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 Tf 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 Tf. Furthermore Tf was found to be systematically higher for MgAl2O4 as substrate and Tf extends to above room temperature. While in some samples Tf seems to be more likely correlated with superparamagnetism, the highest Tf occurs in ZnFe2O4 epitaxial films where experimental signatures of magnetic glassiness can be found. Element-selective x-ray magnetic circular dichroism measure-ments 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.