Enhancing spin-orbit torque via spin transmission through an oxide magnetic insulator membrane

Current-induced spin currents from a heavy-metal layer can transfer spin angular momentum to an adjacent ferromagnetic (FM) layer, showing the potential for efficient electrical manipulation of magnetism. Platinum (Pt) is one of the most popular candidates for generating spin-orbit torque, but the spin-orbit torque efficiency in Pt/FM systems is still 1-2 orders too small to be useful for magnetic memory applications. We have investigated the effects of placing an oxide magnetic insulator membrane, NiFe₂O₄ (NFO), between a Pt spin source layer and a metallic ferromagnet. The NFO membrane is obtained from an atomically-flat epitaxial heterostructure SrTiO₃(sub)/Sr₃Al₂O₆/NiFe₂O₄ where the water-soluble Sr₃Al₂O₆ layer is etched away, making the NFO freestanding and transferable to the Pt layer. Our preliminary results suggest that with the insertion of NFO, the Pt/NFO/FM system exhibits a spin-orbit torque efficiency much greater than Pt alone for temperatures near but less than the Curie temperature of the NFO. We speculate that the presence of a large concentration of magnons in NFO near its Curie temperature might facilitate the vertical transport of spin current.