Relaxation Anomalies of69/71Ga 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 5 f -electron antiferromagnet UGa 3 ( T N =67 K) have been investigated in the paramagnetic state by means of Ga NQR studies of the two naturally abundant isotopes 69 Ga and 71 Ga. A striking and unusual increase of the nuclear longitudinal relaxation rate 1/ T 1 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 5 f electrons. A mechanism of transferred quadrupole interaction is presented, through which the fluctuations among the U 5 f orbitals involve those among the counter-anion Ga 4 p orbitals and thereby cause ...