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

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.

Figures (4)

• Figure 1: (a) Schematic of the RuO$_2$(101)/FM bilayer. The gray bar indicates the etched stripe for spin-pumping detection, positioned between a coplanar waveguide. (b) Experimental arrangement. Microwave excites FMR in the FM layer and spin-charge conversion is read out as a dc voltage across the stripe. (c) Cross-sectional TEM image of the RuO$_2$(101)/YIG bilayer. Inset: FFT of a selected RuO$_2$ region confirming single-crystal quality. (d) Measured voltage of the RuO$_2$(101)(15 nm)/YIG(42 nm) bilayers as a function of applied magnetic field. The arrow marks the peak value $\Delta V_{\rm SP}$. Inset: reference signal from a polycrystalline Pt(4 nm)/Py(6 nm) bilayer recorded under the identical conditions. (e) Corresponding data for the RuO$_2$(101)(15 nm)/Py(6 nm) bilayers; note the sign reversal with respect to (d). (f) Angular dependence of $\Delta V_{\rm SP}$ for both systems. Symbols are the experimental data and the solid lines illustrate $\textrm{cos}^{2}\varphi_c$ fits. The error bars are defined as $(\vert\Delta V_{\rm SP} (H_+ )\vert-\vert\Delta V_{\rm SP} (H_- )\vert)/2$ with $\Delta V_{\rm SP} (H_+)$ and $\Delta V_{\rm SP} (H_-)$ measured at the positive and negative FMR fields, respectively.
• Figure 2: (a) Device sketch. Distinct FMR fields of YIG and Py allow spin current to be injected into the nonmagnetic layer from either side within one measurement. (b) Spin-pumping signals for YIG(42 nm)/RuO$_2$(8 nm)/Py(10 nm) trilayer as a function of applied magnetic field. (c) The same measurement for the reference sample YIG(42 nm)/Pt(6 nm)/Py(10 nm).
• Figure 3: Spin-pumping signals for YIG(42 nm)/RuO$_2$(8 nm)/Py(10 nm) with Au inserted at the (a) YIG/RuO$_2$ and (b) RuO$_2$/Py interfaces. (c, d) Normalized spin-pumping voltage $\Delta V_{\mathrm{SP}}/(R_s P_{\mathrm{rf}})$ as a function of Au thickness, allowing quantitative comparison.
• Figure 4: (a) Schematic Rashba splitting and helical Fermi contours. Opposite momenta carry opposite transverse spins. (b) Spin-projected band structure of RuO$_2$(001). (c) Spin-projected band structure of RuO$_2$(001) with a Ni adsorption layer. (d) DOS of RuO$_2$(001) with Ni adsorption. SOC is turned off for clarity. (e) DOS of RuO$_2$(001) (red lines) and YIG(111) (blue lines), independently calculated with vacuum level aligned. The RuO$_2$ Fermi level (black dashed line) falls inside the energy gap of YIG (light yellow shadow), suppressing hybridization. The RuO$_2$ surface DOS is normalized to the same surface area as the YIG(111) unit cell.