Epitaxial Znx Fe3-x O4 thin films: A spintronic material with tunable electrical and magnetic properties

The ferrimagnetic spinel oxide Znx Fe3-x O4 combines high Curie temperature and spin polarization with tunable electrical and magnetic properties, making it a promising functional material for spintronic devices. We have grown epitaxial Znx Fe3-x O4 thin films (0≤x≤0.9) on MgO(001) substrates with excellent structural properties both in pure Ar atmosphere and an Ar/ O2 mixture by laser molecular beam epitaxy and systematically studied their structural, magnetotransport, and magnetic properties. We find that the electrical conductivity and the saturation magnetization can be tuned over a wide range (102... 104 Ω-1 m-1 and 1.0...3.2 μB /f.u. at room temperature) by Zn substitution and/or finite oxygen partial pressure during growth. Our extensive characterization of the films provides a clear picture of the underlying physics of the spinel ferrimagnet Znx Fe3-x O4 with antiparallel Fe moments on the A and B sublattices: (i) Zn substitution removes both Fe A 3+ moments from the A sublattice and itinerant charge carriers from the B sublattice; (ii) growth in finite oxygen partial pressure generates Fe vacancies on the B sublattice also removing itinerant charge carriers; and (iii) application of both Zn substitution and excess oxygen results in a compensation effect as Zn substitution partially removes the Fe vacancies. Both electrical conduction and magnetism are determined by the density and hopping amplitude of the itinerant charge carriers on the B sublattice, providing electrical conduction and ferromagnetic double exchange between the mixed-valent Fe B 2+ / Fe B 3+ ions on the B sublattice. A decrease (increase) in charge carrier density results in a weakening (strengthening) of double exchange and thereby a decrease (increase) in the conductivity and the saturation magnetization. This scenario is confirmed by the observation that the saturation magnetization scales with the longitudinal conductivity. The combination of tailored Znx Fe3-x O4 films with semiconductor materials such as ZnO in multifunctional heterostructures seems to be particularly appealing. © 2009 The American Physical Society.