Ferromagnetism in defect-ridden oxides and related materials

Abstract

The existence of high-temperature ferromagnetism in thin films and\nnanoparticles of oxides containing small quantities of magnetic dopants remains\ncontroversial. Some regard these materials as dilute magnetic semiconductors,\nwhile others think they are ferromagnetic only because the magnetic dopants\nform secondary ferromagnetic impurity phases such as cobalt metal or magnetite.\nThere are also reports in d0 systems and other defective oxides that contain no\nmagnetic ions. Here, we investigate TiO2 (rutile) containing 1 - 5% of iron\ncations and find that the room-temperature ferromagnetism of films prepared by\npulsed-laser deposition is not due to magnetic ordering of the iron. The films\nare neither dilute magnetic semiconductors nor hosts to an iron-based\nferromagnetic impurity phase. A new model is developed for defect-related\nferromagnetism which involves a spin-split defect band populated by charge\ntransfer from a proximate charge reservoir in the present case a mixture Fe2+\nand Fe3+ ions in the oxide lattice. The phase diagram for the model shows how\ninhomogeneous Stoner ferromagnetism depends on the total number of electrons\nNtot, the Stoner exchange integral I and the defect bandwidth W; the band\noccupancy is governed by the d-d Coulomb interaction U. There are regions of\nferromagnetic metal, half-metal and insulator as well as nonmagnetic metal and\ninsulator. A characteristic feature of the high-temperature Stoner magnetism is\nan an anhysteretic magnetization curve which is practically temperature\nindependent below room temperature. This is related to a wandering\nferromagnetic axis which is determined by local dipole fields. The\nmagnetization is limited by the defect concentration, not by the 3d doping.\nOnly 1-2 % of the volume of the films is magnetically ordered.\n