Magnetic crossover of NiO nanocrystals at room temperature

The nature of room-temperature ferromagnetism in diluted magnetic semiconductors, ZnO:M (M is Ni, Co, Fe, etc.), remains in debate, most likely because previous theoretical and experimental studies have excluded the magnetic contribution from secondary phases of transition metal oxide nanocrystals including NiO and CoO. In this work, the authors initiated a study on NiO nanocrystals that demonstrated room-temperature ferromagnetic behavior with relatively large coercive forces, in apparent contradiction to the previous conjectures in the literature. With size reduction, NiO nanocrystals showed an abnormal magnetic crossover, which is closely related to weakened superexchange interactions in their multisublattices. (c) 2006 American Institute of Physics.