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

TL;DR

This study demonstrates that microsecond magnetic field pulses induce long-lasting magnetoelastic standing waves in UO₂, revealing strong magnetoelastic coupling and novel phase shifts in the antiferromagnetic state.

Contribution

It provides the first observation of magnetoelastic standing waves induced by microsecond magnetic pulses in UO₂, combining experimental measurements with theoretical analysis.

Findings

Magnetic pulses excite persistent mechanical resonances in UO₂.

Resonances are consistent with magnetoelastic coupling and high crystal quality.

A phase shift occurs in lattice oscillations beyond a critical magnetic field.

Abstract

Magnetoelastic measurements in the piezomagnetic antiferromagnet UO$_{2}$ were performed via the fiber Bragg grating method in magnetic fields up to $150\,\mathrm{T}$ generated by a single-turn coil setup. We show that in short timescales, order of a few micro seconds, pulsed-magnetic fields excite mechanical resonances at temperatures ranging from $10\,\mathrm{K}$ to $300\,\mathrm{K}$, in the paramagnetic as well as within the robust antiferromagnetic state of the material. These resonances, which are barely attenuated within the 100 ms observations, are attributed to the strong magnetoelastic coupling in UO$_{2}$ combined with the high crystallographic quality of the single crystal samples. They compare well with mechanical resonances obtained by a resonant ultrasound technique and superimpose on the known non-monotonic magnetostriction background. A clear phase-shift of $\pi$ in the lattice oscillations is, unexpectedly, observed in the antiferromagnetic state when the magnetic field overcomes the piezomagnetic switch-field $H_c \simeq -18\,\mathrm{T}$. We further present simulations and a theoretical argument to explain the observed phenomena.