Piezomagnetic switching and complex phase equilibria in uranium dioxide

Actinide materials exhibit strong spin-lattice coupling and electronic correlations and are predicted to host emergent ground states. Uranium dioxide (UO2), an antiferromagnetic Mott-Hubbard insulator, has shown piezomagnetism and a magneto-elastic memory effect, though the microscopic origin is debated. Here, single-crystal X-ray diffraction under strong pulsed magnetic fields is used to probe oriented UO2 crystals. In the antiferromagnetic state, a [888] Bragg peak tracks bulk magnetostriction and expands with field. Upon field reversal the expansion turns to contraction before the [888] peak follows a switching effect and piezomagnetic butterfly behavior, characteristic of two structures connected by time-reversal symmetry. An unexpected splitting of the [888] peak reveals simultaneous presence of time-reversed domains of the 3-k structure and a complex field-induced microstructural evolution. These results provide crystallographic evidence for piezomagnetism and strong magneto-elastic coupling in UO2.