First-order phase transition inUO2:235Uand17ONMR study

Abstract

A nuclear magnetic resonance (NMR) study is reported for a $5f$-cooperative Jahn-Teller (JT) system with antiferromagnetic ordering, ${\mathrm{UO}}_{2},$ where we have performed magnetic actinide ${(}^{235}\mathrm{U})$ as well as nonmagnetic ligand ${(}^{17}\mathrm{O})$ NMR studies in a $5f$-electron system. The observed ${}^{235}\mathrm{U}$ hyperfine interaction is consistent with an axially symmetric $5f$-wave-function character for ${\mathrm{U}}^{4+}$ below a first-order transition temperature ${T}_{\mathrm{N}}=30.8\mathrm{K}.$ Thus, the orbital degeneracy in the cubic crystalline field is lifted by a magnetic ordering combined with a JT distortion. On the other hand, modulation phenomena in the ${}^{17}\mathrm{O}$ spin-echo decay provide evidence of a lattice distortion just below ${T}_{\mathrm{N}},$ which gives rise to an axially symmetric electric-field gradient at the oxygen sites. These results indicate that, in a cooperative JT transition which occurs as a result of coupling between $5f$ quadrupoles and lattice distortion, the behavior of the $5f$ quadrupoles can be investigated using uranium NMR, while the lattice distortion can be studied with the nonmagnetic ligand NMR. Among magnetic structures and JT distortions, proposed on the basis of neutron scattering experiments and theoretical work, the noncollinear $3\mathit{k}$-type structure is the most likely, because either the $1\mathit{k}$ or $2\mathit{k}$ would cause an orthorhombic distortion. With respect to low-energy spin-wave excitations, nuclear spin-lattice relaxation rates ${T}_{1}^{\ensuremath{-}1}$ at both sites show a ${T}^{7}$ behavior below ${T}_{\mathrm{N}},$ which suggests the presence of gapless excitations due to strong magnon-phonon coupling.