Current direction anisotropy of the spin Hall magnetoresistance in nickel ferrite thin films with bulk-like magnetic properties

Abstract: We utilize spin Hall magnetoresistance (SMR) measurements to experimentally investigate the pure spin current transport and magnetic properties of nickel ferrite (NiFe2O4,NFO)/normal metal (NM) thin film heterostructures. We use (001)-oriented NFO thin films grown on lattice-matched magnesium gallate substrates by pulsed laser deposition, which significantly improves the magnetic and structural properties of the ferrimagnetic insulator. The NM in our experiments is either Pt or Ta. A comparison of the obtained SMR magnitude for charge currents applied in the [100]- and [110]-direction of NFO yields a change of 50% for Pt at room temperature. We also investigated the temperature dependence of this current direction anisotropy and find that it is qualitatively different for the conductivity and the SMR magnitude. From our results we conclude that the observed current direction anisotropy may originate from an anisotropy of the spin mixing conductance or of the spin Hall effect in these Pt and Ta layers, and/or additional spin-galvanic contributions from the NFO/NM interface.