Ferromagnetic interface engineering of spin-charge conversion in RuO$_2$

TL;DR

This study demonstrates that ferromagnetic interfaces can control the magnitude and sign of spin-charge conversion in RuO$_2$, revealing a new way to engineer spintronic devices using interface effects and altermagnetic oxides.

Contribution

It shows that the adjacent ferromagnet can dictate spin-charge conversion in RuO$_2$, highlighting interface engineering as a novel control mechanism.

Findings

Opposite spin-Hall angles in RuO$_2$/YIG and RuO$_2$/Py bilayers.

Reversal of signal with ultrathin Au spacer at RuO$_2$/YIG interface.

Interface-selective band hybridization explains the dichotomy.

Abstract

Spin-orbit torque efficiency is conventionally fixed by bulk materials. $D$-wave altermagnets introduce an additional nonrelativistic spin-charge conversion channel beyond inverse spin-Hall effect. Using prototypical candidate RuO$_2$ as an example, we show that the adjacent ferromagnet alone can dictate both the magnitude and sign of spin-charge conversion. Spin-pumping measurements on RuO$_2$/Y$_3$Fe$_5$O$_{12}$ (YIG) and RuO$_2$/Ni$_{80}$Fe$_{20}$ (Py) bilayers yield opposite effective spin-Hall angles that persist across crystalline and polycrystalline RuO$_2$. Inserting an ultrathin Au spacer at the RuO$_2$/YIG interface reverses the signal, envidencing a dominant interfacial inverse Rashba-Edelstein effect, whereas RuO$_2$/Py is governed by bulk inverse spin-Hall effect. First-principles calculations trace this dichotomy to interface-selective band hybridization: Rashba surface states survive at the insulating YIG contact yet are quenched by metallic Py. Our findings establish ferromagnetic interfacing as a deterministic knob for tailoring spin-charge conversion in altermagnetic oxides, paving the way to field-free, low-dissipation spintronic memory devices.