Anisotropic spin fluctuations and anomalies of nuclear quadrupole interactions in the itinerant antiferromagnet NpCoGa5: 59Co NMR and 69,71Ga NMR/NQR studies

Nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) experiments have been performed on a single crystal of tetragonal NpCoGa5, an itinerant antiferromagnet with a Neel temperature T-N=47 K. The antiferromagnetic phase is inverted to a field-induced ferromagnetic (FIF) phase with an applied field (H-0) above H-m (T -> 0) similar to 40 kOe oriented along the c axis. NMR spectra have been measured above and below T-N with H-0 parallel to c and a axes and have been assigned to Ga-69,Ga-71 nuclei on two crystallographically inequivalent 1c and 4i sites and to Co-59 nuclei on the 1b site. Using second-order perturbation calculations, Knight shift (K), electric field gradient (EFG), frequency (nu(Q)), and asymmetry parameter (eta) of the EFG are deduced for each site. These parameters for the Ga-69(1c) and Ga-69,Ga-71(4i) sites are confirmed by NQR measurements in zero field. The Knight shifts obtained in the paramagnetic (PM) state obey a Curie-Weiss law, which scales with the bulk susceptibility (chi). Hyperfine tensors for each site are deduced from K-chi plots with temperature as an implicit parameter. Antiferromagnetic NMR spectra in zero field were also observed, finding an internal field of similar to 20 kOe at the Ga(4i) site at the lowest temperature. The ordered moment can be estimated from this to be 0.81 mu(B)/Np. The nuclear quadrupolar parameters (nu(Q) and eta) are found to exhibit an anomaly just below T-N in the FIF phase. T-1 and T-2 have been measured for each site. For H-0 parallel to c, T-1 similar to constant behavior suggests localized 5f character for T > 100 K and itinerant (1/T-1 proportional to T) behavior for T-N < T