Piezomagnetic switching and complex phase equilibria in uranium dioxide

Abstract

Actinide materials exhibit strong spin–lattice coupling and electronic correlations, and are predicted to host new emerging ground states. One example is piezomagnetism and magneto-elastic memory effect in the antiferromagnetic Mott-Hubbard insulator uranium dioxide, though its microscopic nature is under debate. Here, we report X-ray diffraction studies of oriented uranium dioxide crystals under strong pulsed magnetic fields. In the antiferromagnetic state a [888] Bragg diffraction peak follows the bulk magnetostriction that expands under magnetic fields. Upon reversal of the field the expansion turns to contraction, before the [888] peak follows the switching effect and piezomagnetic ‘butterfly’ behaviour, characteristic of two structures connected by time reversal symmetry. An unexpected splitting of the [888] peak is observed, indicating the simultaneous presence of time-reversed domains of the 3-k structure and a complex magnetic-field-induced evolution of the microstructure. These findings open the door for a microscopic understanding of the piezomagnetism and magnetic coupling across strong magneto-elastic interactions.

Thermal Energy Transport in Oxide Nuclear Fuel

“…Elastic constants have been studied experimentally using a pulse-echo technique on single crystals of ThO 2 and UO 2 at room temperature. , The thermal expansion of UO 2 single crystals was measured by Brandt and Walker in 1967. , That was the first direct demonstration that the phase transition is first-order in the lattice properties. The volume collapse at T N was first observed in 1974 and, more recently, by X-ray diffraction . Furthermore, the measurements of elastic constants versus temperature have not only exhibited changes at the magnetic transition but also confirmed that the elastic properties at high temperature (up to room temperature) are much more complicated than anticipated.…”

“…That was the first direct demonstration that the phase transition is first-order in the lattice properties. The volume collapse at TN was first observed in 1974 [190] and, more recently, by X-ray diffraction [191]. Furthermore, the measurements of elastic constants versus temperature have not only exhibited changes at the magnetic transition but also confirmed that the elastic properties at high temperature (up to room temperature) are much more complicated than anticipated.…”

“…The volume collapse at T N was first observed in 1974 190 and, more recently, by X-ray diffraction. 191 Furthermore, the measurements of elastic constants versus temperature have not only exhibited changes at the magnetic transition but also confirmed that the elastic properties at high temperature (up to room temperature) are much more complicated than anticipated. Similar indications have come from susceptibility measurements 192 and polarized neutron studies 193 that showed short-range order up to at least 200 K. Elastic constants are directly related to low-frequency dispersion of the three acoustic phonon modes.…”

Synchrotron Radiation Techniques and their Application to Actinide Materials

“…A number of experiments using pulsed magnetic fields have been performed with SR, e.g., (Detlefs et al, 2008b), but the first one on an actinide system, was reported from the APS on UO 2 (Antonio et al, 2021). The results are shown in Fig.…”

Magnetoelastic standing waves induced in UO$_{2}$ by microsecond magnetic field pulses

“…A recent X-ray study on UO 2 single crystals evidences the presence of AFM domains and subsequently the coexistence of AFM phases L 0 and −L 0 connected by timereversal even in magnetic fields beyond the piezomagnetic switching field [22]. This is also supported by magnetostriction measurements [2], which show that the first pulse taken below T N always has a smaller magnetostriction slope for fields below the switching field.…”