Local aging effects in PuB$_{4}$: Growing inhomogeneity and slow dynamics of local-field fluctuations probed by $^{239}$Pu NMR

TL;DR

This study uses $^{239}$Pu NMR to investigate how self-irradiation damage affects local magnetic environments in PuB$_{4}$ over six years, revealing increased inhomogeneity and slow fluctuations without altering local site symmetry.

Contribution

It provides the first detailed NMR analysis of long-term aging effects in a plutonium-based topological insulator, highlighting changes in relaxation dynamics and inhomogeneity due to radiation damage.

Findings

Aging increases spin-lattice relaxation time $T_{1}$ and its distribution.

Signal intensity decreases linearly by 20% annually.

Growth of slow hyperfine field fluctuations linked to damage.

Abstract

The effect of self-irradiation damage can influence many properties of a radioactive material. Actinide materials involving the decay through alpha radiation have been frequently studied using techniques such as transport, thermodynamics, and x-ray diffraction. The use of nuclear magnetic resonance (NMR) spectroscopy to study such effects, however, has seen relatively little attention. Here, we use $^{239}$Pu NMR to study the local influence of self-damage in a single crystal of the candidate topological insulator plutonium tetraboride (PuB$_{4}$). We first characterize the anisotropy of the $^{239}$Pu resonance in a single crystal and confirm the local axial site symmetry inferred from previous polycrystalline measurements. Aging effects are then evaluated over the timeframe of six years. We find that, though the static NMR spectra may show a slight modulation in their shape, their field-rotation pattern reveals no change in Pu local site symmetry over time, suggesting that aging has a surprisingly small impact on the spatial distribution of the static hyperfine field. By contrast, aging has a prominent impact on the NMR relaxation processes and signal intensity. Specifically, aging-induced damage manifests itself as an increase in the spin-lattice relaxation time $T_{1}$, an increased distribution of $T_{1}$, and a signal intensity that decreases linearly by 20 % per year. The spin-spin relaxation time $T_{2}$ in the aged sample shows a strong variation across the spectrum as well as a drastic shortening towards lower temperature, suggesting growth of slow fluctuations of the hyperfine field that are linked to radiation-damage-induced inhomogeneity and could be responsible for the signal wipeout that develops over time.