CPM Seminar

Dilute Ferromagnetic Oxides - Fact or Fantasy?

Michael Coey

Trinity College Dublin

There are many reports in the literature of ferromagnetism in thin films and nanoparticles of nonmagnetic oxides doped with a few percent of transition metal cations. In some cases, samples are ferromagnetic, even when undoped. The reports are controversial, not least because they fly in the face of received wisdom regarding the magnetism of oxides. Based on consideration of superexchange, the materials would normally be expected to be paramagnetic when doped below the percolation threshold. The ferromagnetism is unusual, in that it a high-temperature phenomenon which is largely anhysteretic. The magnetization process is controlled by dipolar interactions, and it will be shown that only a few percent of the volume of the films actually orders magnetically. Based on this, a new Stoner model of wandering axis ferromagnetism is developed, where the magnetism resides in defects such as grain boundaries, and the associate density of states is poulated by charge transfer from a proximate charge reservoir. Prospects for making use of the phenomenon will be discussed.