Epitaxial Zn_xFe_3−xO₄ thin films : A spintronic material with tunable electrical and magnetic properties

Abstract

The ferrimagnetic spinel oxide ZnxFe3−xO4 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 ZnxFe3−xO4 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 ZnxFe3−xO4 with antiparallel Fe moments on the A and B sublattices: (i) Zn substitution removes both Fe3+A 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 Fe2+B/Fe3+B 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 ZnxFe3−xO4 films with semiconductor materials such as ZnO in multifunctional heterostructures seems to be particularly appealing.