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 $\alpha$ 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.

• Received 18 April 2014
• Revised 27 May 2014

DOI:https://doi.org/10.1103/PhysRevB.89.214416