Thin films (250–4500 Å) of epitaxial UO${}_2$ were produced by reactive sputtering on two different substrate materials: LaAlO${}_3$ and CaF${}_2$. Using the large enhancement present with resonant x-ray scattering using photons at the uranium $M$${}_4$ absorption edge, antiferromagnetic (AF) order was found in all films. The ordering temperature $T$${}_N$ is the same as the bulk, but the films show second-order (continuous) transitions in contrast to the first-order bulk transition. For LaAlO${}_3$-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 $M$${}_4$ resonances can be related to the thickness of the AF region. The LaAlO${}_3$-based films do not order magnetically over more than $\sim$600 Å, whereas the CaF${}_2$-based film orders throughout. Further, for thicker films ($>$1000 Å) the fitting procedure shows that the AF order is located at the top of the LaAlO${}_3$-based film. This points to the formation in thicker films of a nonmagnetic layer of UO${}_2$ adjacent to the substrate, which may have tetragonal symmetry.

• Received 9 June 2013

DOI:https://doi.org/10.1103/PhysRevB.88.134426