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

Zinc ferrite (ZnFe₂O₄) epitaxial thin films were grown by reactive magnetron sputtering on MgAl₂O₄ and Al₂O₃ 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 ZnFe₂O₄ 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:O₂ ratio did not systematically influence T_f. Furthermore T_f was found to be systematically higher for MgAl₂O₄ 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 ZnFe₂O₄ 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 ZnFe₂O₄.