Room-temperature spin glass behavior in zinc ferrite epitaxial thin films

Zinc ferrite (ZnFe$_{\text{2}}$O$_{\text{4}}$) epitaxial thin films were grown by reactive magnetron sputtering on MgAl$_{\text{2}}$O$_{\text{4}}$ and Al$_{\text{2}}$O$_{\text{3}}$ substrates varying a range of preparation parameters. The resulting structural and magnetic properties were investigated using a range of experimental techniques confirming epitaxial growth of ZnFe$_{\text{2}}$O$_{\text{4}}$ with the nominal stoichiometric composition and long range magnetic order at and above room temperature. The main preparation parameter influencing the temperature $T_{\text{f}}$ of the bifurcation between $M(T)$ curves under field cooled and zero-field cooled conditions was found to be the growth rate of the films, while growth temperature or the Ar:O$_2$ ratio did not systematically influence $T_{\text{f}}$. Furthermore $T_{\text{f}}$ was found to be systematically higher for MgAl$_{\text{2}}$O$_{\text{4}}$ as substrate and $T_{\text{f}}$ extends to above room temperature. While in some samples $T_{\text{f}}$ seems to be more likely correlated with superparamagentism, the highest $T_{\text{f}}$ occurs in ZnFe$_{\text{2}}$O$_{\text{4}}$ epitaxial films where experimental signatures of magnetic glassiness can be found. Element-selective X-ray magnetic circular dichroism measurements aim at associating the magnetic glassiness with the occurrence of a different valence state and lattice site incorporation of Fe pointing to a complex interplay of various competing magnetic interactions in ZnFe$_{\text{2}}$O$_{\text{4}}$.