Relaxation Anomalies of ^69⁄71Ga NQR in the Paramagnetic State of the Itinerant Antiferromagnet UGa₃: Possible Evidence for a Crossover to Localized Spin Fluctuations and Development of Orbital Fluctuations of 5f-electrons at High Temperatures

Abstract

The microscopic magnetic properties of the itinerant 5f-electron antiferromagnet UGa₃ (T_N=67 K) have been investigated in the paramagnetic state by means of Ga NQR studies of the two naturally abundant isotopes ⁶⁹Ga and ⁷¹Ga. A striking and unusual increase of the nuclear longitudinal relaxation rate 1⁄T₁ has been observed above 230 K, which includes a strong component of nuclear quadrupolar relaxation. Both the magnetic and quadrupolar relaxation rates have been found to increase simultaneously above 230 K. Correspondingly, the NQR frequencies also show a clear deviation from lower-T T^3⁄2-dependence above 230 K. These results are discussed in terms of a crossover from lower-T itinerant to higher-T more localized spin fluctuations and associated orbital fluctuations on the part of the U 5f electrons. A mechanism of transferred quadrupole interaction is presented, through which the fluctuations among the U 5f orbitals involve those among the counter-anion Ga 4p orbitals and thereby cause quadrupolar relaxation of the Ga nuclear spins.