Signatures of room-temperature magnetic glassiness in zinc ferrite epitaxial thin films

Zinc ferrite (${\mathrm{ZnFe}}_{\text{2}}{\mathrm{O}}_{\text{4}}$) epitaxial thin films were grown by reactive magnetron sputtering on ${\mathrm{MgAl}}_{\text{2}}{\mathrm{O}}_{\text{4}}$ and ${\mathrm{Al}}_{\text{2}}{\mathrm{O}}_{\text{3}}$ 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 ${\mathrm{ZnFe}}_{\text{2}}{\mathrm{O}}_{\text{4}}$ with the nominal stoichiometric composition and long-range magnetic order at and above room temperature. The main preparation parameter influencing the temperature ${T}_f$ 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:${\mathrm{O}}₂$ ratio did not systematically influence ${T}_f$. Furthermore ${T}_f$ was found to be systematically higher for ${\mathrm{MgAl}}_{\text{2}}{\mathrm{O}}_{\text{4}}$ as substrate and ${T}_f$ extends to above room temperature. While in some samples ${T}_f$ seems to be more likely correlated with superparamagnetism, the highest ${T}_f$ occurs in ${\mathrm{ZnFe}}_{\text{2}}{\mathrm{O}}_{\text{4}}$ 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 ${\mathrm{ZnFe}}_{\text{2}}{\mathrm{O}}_{\text{4}}$.