Actinide materials have uses in a range of modern commercial and industrial applications: nuclear fuels (uranium-235, plutonium-239), smoke detectors (americium-241), radioisotope thermoelectric generators (plutonium-238), medical isotopes (actinium-225), neutron sources for well logging (americium-241), and numerous other specialized applications. Recent studies have also shown that actinide materials may have applications in quantum computing. Due to strong electronic correlations, actinide materials exhibit wide range of exotic properties but owing to their general scarcity, inherent radioactivity, and synthetic challenges, they represent a poorly understood section of the periodic table. Uranium, however, is readily available and in its depleted form (uranium-238) represents an extremely low radiation risk.
During the past several decades, we have witnessed tremendous breakthroughs in the science of complex oxides enabled by thin-film growth technology.
Strain engineering, interface engineering, and defect engineering have been widely used to tune functional properties or create exotic phenomena in thin films. By using the actinide thin film capability established at Los Alamos National Laboratory, we have explored strain engineering of epitaxial (lattice matched) actinide thin films, specifically, uranium dioxide (UO₂).
We have synthesized high quality epitaxial thin films of UO₂ by pulsed laser deposition (PLD) and explored their magnetic properties using a strain engineering approach. The PLD method uses laser pulses focused at a 45° angle on the target (UO₂ in our case) in a high-vacuum chamber and consequently produces a plasma plume—an ionic cloud of constituent atoms (Fig. 1). Depending on the substrate temperature and oxygen pressure inside the chamber, the oxidation state of the uranium metal can be controlled, and hence for a certain growth condition we can create high quality epitaxial thin films on a substrate of our choice.
Strain engineering
Strain engineering in epitaxial thin films has been applied to a variety of non-actinide thin films in the past decade. A lattice mismatch of a few percent between a film and the underlying substrate can produce several GPa of biaxial pressure. This mechanical pressure achieved through epitaxial strain can be used to manipulate the electronic and magnetic ground states in actinide thin films. To apply in-plane tensile and compressive strain, we used the same growth condition to deposit UO₂ films on different substrates by PLD. We selected substrates based on the required strain condition (Fig. 2a). Based on the cubic lattice parameter of 5.471 Å in UO₂, we selected a few available substrates including YAlO₃, LaAlO₃, (La,Sr)(Al,Ta)O₃, and SrTiO₃ to tune the epitaxial strain in UO₂.
During the growth of epitaxial heterostructures, the lattice constant difference between the film and the substrate could induce epitaxial biaxial strain. The lattice mismatch (f) can be calculated for these substrates (Fig. 2a). Taking the SrTiO₃
Figure 2. (a) Lattice mismatch between bulk UO₂ and various perovskite substrates.
(b) Crystal structure schematic of (001) UO₂ epitaxially grown on (001) SrTiO₃, depicting the 45° rotation of the UO₂ unit cell (above) with respect to the cubic SrTiO₃ lattice (below). (c) Epitaxial orientation relationship between the surface mesh of the (001) UO₂ film and underlying (001) SrTiO₃ substrate, with one UO₂ unit cell highlighted, which corresponds to a 0.92% lattice mismatch.
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