Piezomagnetism in Uranium Dioxide: Actinide Science at High Magnetic Fields

The spin-lattice coupling in uranium dioxide remains an unsolved puzzle resulting from the lack of a thorough understanding of the strong coupling between 5<i>f</i>-electron magnetism and lattice vibrations. Besides being the main nuclear fuel material, UO₂ is a Mott-Hubbard insulator with well-localized 5<i>f</i>-electrons (U⁴⁺ electronic configuration) and its magnetic state is characterized by a non-collinear antiferromagnetic structure of 3<b><i>k</i></b> type and multidomain Jahn-Teller distortions. In the magnetic state, a UO₂ single crystal subjected to strong magnetic fields exhibits the abrupt appearance of positive linear magnetostriction leading to a trigonal distortion and piezomagnetism. This is the first example of piezomagnetism in the <i>f</i>-electron spin system. The unusually strong correlations between the magnetic moments in U-atoms and lattice distortions are a direct consequence of the non-collinear symmetry of the magnetic state that breaks time-reversal symmetry in a non-trivial way. The microscopic nature of these interactions, however, remains unclear. During the talk, we will demonstrate how detailed thermodynamic and structural measurements, performed in high and ultra-high magnetic fields, can be used to study these interactions and their couplings. We will discuss the implications of these results in the context of the origin of the piezomagnetic ground state in this material.