Incorporation of Ti in epitaxial Fe₂TiO₄thin films

The titanomagnetites (Fe_2-xTi_xO₄,x⩽ 1) are a family of reducible spinel-structure oxides of interest for their favorable magnetic, catalytic, and electrical transport properties. To understand the stability of the system during low temperature deposition, epitaxial thin films of Fe₂TiO₄were deposited by molecular beam epitaxy (MBE) on MgO(001) at 250-375 °C. The homogeneous incorporation of Ti, Fe valence state, and film morphology were all found to be strongly dependent on the oxidation conditions at the low substrate temperatures employed. More oxidizing conditions led to phase separation into epitaxial, faceted Fe₃O₄and rutile TiO₂. Less oxidizing conditions resulted in polycrystalline films that exhibited Ti segregation to the film surface, as well as mixed Fe valence (Fe³⁺, Fe²⁺, Fe⁰). A narrow window of intermediate oxygen partial pressure during deposition yielded nearly homogeneous Ti incorporation and a large fraction of Fe²⁺. However, these films were poorly crystallized, and no occupation of tetrahedral sites in the spinel lattice by Fe²⁺was detected by x-ray magnetic circular dichroism at the Fe L-edge. After vacuum annealing, a small fraction of Fe²⁺was found to occupy tetrahedral sites. Comparison of these results with previous work suggests that the low temperature deposition conditions imposed by use of MgO substrates limits the incorporation of Ti into the spinel lattice. This work suggests a path towards obtaining stoichiometric, well-crystallized Fe₂TiO₄by MBE by utilizing high substrate temperature and low oxygen partial pressure during deposition on thermally stable substrates.