Magnetic properties and microstructure of Fe-O and Co-O thin films

Iron oxide and cobalt oxide thin films were prepared by reactive DC magnetron sputtering in a mixture of Ar and O₂ gases. The stoichiometry of the oxide films was studied as a function of the amount of O₂ during deposition. Sputtering conditions, including dc power and oxygen-to-argon ratio, were found which allowed pure FeO, Fe ₃O₄, α-Fe₂O₃, CoO, and Co₃O₄ films to be fabricated. The microstructure of the films, and especially the grain shape and size distribution, were quite different for oxide films with different stoichiometry. The crystallographic axes of Fe-O films were randomly oriented in contrast to Co-O films in which a strong <111> texture was found. A strong ferromagnetic-like behavior was observed in FeO and α-Fe₂ O₃ films, in contrast to the well-established antiferromagnetic behavior in bulk. The reason for this anomalous behavior is attributed to clusters of defects in FeO films and uncompensated surface spins in α-Fe₂O₃ films. A large shift (3800 Oe) was observed in the low temperature hysteresis loop of a field cooled sample consisting of 50% Co and 50% CoO. A similar but less pronounced effect was observed in FeO and α-Fe₂O₃ films. This result is believed to be caused by a strong exchange coupling between the ferromagnetic and antiferromagnetic constituents in these composite films