Green function theory of electron correlations above and below the Verwey transition in magnetite doped with impurities

Abstract

The electron correlation effects in magnetite doped with impurities (Me_xFe_3-xO₄ with Me = Zn, Ni, Co, Ti, Sn, Li, Cr, Mg, Al, B-site vacancy) are studied on the basis of a Hamiltonian which describes the electron-electron and the electron-impurity interactions and is an extension of the Cullen-Callen model of magnetite. Treating this model in the case of low impurity concentrations x by means of a cluster approximation for Green functions the temperature dependence of the charge density oscillations around an isolated impurity and the Verwey transition temperature T_v(x) are calculated. The theoretical results for T_v(x) agree well with experiments except for titanium-substituted magnetite. For this ferrite the theory predicts an increase of T_v(x) with increasing x which represents a combined electron correlation and lattice structure effect.