Room temperature ferromagnetic ordering of NiO films through exchange coupling

NiO films were deposited by sputtering with a relatively high oxygen partial pressure, followed by subsequent annealing in air. The as-deposited film is amorphous. The amorphous film gradually turns to crystalline structure with the increase in annealing temperature. The film annealed at 673 K shows strong ferromagnetism at room temperature, whereas, there is no room-temperature ferromagnetic ordering for the as-deposited amorphous NiO film or the film after annealed at 873 K. The ferromagnetism is due to the finite size effect and the high Curie temperature above room temperature is associated with the interaction between ferromagnetic NiO crystalline nanoclusters and antiferromagnetic amorphous matrix.