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

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 UO₂ (bulk Néel temperature 30.8 K) with ferrimagnetic Fe₃O₄. A reference UO₂/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 Fe₃O₄ thicknesses while the thickness of the UO₂ layer was kept constant. The samples were grown on commercially available CaF₂ (100) and (111) substrates (CaF₂ has an identical crystal structure and a lattice parameter within 0.5% of UO₂). The layer of UO₂ was deposited onto the substrate at elevated temperature (850 K), using a partial oxygen pressure of 1.2×10^-6 mbar (Ar pressure of 6×10^-3 mbar). Fe₃O₄ was deposited on the top of UO₂ 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.