Simultaneous High Charge-Spin Conversion Efficiency and Large Spin Diffusion Length in Altermagnetic RuO₂

Efficient spin sources with large spin Hall angle (θ_SH) and long spin diffusion length (λ_SD) are highly desired in the application of spintronic devices. However, due to strong spin-orbit coupling, the two targets can hardly be achieved simultaneously in conventional relativistic spin sources, like heavy metals. Here, it is proven that collinear antiferromagnetic RuO₂ films are able to address this key issue and break the inverse relationship of θ_SH versus λ_SD. Based on spin torque-ferromagnetic resonance and spin pumping measurements, the authors demonstrate that the θ_SH of RuO₂(100) films is 0.183, and the λ_SD is over 12 nm being an order of magnitude longer than that of Pt, β-W, and β-Ta. Meanwhile, sizable λ_SD is also obtained in spin current with out-of-plane spin polarization generated by RuO₂(101) films. By conducting a control experiment, the nonrelativistic altermagnetic spin splitting effect (ASSE) is ascertained to be the crucial mechanism accounting for the simultaneous large θ_SH and long λ_SD in RuO₂. Besides the fundamental significance, these findings will advance the development of spintronics towards higher efficiency and lower power consumption.