Antiferromagnetism in UO2 thin epitaxial films

Thin films (250-4500 Å) of epitaxial UO2 were produced by reactive sputtering on two different substrate materials: LaAlO3 and CaF2. Using the large enhancement present with resonant x-ray scattering using photons at the uranium M4 absorption edge, antiferromagnetic (AF) order was found in all films. The ordering temperature TN is the same as the bulk, but the films show second-order (continuous) transitions in contrast to the first-order bulk transition. For LaAlO3-based films, an additional strong diffuse magnetic disorder is observed, which is reminiscent of the second-length scale, associated with structural disorder and/or strain. By using a formulation accounting for the strong absorption and coherent nature of the photons, the energy widths at the U M4 resonances can be related to the thickness of the AF region. The LaAlO3-based films do not order magnetically over more than ∼600 Å, whereas the CaF2-based film orders throughout. Further, for thicker films (>1000 Å) the fitting procedure shows that the AF order is located at the top of the LaAlO3-based film. This points to the formation in thicker films of a nonmagnetic layer of UO2 adjacent to the substrate, which may have tetragonal symmetry. © 2013 American Physical Society