Giant orbital Hall effect and orbital-to-spin conversion in 3d, 5d, and 4f metallic heterostructures

TL;DR

This paper demonstrates giant orbital Hall effects in various metallic heterostructures and explores how orbital-to-spin current conversion can be modulated, revealing new pathways for spin-orbit torque control in spintronics.

Contribution

It provides a systematic experimental and theoretical study of orbital Hall effects and their conversion to spin currents in 3d, 5d, and 4f metal heterostructures, including the role of 4f spacers.

Findings

Cr exhibits giant orbital Hall conductivity (~5×10^5 Ω^{-1} m^{-1})

Gd and Tb efficiently convert orbital to spin currents, enhancing spin-orbit torques

Introduction of 4f spacers can reverse and amplify spin-orbit torques

Abstract

The orbital Hall effect provides an alternative means to the spin Hall effect to convert a charge current into a flow of angular momentum. Recently, compelling signatures of orbital Hall effects have been identified in 3d transition metals. Here, we report a systematic study of the generation, transmission, and conversion of orbital currents in heterostructures comprising 3d, 5d, and 4f metals. We show that the orbital Hall conductivity of Cr reaches giant values of the order of 5*10^5 Ohm^{-1} m^{-1} and that Pt presents a strong orbital Hall effect in addition to the spin Hall effect. Measurements performed as a function of thickness of nonmagnetic Cr, Mn, and Pt layers and ferromagnetic Co and Ni layers reveal how the orbital and spin currents compete or assist each other in determining the spin-orbit torques acting on the magnetic layer. We further show how this interplay can be drastically modulated by introducing 4 f spacers between the nonmagnetic and magnetic layers. Gd and Tb act as very efficient orbital-to-spin current converters, boosting the spin-orbit torques generated by Cr by a factor of 4 and reversing the sign of the torques generated by Pt. To interpret our results, we present a generalized drift-diffusion model that includes both spin and orbital Hall effects and describes their interconversion mediated by spin-orbit coupling.