Superexchange in Dilute Magnetic Dielectrics: Application to (Ti,Co)O_2

TL;DR

This paper extends the superexchange model to dilute magnetic dielectrics, explaining intrinsic ferromagnetism in oxygen-deficient TiO_2 doped with transition metals without free carriers, highlighting the role of vacancies and charge transfer.

Contribution

It introduces a superexchange-based explanation for ferromagnetism in insulating TiO_2 doped with transition metals, emphasizing vacancy complexes and charge transfer effects.

Findings

Ferromagnetism arises from superexchange between vacancy-impurity complexes.

Curie temperature is highly sensitive to sample preparation methods.

The exchange mechanism does not require free carriers.

Abstract

We extend the model of ferromagnetic superexchange in dilute magnetic semiconductors to the ferromagnetically ordered highly insulating compounds (dilute magnetic dielectrics). The intrinsic ferromagnetism without free carriers is observed in oxygen-deficient films of anatase TiO_2 doped with transition metal impurities in cation sublattice. We suppose that ferromagnetic order arises due to superexchange between complexes [oxygen vacancies + magnetic impurities], which are stabilized by charge transfer from vacancies to impurities. The Hund rule controls the superexchange via empty vacancy related levels so that it becomes possible only for the parallel orientation of impurity magnetic moments. The percolation threshold for magnetic ordering is determined by the radius of vacancy levels, but the exchange mechanism does not require free carriers. The crucial role of the non-stoichiometry in formation of the ferromagnetism makes the Curie temperatures extremely sensitive to the methods of sample preparation.