Field-like spin\u2013orbit torque induced by bulk Rashba channels in GeTe/NiFe bilayers

Jeehoon Jeon,Seungwu Han,Hye-Won Ko,Kyung-Jin Lee,Joon Young Kwak,Oukjae Lee,Soonho Jung,Yunseok Kim,Jinki Hong,Seong Won Cho,Suyoun Lee,Hyun Cheol Koo

doi:10.1038/s41427-021-00344-6

Abstract

Most studies of the Rashba effect have focused on interfacial Rashba spin–orbit coupling. Recently, bulk Rashba materials have attracted considerable interest owing to their potential to enhance the Rashba spin–orbit torque. By employing a bulk Rashba material, GeTe, as a spin–orbit channel in GeTe/NiFe bilayers, a large field-like spin–orbit torque up to 15.8 mT/(107 A cm−2) is measured. This value is one of the largest reported field-like torques and is attributed to the interfacial spin–orbit coupling being enhanced by the bulk Rashba effect in the GeTe channel. Furthermore, the large field-like torque is maintained even for a 20-nm-thick NiFe layer. This unconventional dependence on the thickness of both the GeTe and NiFe layers cannot be described by conventional theory, but it is believed to stem from the additional bulk Rashba effect-induced term. The large field-like torque over a wide range of ferromagnet thicknesses results in scalable in-plane spin–orbit torque devices. This result calls for a further theoretical study on spin transport in heterostructures, including bulk Rashba materials.

Highlights

Rashba spin–orbit coupling (RSOC) is a crucial element for spin-based information devices
In addition to gate-controlled spin precession, the RSOC contributes to spin–orbit torques (SOTs) in a normal metal (NM)/ferromagnet (FM) bilayer, the basic structure for SOT-active devices, where inversion symmetry breaks at the NM/FM interface gives rise to RSOC6–10
An important question has remained unexplored: what happens to the SOT of NM/FM bilayers when the NM layer is a bulk Rashba material? In this work, we address this question by investigating the SOTs in GeTe/ Ni81Fe19 bilayers using the second-harmonic Hall voltage measurement[24], from which we extract SOT magnitudes

Summary

Introduction

Rashba spin–orbit coupling (RSOC) is a crucial element for spin-based information devices. The control of spin precession concomitant with the modulation of RSOC by an electric field has been theoretically suggested and experimentally demonstrated using a two-dimensional quantum well[1,2,3,4,5,6]. In addition to gate-controlled spin precession, the RSOC contributes to spin–orbit torques (SOTs) in a normal metal (NM)/ferromagnet (FM) bilayer, the basic structure for SOT-active devices, where inversion symmetry breaks at the NM/FM interface gives rise to RSOC6–10. As the SOT serves as a writing scheme for energy-efficient spintronic devices[11,12,13], enhancing the RSOC effect in NM/ FM bilayers is an important task for practical applications. Most RSOC studies have focused on interfacial effects in two-dimensional systems or bilayers.

Methods

Results

Conclusion