Crystallinity control of spin torque efficiency in CuNi alloys

Light metals are gaining increasing attention in spintronics due to their advantageous properties over traditional heavy metals. This work investigates the origin of large spin torque efficiency ( ξ SOT ) in CuNi alloys using spin-torque ferromagnetic resonance in FeNi/CuNi bilayers. Polycrystalline CuNi shows a ξ SOT comparable to heavy metal Pt. By controlling crystallinity, we observe a significant modulation of ξ SOT , as epitaxial CuNi (001) and (111) films exhibit similar but markedly lower values than their polycrystalline counterparts. This suggests that the charge-spin conversion in CuNi is strongly influenced by crystallinity and primarily dominated by extrinsic scattering. Temperature-dependent studies further support a dominant contribution from extrinsic scattering in both paramagnetic and ferromagnetic phases, with minimal contribution from spin fluctuations across the Curie temperature. Our findings deepen the understanding of charge-spin interconversion in light metals and highlight their potential for spintronic applications.