Strain-Driven Altermagnetic Spin Splitting Effect in RuO$_2$

TL;DR

This study explores how strain and crystal orientation induce altermagnetic spin splitting in RuO$_2$, clarifying previous experimental inconsistencies and guiding future spintronic device design.

Contribution

It demonstrates strain-induced altermagnetic spin splitting in RuO$_2$ without the need for Hubbard $U$, reconciling prior experimental discrepancies.

Findings

Strain induces spin splitting in (100) and (110) RuO$_2$ films.

Bulk and (001)/(101) films are nonmagnetic without extrinsic effects.

Strain and orientation influence the spin Hall response significantly.

Abstract

The non-relativistic spin-momentum locking in altermagnets gives rise to a time-reversal-odd spin Hall effect, known as the altermagnetic spin-splitting effect (ASSE). Although ASSE was first reported in RuO$_2$, subsequent experiments have yielded inconsistent results, leaving its spin-transport mechanism unclear. Here, we systematically investigate how strain, crystal orientation, and the Hubbard $U$ parameter influence the magnetic ground state and spin Hall response of RuO$_2$. Guided by recent experimental observations, we find that $U$ is likely smaller than the value required to induce intrinsic magnetism, suggesting that bulk RuO$_2$ and (001)/(101) RuO$_2$ thin films grown on TiO$_2$ are nonmagnetic in the absence of extrinsic effects. In contrast, (100) and (110) films exhibit strain-induced altermagnetic spin splitting, leading to a strong ASSE even without Hubbard $U$ corrections. These results reconcile previous experimental discrepancies and provide design guidelines for RuO$_2$-based spintronic devices.