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

The electron correlation effects in magnetite doped with impurities (MexFe3-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 Tv(x) are calculated. The theoretical results for Tv(x) agree well with experiments except for titanium-substituted magnetite. For this ferrite the theory predicts an increase of Tv(x) with increasing x which represents a combined electron correlation and lattice structure effect.