Epitaxial ${\text{Zn}}_x{\text{Fe}}_{3-x}{\text{O}}_4$ thin films: A spintronic material with tunable electrical and magnetic properties

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.

Figure 1

(Color online) (a) Inverse spinel structure of magnetite with ${\text{Fe}}_A^{3+}$ on the tetrahedrally coordinated $A$ site and mixed-valent ${\text{Fe}}_B^{2+}\left(3d^6,S=2\right)/{\text{Fe}}_B^{3+}\left(3d^5,S=5/2\right)$ ions on the octahedrally coordinated $B$ site. Zn substitution replaces ${\text{Fe}}_A^{3+}$ on the $A$ site by nonmagnetic ${\text{Zn}}^{2+}\left(3d^{10},S=0\right)$ ions. Also shown is the occupation of the ${\text{Fe}}_B^{2+}$ and ${\text{Fe}}_B^{3+}3d$ states with $t_{2g}$ and $e_g$ symmetry separated by the crystal-field splitting due to the octahedral oxygen surrounding. The $t_{2g}$ spin-down electron can hop between ${\text{Fe}}_B^{2+}$ and ${\text{Fe}}_B^{3+}$ ions with its spin coupled antiparallel to the local moment formed by the spin-up electrons owing to Hund’s rule coupling. (b)–(e) show the $A$ and $B$ sublattice configuration for ${\text{Zn}}_x{\text{Fe}}_{3-x}{\text{O}}_4$ for (b) $x=0$, (c) $x=1/8$, (d) $x=0.5$, and (e) $x=1$.

Figure 2

(Color online) X-ray diffraction diagrams ($\omega \text{−}2\theta$ scans) of the out-of-plane reflections from ${\text{Zn}}_x{\text{Fe}}_{3-x}{\text{O}}_4$ thin films deposited in different growth atmospheres of $\text{Ar}/{\text{O}}_2$ (99:1) (left panels) and pure Ar (right panels). With increasing $x$ the ${\text{Zn}}_x{\text{Fe}}_{3-x}{\text{O}}_4$ (004) reflection shifts across the (002) reflection of the MgO substrates marked by the vertical line. The insets show the corresponding rocking curves of the ${\text{Zn}}_x{\text{Fe}}_{3-x}{\text{O}}_4$ (004) reflection, except for (f) where the ${\text{Zn}}_x{\text{Fe}}_{3-x}{\text{O}}_4$ (008) reflection is displayed.

Figure 3

(Color online) (a) Change in the $c$-axis lattice parameter of ${\text{Zn}}_x{\text{Fe}}_{3-x}{\text{O}}_4$ films grown on MgO(001) in pure Ar atmosphere (squares) and an $\text{Ar}/{\text{O}}_2$ mixture (circles) with Zn content $x$. The bulk $c$-axis lattice constant of ${\text{Fe}}_3{\text{O}}_4$ is marked by the dashed horizontal line. (b) Reciprocal space map around the (204) reflection of the MgO(001) substrate. The (408) reflection from the ${\text{Zn}}_{0.1}{\text{Fe}}_{2.9}{\text{O}}_4$ film appears at the same in-plane scattering vector $q_{\parallel }$ marked by the dashed vertical line.

Figure 4

(Color online) (a) Longitudinal resistivity ${\rho }_{xx}$ as a function of temperature $T$ for epitaxial ${\text{Zn}}_x{\text{Fe}}_{3-x}{\text{O}}_4$ thin films, grown in pure Ar atmosphere (solid lines) or an $\text{Ar}/{\text{O}}_2$ (99:1) mixture (dashed lines) on MgO(001) substrates. In (b) and (c) ${\rho }_{xx}$ and ${\rho }_{xx}/T$ of the same samples are plotted versus a reciprocal temperature scale. Fitting the linear parts of the curves in (b) and (c) by ${\rho }_{xx}\propto \text{exp}\left(E_a/k_{B}T\right)$ and ${\rho }_{xx}/T\propto \text{exp}\left(E_{\text{pol}}/k_{B}T\right)$, respectively, gives the listed activation energies $E_a$ and $E_{\text{pol}}$.

Figure 5

(Color online) (a) ${\text{MR}}_{xx}$ plotted versus the magnetic field $H$ applied perpendicular to the film plane at different temperatures for ${\text{Zn}}_{0.1}{\text{Fe}}_{2.9}{\text{O}}_4$ thin films grown in pure Ar atmosphere. (b) ${\text{MR}}_{xx}\left(H\right)$ curves measured at 300 K for different ${\text{Zn}}_x{\text{Fe}}_{3-x}{\text{O}}_4$ grown in pure Ar atmosphere (left) or an $\text{Ar}/{\text{O}}_2$ mixture (right). The dash-dotted gray lines are fits to the data according to Eq. (6).

Figure 6

(Color online) Room-temperature magnetization $M$ versus magnetic field $H$ applied in the film plane for ${\text{Zn}}_x{\text{Fe}}_{3-x}{\text{O}}_4$ thin films grown in (a) pure Ar atmosphere and in (b) $\text{Ar}/{\text{O}}_2$ (99:1) mixture. The insets show the saturation magnetization $M_S$ (full symbols) and the remanence $M_R$ (open symbols) in units of ${\mu }_B/\text{f}\text{.u}\text{.}$ as a function of the Zn substitution level $x$. The lines are guides to the eyes.

Figure 7

(Color online) Remanent magnetization $M_R$ plotted versus temperature $T$ for a ${\text{Zn}}_{0.5}{\text{Fe}}_{2.5}{\text{O}}_4$ film grown in an $\text{Ar}/{\text{O}}_2$ (99:1) mixture. The data were taken after field cooling at 7 T upon increasing the temperature at zero applied magnetic field. The inset schematically shows the hump in the $M\left(T\right)$ curve as the result of two opposite sublattice magnetizations with different $T$ dependence.

Figure 8

(Color online) Saturation magnetization $M_S$ plotted versus the longitudinal conductivity ${\sigma }_{xx}$ for epitaxial ${\text{Zn}}_x{\text{Fe}}_{3-x}{\text{O}}_4$ thin films with different Zn substitution $x\le 0.5$ grown in pure Ar atmosphere (squares) or $\text{Ar}/{\text{O}}_2$ (99:1) mixture (circles).