Giant Internal Spin-Hall Ratio of Order Unity in Highly Crystalline Pt

Identifying the origins of spin-orbit torques (SOTs) in heterostructures is an outstanding challenge in spintronics. Here, we examine SOTs in a model system of all-epitaxial ferrite/Pt bilayers. With no electronic transport in the insulating ferrite, the highly crystalline Pt film is the sole source of charge-to-spin conversion. A small field-like SOT independent of Pt thickness suggests a modest Rashba-Edelstein effect at the ferrite/Pt interface. By contrast, a sizable Pt-thickness-dependent damping-like SOT is quantitatively accounted for by a bulk spin-Hall effect (skew scattering) and Dyakonov-Perel spin relaxation, along with a remarkably large internal spin-Hall ratio $\theta_\mathrm{int}$ of order unity.