Epitaxial strain effect on site occupancy and conductivity of titanomagnetite

Magnetite, Fe₃O₄, exhibits metallic conductivity via electron hopping between Fe²⁺ and Fe³⁺ occupying octahedral sites in the spinel lattice. As Ti⁴⁺ is doped into the octahedral sites of magnetite (the titanomagnetite series), an equal fraction of Fe³⁺ is reduced to Fe²⁺ to maintain charge neutrality. The site occupancies of Fe²⁺ and Fe³⁺ determine the transport properties of the titanomagnetite series; the end-member ulvöspinel, Fe₂TiO₄, exhibits p-type semiconducting transport properties. The Fe²⁺/Fe³⁺ site occupancy remains controversial, but is likely in part a function of the lattice strain induced by doping smaller Ti⁴⁺ into the lattice. Here, we have deposited titanomagnetites and ulvöspinel as well-defined epitaxial thin films on both MgO and MgAl₂O₄ substrates. We have characterized the Fe valence state and site occupancy with XPS, XANES, and EXAFS, and related them to the epitaxial strain induced by the substrate. The impact of these factors on the electrical transport properties of the films will be discussed.