EpitaxialZnxFe3−xO4thin films: A spintronic material with tunable electrical and magnetic properties

Abstract

The ferrimagnetic spinel oxide ${\text{Zn}}_x{\text{Fe}}_{3-x}{\text{O}}_4$ 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 ${\text{Zn}}_x{\text{Fe}}_{3-x}{\text{O}}_4$ thin films $\left(0\le x\le 0.9\right)$ on MgO(001) substrates with excellent structural properties both in pure Ar atmosphere and an $\text{Ar}/{\text{O}}_2$ 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 (${10}^2\dots {10}^4{\Omega }^{-1}{\text{m}}^{-1}$ and $1.0\dots 3.2{\mu }_B/\text{f}\text{.u}\text{.}$ 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 ${\text{Zn}}_x{\text{Fe}}_{3-x}{\text{O}}_4$ with antiparallel Fe moments on the $A$ and $B$ sublattices: (i) Zn substitution removes both ${\text{Fe}}_A^{3+}$ 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 ${\text{Fe}}_B^{2+}/{\text{Fe}}_B^{3+}$ 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 ${\text{Zn}}_x{\text{Fe}}_{3-x}{\text{O}}_4$ films with semiconductor materials such as ZnO in multifunctional heterostructures seems to be particularly appealing.