Disorder-driven ferromagnetic insulator phase in manganite heterostructures

Ferromagnetic insulator plays a pivotal role in various emerging spintronic effects and can be integrated as an essential component into future spintronic devices. However, ferromagnetic insulating perovskite oxides are rather scarce due to the strong coupling between ferromagnetism and metallicity via double exchange and superexchange mechanisms, severely restricting the development for oxide-based spintronic devices. Here, the disorder of manganite La2/3Sr1/3MnO3 (LSMO) films grown on MgAl2O4 substrates is utilized to decouple ferromagnetism and metallicity, thereby inducing a ferromagnetic insulating state over a rather broad temperature and thickness range. These films are non-crystalline with enormous disorder. The films with thicknesses less than 16 nm are in completely insulating state, and the films with thicknesses larger than 16 nm are in essentially insulating state with high resistivity. This insulating state is ascribed to the disorder in LSMO films and can be fitted by a localization model. In contrast, all LSMO films exhibit ferromagnetic transitions with considerable saturated magnetic moments and the Curie temperatures near room temperature, leading to the ferromagnetic insulator phase in our manganite heterostructures over a large temperature and thickness interval. Our results broaden the mind for inducing new ferromagnetic insulating state for potential applications in spintronic devices.