55MnNMR and NQR study of the cubic Laves-phase compoundUMn2

Abstract

We performed 55 Mn NMR and nuclear quadrupole resonance ͑NQR͒ measurements of a cubic Laves-phase compound 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.

Magnetocrystalline anisotropy inUMn2Ge2and related Mn-based actinide ferromagnets

“…Interestingly, the calculated results for UMn 2 , while agreeing with the trend of decreasing moment magnitudes with decreasing nearest neighbor distances, are apparently at odds with the experimental situation 28 , which does not find magnetic order in this compound. We ascribe this inconsistency to the well-known difficulties that actinide compounds present to first principles calculations.…”

Dynamic lattice distortions in Sr2RuO4: microscopic studies by perturbed angular correlation spectroscopy andab initiocalculations

“…Instead, the ω S 0 measured for 111 Cd in Sr 2 RuO 4 increases with temperature and approximately shows a √ T dependence. A decrease of quadrupole interaction frequency with lowering of the sample temperature has earlier been reported for UMn 2 and HfV 2 and has been suggested to be due to lattice instabilities in these materials [29,37]. We believe that the anomalous temperature dependence of ω 0 observed for Sr 2 RuO 4 also arises due to lattice instabilities, but of different origin.…”

Exchange bias effect in alloys and compounds

“…This asymmetry was proposed to be a manifestation of the EB effect. UMn 2 was described as an AFM compound associated with a small parasitic FM component which was attributed to uncompensated sublattice moment or uncompensated domain wall magnetization, although the absence of long range magnetic ordering was confirmed recently by 55 Mn NMR and NQR investigations [142]. Very recently, an interesting observation of the EB effect has also been reported for R 1−x Gd x Al 2 [143,144] and Sm 0.975 Gd 0.025 Cu 4 Pd [145] in the vicinity of the compensation temperature, at which the ZFC magnetization changes its sign.…”