Nonrelativistic Spin Current Generated Unconventional Spin Hall Magnetoresistance in Altermagnetic RuO₂

The intrinsic spin-split band structure and compensated magnetic moments of altermagnets make them promising candidates for spintronic applications. Recently, a magnetic spin Hall effect has been discovered in spin-split antiferromagnets, including noncollinear antiferromagnets and altermagnets. This effect enables the efficient generation of unconventional spin currents even without spin-orbit coupling. However, although such nonrelativistic spin currents are proposed to have a magnetic origin, the direct connection between the Néel vector and the spin current polarization is still missing. Here, we report an unconventional spin Hall magnetoresistance (SMR) generated from nonrelativistic spin currents, which exhibits a fundamentally distinct angular- and temperature-dependences compared to conventional SMR. Based on this mechanism, we investigate RuO₂, an altermagnetic candidate under debate recently, and unveil its spin current polarization by disentangling the conventional and magnetic spin Hall effects. The results suggest that the nonrelativistic spin current hosts a polarization very close to the Néel vector, unambiguously indicating a magnetic origin. The unconventional SMR not only presents solid evidence helpful to solve the puzzle of state in RuO₂, but also paves a straightforward route to harnessing nonrelativistic spin currents in altermagnets for advanced spintronic applications.