${}^{55}\mathrm{Mn}$ NMR and NQR study of the cubic Laves-phase compound ${\mathrm{UMn}}_2$

We performed ${}^{55}\mathrm{Mn}$ NMR and nuclear quadrupole resonance (NQR) measurements of a cubic Laves-phase compound ${\mathrm{UMn}}_2,$ which shows structural instability in the temperature range 210–240 K. NMR and NQR signals of two different crystallographic Mn sites, which are in nearly axial and nonaxial electric field gradients, were found below structural transitions, being consistent with the crystal structure determined from diffraction analysis and confirming the absence of magnetic ordering. Analysis of the Knight shift shows that the spin component of the isotropic hyperfine coupling constant does not change at the phase transitions, and the anisotropy of the Knight shift appreciably depends on temperature. The nuclear spin-lattice relaxation is not enhanced except around the structural transitions and is explained as simple conduction-electron relaxation, indicating absence of localized moments at both U and Mn sites. The relaxation behavior also suggests neither critical behavior nor strong electron correlation in the low-temperature range.