Dilute magnetic oxides

Dilute magnetic oxides are transparent, wide-bandgap, materials that behave ferromagnetically when doped with a few percent of a magnetic 3d cation. The magnetism, which appears well below the cation percolation threshold, cannot be understood in terms of the conventional theory of magnetism in insulators; nor can a carrier-mediated ferromagnetic exchange mechanism account for the magnitude of the Curie temperatures, which are well in excess of 400 K. The phenomenon is observed in thin films and nanocrystals, but not in well-crystallized bulk material. Experimental artefacts and segregation of secondary ferromagnetic phases can explain some observations, but the existence of a novel type of magnetism related to defects other than the magnetic dopants is a likely inference from the data. (c) 2007 Published by Elsevier Ltd.