Effect of Post‐Annealing on the Interface Quality and Spin Hall Magnetoresistance of Altermagnetic RuO ₂ Heterostructures on TiO ₂

Altermagnets hold promise for next‐generation spintronics, with ruthenium dioxide (RuO 2 ) as a leading candidate due to its high conductivity and large spin splitting. However, discrepancies in reported magnetic properties of RuO 2 films, often stemming from uncontrolled interfaces and surfaces, remain a major challenge. Here, we systematically investigate how post‐growth processing affects the interface quality and spin Hall magnetoresistance (SMR) of TiO 2 /RuO 2 /Co heterostructures. We find that dry etching of TiO 2 substrates creates a conductive n‐type surface layer, while as‐grown RuO 2 films exhibit rough surfaces, both of which shunt current and degrade device performance. We demonstrate that post‐annealing at 600 °C in oxygen effectively heals these defects, restoring the insulating TiO 2 surface and smoothing the RuO 2 film. Crucially, the annealed samples exhibit a temperature‐dependent SMR ratio that follows the theoretical scaling law (decreasing with temperature), in stark contrast to the unannealed samples whose SMR ratio increases anomalously with temperature. Our findings establish a reliable fabrication protocol for uncovering the intrinsic properties of altermagnetic RuO 2 and highlight the critical importance of interface engineering in interpreting altermagnetic transport phenomena.