Giant, Temperature-Dependent Spin Hall Torque from Rare-Earth Mixed-Valence YbAl3*

The spin Hall effect occurs in non-magnetic systems and gives rise to a spin current that flows transverse to an applied electric field. Theoretical predictions and recent experimental work reveal that the strength of the spin Hall effect can be enhanced by the presence of 4f-derived states at the Fermi level. YbAl3 is a rare-earth mixed-valence metal in which the Yb 4f-derived states become increasingly itinerant and shift towards the Fermi level as temperature is lowered below room temperature. Using temperature-dependent spin torque ferromagnetic resonance measurements of the spin Hall effect in YbAl3/Fe bilayers, we find an effective spin Hall conductivity that increases in concert with the increase in the 4f density of states near the Fermi level. At the peak, the effective spin Hall conductivity is enormous, at least an order of magnitude larger than in any other material system reported previously. We discuss the implications of this result for optimizing the spin Hall effect in light of the evolution of the electronic structure of YbAl3 with temperature.