Unveiling the mechanisms of the spin Hall effect
Prof. Felix Casanova (Ikerbasque, San Sebastian, Spain)

The discovery of new phenomena leading to spin-to-charge current conversion (spin Hall effect (SHE) in heavy metals, Edelstein effect in Rashba interfaces, spin-momentum locking in topological insulators) is expanding the potential of applications such as the magnetization switching of ferromagnetic elements for memories [1] or the recent proposal of a spin-orbit logic [2] which can have a strong technological impact. Finding routes to maximize the SHE is not possible as long as it remains unclear which is the dominant mechanism in a material. I will present a systematic study in Pt, the prototypical SHE material, using the spin absorption method in lateral spin valve devices. We find a single intrinsic spin Hall conductivity in a wide range of resistivities, in good agreement with theory. By tuning the resistivity, we observe for the first time the crossover between the moderately dirty (where the intrinsic mechanism dominates) and the superclean (where the extrinsic mechanism governs) scaling regimes of the SHE, equivalent to that obtained for the anomalous Hall effect. Our results explain the dispersion of values in the literature and find a route to maximize this important effect [3]. We also studied highly-resistive Ta, a material with a claimed giant SHE. In this case, the intrinsic mechanism in Ta dominates the SHE and allows us to linearly enhance the spin Hall angle by further increasing the resistivity of Ta, reaching up to -35±3 %, the largest value reported for a pure metal [4]. Finally, I will show how to optimize the spin-to-charge current conversion at room temperature by combining Pt with a graphene channel [5], opening up exciting opportunities towards the implementation of spin-orbit-based logic circuits.