High-field magnetostriction and magneto-elastic properties of UO_2+x single crystals

The thermal and magnetic properties of uranium dioxide, a prime nuclear fuel and thoroughly studied actinide material, remain a long-standing puzzle, a result of strong coupling between magnetism and lattice vibrations. The magnetic state of this cubic material is characterized by a 3k non-collinear antiferromagnetic order, that breaks time-reversal symmetry in a non-trivial way, and dynamic Jahn-Teller interactions. It has been shown recently that single crystals of uranium dioxide subjected to strong magnetic fields along threefold axes in the magnetic state exhibit the abrupt appearance of positive linear magnetostriction, leading to a trigonal distortion. Upon reversal of the field the linear term also reverses sign, a hallmark of piezomagnetism [M. Jaime et al., Nature Communications 8, 99 (2017)]. Here we show our recent studies on high-field magnetostriction of oriented (along <111>) UO_2+x (x = 0, 0.033, and 0.11) single crystals. The influence of excess oxygen on magneto-elastic properties and its relationship to piezomagnetism in UO₂ is examined. We will discuss implications of these new results.