High-field ultrasound study of elastic constants and possible magnetic symmetry transformations in UO2

TL;DR

This study investigates the coupling between magnetism and lattice dynamics in UO2 under high magnetic fields, revealing a crossover in elastic properties and predicting magnetic structure transitions at very high fields.

Contribution

It combines ultrasound measurements with ab initio modeling to identify magnetic symmetry transformations in UO2 at high magnetic fields.

Findings

Ultrasound velocity crossover observed at >50 T

Magnetic structure remains 3k in the plane but becomes ferromagnetic along z

Predicted transition to 1k structure at ~104 T

Abstract

In this study, we probe the coupling between magnetism and lattice dynamics in UO2, a 3k antiferromagnet that undergoes magnetic ordering below its Neel temperature (TN) of 30.8 K. Ultrasound measurements provide insights into the interplay between the material's magnetic properties and lattice vibrations in response to the applied high magnetic field. A model analysis based on ab initio calculated superexchange interactions predicts a change in the magnetic structure from 3k to 2k at around 50 T. Although this change is not evident in the magnetization curve, we observe a crossover of ultrasound velocity Dv/v using a phase-sensitive pulse-echo technique in pulsed magnetic fields of up to 65 T. We show that at Hz > 50 T, the structure remains antiferromagnetic in the (x, y) plane and becomes ferromagnetic in the z direction. A further transition into the 1k structure is theoretically predicted to take place at a magnetic field of ~104 tesla.