Speaker: J. M. D. Coey
Affiliation: School of Physics and CRANN, Trinity College, Ireland
Abstract Details: 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, the d0 materials, 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, these 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 can be shown that no only small fraction of the volume of the films or nanoparticles is actually magnetic. Based on this experimental analysis, two quite different models are proposed. One is a Stoner model of wandering axis ferromagnetism, where the magnetism resides in an impurity band associated with defects such as grain boundaries, and the associate density of states is poulated by charge transfer from a proximate charge reservoir. The other is quite different; it depends on giant orbital moments associated with pseudospin excitations in a graphene-like lattice with topological defects.

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