Article abstract
Nature Materials 4, 173 - 179 (2005)
Published online: 16 January 2005 | doi:10.1038/nmat1310
Subject Categories: Molecular electronics | Semiconductors | Magnetic materials | Computation, modelling and theory
Donor impurity band exchange in dilute ferromagnetic oxides
J. M. D. Coey1, M. Venkatesan1 & C. B. Fitzgerald1
Abstract
Dilute ferromagnetic oxides having Curie temperatures far in excess of 300 K and exceptionally large ordered moments per transition-metal cation challenge our understanding of magnetism in solids. These materials are high-k dielectrics with degenerate or thermally activated n-type semiconductivity. Conventional super-exchange or double-exchange interactions cannot produce long-range magnetic order at concentrations of magnetic cations of a few percent. We propose that ferromagnetic exchange here, and in dilute ferromagnetic nitrides, is mediated by shallow donor electrons that form bound magnetic polarons, which overlap to create a spin-split impurity band. The Curie temperature in the mean-field approximation varies as (x
)1/2 where x and are the concentrations of magnetic cations and donors, respectively. High Curie temperatures arise only when empty minority-spin or majority-spin d states lie at the Fermi level in the impurity band. The magnetic phase diagram includes regions of semiconducting and metallic ferromagnetism, cluster paramagnetism, spin glass and canted antiferromagnetism.
- Physics Department, Trinity College, Dublin 2, Ireland
Correspondence to: J. M. D. Coey1 e-mail: jcoey@tcd.ie
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