CaF2-based UO2/Fe3O4 thin films: Crystal structure and magnetic exchange bias effect

摘要

Exchange interaction through interface between a ferromagnet and an antiferromagnet may result in the magnetic exchange bias (EB) effect [1]. The EB manifests itself as a shift of a magnetic hysteresis loop along the field direction. This property has become of a great technological value for applications in magnetic sensors based on spin-valves or tunnel junctions [2]. In this work, we studied crystal structure and magnetic EB effect in bilayers of antiferromagnetic UO (bulk Néel temperature 30.8 K) with ferrimagnetic FeO. A reference UO/Fe sample has been also prepared and studied. We used reactive sputter deposition from metallic U and Fe targets to prepare a set of samples with different FeO thicknesses while the thickness of the UO layer was kept constant. The samples were grown on commercially available CaF (100) and (111) substrates (CaF has an identical crystal structure and a lattice parameter within 0.5% of UO). The layer of UO was deposited onto the substrate at elevated temperature (850 K), using a partial oxygen pressure of 1.2×10 mbar (Ar pressure of 6×10 mbar). FeO was deposited on the top of UO at the same partial oxygen pressure but at room temperature, in order to avoid interdiffusion. The stoichiometry of each deposited layer was controlled by X-ray Photoelectron Spectroscopy (XPS). A magnesium cap was deposited for protection on top of each sample. The samples have been characterized using X-ray diffraction.