Abstract
The magnetic phase transition near ${T}_{0}=8.5$ K in americium dioxide (${\mathrm{AmO}}_{2}$) has been investigated microscopically by means of $^{17}\mathrm{O}$ NMR. To avoid complexities arising from sample aging associated with the $\ensuremath{\alpha}$ decay of $^{243}\mathrm{Am}$, all measurements have been performed within 40 days after sample synthesis. Even during such a short period, however, a rapid change of NMR line shape has been observed at 1.5 K, suggesting that the ground state of ${\mathrm{AmO}}_{2}$ is very sensitive to disorder. We have also confirmed the loss of $^{17}\mathrm{O}$ NMR signal intensity over a wide temperature range below ${T}_{0}$, and more than half of oxygen nuclei are undetectable at 1.5 K. This behavior reveals the persistence of slow and distributed spin fluctuations down to temperatures well below ${T}_{0}$. In the paramagnetic state, strong NMR line broadening and spatially inhomogeneous spin fluctuations have been observed. The results are all indicative of short-range, spin-glass-like character for the magnetic transition in this system.
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