A model for ferromagnetic super-exchange in dilute magnetic semiconductors was extended to ferromagnetically ordered highly insulating compounds (dilute magnetic dielectrics). Intrinsic ferromagnetism without free carriers was observed in O-deficient films of anatase TiO2 doped with transition metal impurities on the cation sub-lattice. It was proposed that ferromagnetic order arose due to super-exchange between complexes [O vacancies+magnetic impurities] which were stabilized by charge transfer from the vacancies to the impurities. The Hund rule controlled super-exchange via the empty vacancy-related levels in the energy gap, so that it became possible only for a parallel orientation of impurity magnetic moments. The percolation threshold for magnetic ordering was determined by the radius of vacancy levels, but the exchange mechanism did not require free carriers. The crucial role played by non-stoichiometry in the formation of ferromagnetism made the Curie temperature extremely sensitive to the method of sample preparation.

Vacancy Mediated Ferromagnetic Interaction in TiO2 Doped with Magnetic Ions. G.Cohen, V.Fleurov, K.Kikoin: Journal of Applied Physics, 2007, 101[9], 098106 (3pp)