CaF<inf>2</inf>-based UO<inf>2</inf>/Fe<inf>3</inf>O<inf>4</inf> thin films: Crystal structure and magnetic exchange bias effect

Abstract

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 2 (bulk Neel temperature 30.8 K) with ferrimagnetic Fe 3 O 4 . A reference UO 2 /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 Fe 3 O 4 thicknesses while the thickness of the UO 2 layer was kept constant. The samples were grown on commercially available CaF 2 (100) and (111) substrates (CaF 2 has an identical crystal structure and a lattice parameter within 0.5% of UO 2 ). The layer of UO 2 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). Fe 3 O 4 was deposited on the top of UO 2 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.