Ferromagnetic Interface Engineering of Spin-Charge Conversion in RuO_{2}.

Spin-orbit torque efficiency is conventionally fixed by bulk materials. D-wave altermagnets introduce an additional nonrelativistic spin-charge conversion channel beyond the 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, evidencing a dominant interfacial inverse Rashba-Edelstein effect, whereas RuO_{2}/Py is governed by the 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.