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 UO
₂
(bulk Néel 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.