Reversal of Spin-torque Polarity with Inverting Current Vorticity in Composition-graded Layer at the Ti/W Interface

TL;DR

This study demonstrates that the polarity of compositional gradients at the Ti/W interface controls the sign of spin torque, offering a new approach for efficient spin-torque generation independent of strong spin Hall materials.

Contribution

It reveals that reversing the compositional gradient at the Ti/W interface inverts the spin-torque polarity, a novel mechanism for controlling spin currents.

Findings

Moderate compositional gradient suppresses negative spin torque from W.

Reversing Ti/W stacking suppresses positive spin torque from Ti.

Polarity of gradient governs the sign of spin torque.

Abstract

While compositional gradient-induced spin-current generation has been explored, its microscopic mechanisms remain poorly understood. Here, the contribution of polarity of compositional gradient on spin-current generation is explored. A nanoscale compositional gradient, formed by in-situ atomic diffusion of ultrathin Ti and W layers, is introduced between 10-nm-thick W and Ti layers. Spin-torque ferromagnetic resonance in ferromagnetic Ni95Cu5 deposited on this gradient reveals that a moderate compositional gradient suppresses negative spin torque from the spin Hall effect in W. In contrast, reversing the Ti/W stacking order, which inverts the gradient, suppresses positive spin torque from the orbital Hall effect in Ti. These findings suggest that the sign of spin torque is governed by the polarity of compositional gradient, providing a novel strategy for efficient spin-torque generation without relying on materials with strong spin or orbital Hall effect.