Rocksalt rare earth monoxides as electronic and magnetic materials

Abstract Stable binary rare earth (RE) oxides are usually trivalent RE ion sesquioxides (RE₂O₃), that are highly insulating and either nonmagnetic or antiferromagnetic. However, rocksalt-type divalent RE ion monoxides (REOs) have been scarcely synthesized owing to their metastable nature. Accordingly, their fundamental properties have not been unveiled. Recently, thin film epitaxy was successfully applied to synthesize various REOs. In stark contrast with RE₂O₃, REOs are highly electrically conducting, and exhibit superconductivity, room temperature ferromagnetism, and so on. Therefore, REOs are a new and simple f-electron system, showing promise as electronic, magnetic, and spintronic materials. In this review, their fundamental properties are introduced and their significance and future prospects are discussed toward a new paradigm in f-electron systems.