Crystallographic dependence of the spin Hall angle in epitaxial Pt films: Comparison of optical and electrical detection of spin-torque ferromagnetic resonance techniques

The spin Hall effect appears in nature in two forms. Its intrinsic form is highly dependent on the crystal symmetry while its extrinsic form stems from impurity scattering. Its efficiency is defined by the spin Hall angle, θ S H, and has profound impact on spintronic technologies. However, an accurate measurement of θ S H is not straightforward nor the identification of its origin. In this work, we apply a spin-torque driven ferromagnetic resonance method that is probed in two different ways, optically and electrically, to study the dependence of θ S H in the crystallographic direction in epitaxial Al2O3/Pt (111), MgO(110)/Pt (110), and MgO(001)/Pt (001) films. We show that the electrical technique is limited in its ability to accurately quantify θ S H at high current densities, and in some cases, it may even result in erroneous θ S H values. Such cases include films that exhibit a large inhomogeneous broadening. We find that θ S H is strongly affected by the crystallographic direction. Our study extends the understanding of one of the most commonly used methods for the exploration of the spin Hall effect.