Spin pumping and inverse spin Hall effect in Platinum and other 5d metals: The essential role of spin-memory loss and spin-current discontinuities at interfaces

It was recently realized that the spin Hall effect (SHE) can be very useful in the area of spintronics, due to its ability to generate pure spin current from charge current, without the use of any magnetic materials or magnetic field. The maximum conversion factor is given by the spin Hall angle θ_SH, which can take rather important values (above 10% in absolute value was reported for β-Ta and β-W). This phenomenon is usually observed in materials with large spin-orbit coupling, either intrinsic (Pt, Ta, W, etc.) or induced by heavy impurities (Cu doped with Bi or Ir). To investigate this property, several groups studied the reciprocal effect, the so-called inverse spin Hall effect (ISHE), converting a pure "pumped" spin current into a charge current (measured by voltage detection in an "open circuit"). We focus here on the 5d Pt material. Values published nowadays for θ_SH in Pt are scattered over one order of magnitude, with a clear correlation between the spin diffusion length ℓ_sf and the θ_SH, both quantities being related to the spin-orbit strength or its inverse. We performed measurements of spin pumping in a cavity and measured the resulting ISHE voltage. We propose a model including spin-current discontinuity or spin memory loss at the interfaces that may reconcile all the different observations. In particular, we demonstrate consistent values of spin diffusion length (ℓ_sf = 3.4 ± 0.4 nm) and spin Hall angle (θ_SH = 0.056 ± 0.010) for Pt in different metallic multilayer systems.