoB bilayers for spin Hall conductivity determination

TL;DR

This paper introduces a precise measurement protocol for determining spin Hall conductivity in heavy-metal/ferromagnet bilayers, validated on Pt and Pt-Ti underlayers with FeCoB, revealing high conductivity values.

Contribution

The work presents a novel, comprehensive measurement protocol combining static and dynamic techniques for accurate spin Hall conductivity determination in HM/FM bilayers.

Findings

Achieved spin Hall conductivity up to 3.3×10^5 S/m in Pt and Pt-Ti/FeCoB bilayers.

Validated the measurement protocol with static and dynamic transport measurements.

Found higher spin Hall conductivity than in other heavy metals like W or Ta.

Abstract

Materials with significant spin-orbit coupling enable efficient spin-to-charge interconversion, which can be utilized in novel spin electronic devices. A number of elements, mainly heavy-metals (HM) have been identified to produce a sizable spin current ($j_\mathrm{s}$), while supplied with a charge current ($j$), detected mainly in the neighbouring ferromagnetic (FM) layer. Apart from the spin Hall angle $\theta_\mathrm{SH}$ = $j_\mathrm{s}$/$j$, spin Hall conductivity ($\sigma_\mathrm{SH}$) is an important parameter, which takes also the resistivity of the material into account. In this work, we present a measurement protocol of the HM/FM bilayers, which enables for a precise $\sigma_\mathrm{SH}$ determination. Static transport measurements, including resistivity and magnetization measurements are accompanied by the angular harmonic Hall voltage analysis in a dedicated low-noise rotating probe station. Dynamic characterization includes effective magnetization and magnetization damping measurement, which enable HM/FM interface absorption calculation. We validate the measurement protocol in Pt and Pt-Ti underlayers in contact with FeCoB and present the $\sigma_\mathrm{SH}$ of up to 3.3$\times$10$^5$ S/m, which exceeds the values typically measured in other HM, such as W or Ta.