Room-temperature insulating ferromagnetic (Ni,Co)1+2xTi1-xO3 thin films

Abstract

Insulating uniaxial room-temperature ferromagnets are a prerequisite for commonplace spin wave-based devices, the obstacle in contemporary ferromagnets being the coupling of ferromagnetism with large conductivity. We show that the uniaxial $A^{1+2x}$Ti${}^{4+}$${}_{1-x}$O${}_3$ (ATO), $A=$Ni${}^{2+}$,Co${}^{2+}$ and $0.6<x\le 1$, thin films are electrically insulating ferromagnets already at room-temperature. The octahedra network of the ATO and ilmenite structures are similar yet different octahedra-filling proved to be a route to switch from the antiferromagnetic to ferromagnetic regime. Octahedra can continuously be filled up to $x=1$, or vacated $(-0.24<x<0)$ in the ATO structure. TiO-layers, which separate the ferromagnetic (Ni,Co)O-layers and intermediate the antiferromagnetic coupling between the ferromagnetic layers in the NiTiO${}_3$ and CoTiO${}_3$ ilmenites, can continuously be replaced by (Ni,Co)O-layers to convert the ATO-films to ferromagnetic insulator with abundant direct cation interactions.