Anisotropic spin fluctuations and anomalies of nuclear quadrupole interactions in the itinerant antiferromagnet NpCo Ga5: Co59 NMR and Ga 69,71 NMR/NQR studies

Nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) experiments have been performed on a single crystal of tetragonal NpCo Ga5, an itinerant antiferromagnet with a Néel temperature TN =47 K. The antiferromagnetic phase is inverted to a field-induced ferromagnetic (FIF) phase with an applied field (H0) above Hm (T→0) ∼40 kOe oriented along the c axis. NMR spectra have been measured above and below TN with H0 c and a axes and have been assigned to Ga 69,71 nuclei on two crystallographically inequivalent 1c and 4i sites and to Co59 nuclei on the 1b site. Using second-order perturbation calculations, Knight shift (K), electric field gradient (EFG), frequency (νQ), and asymmetry parameter (η) of the EFG are deduced for each site. These parameters for the Ga69 (1c) and Ga 69,71 (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 (χ). Hyperfine tensors for each site are deduced from K-χ plots with temperature as an implicit parameter. Antiferromagnetic NMR spectra in zero field were also observed, finding an internal field of ∼20 kOe at the Ga (4i) site at the lowest temperature. The ordered moment can be estimated from this to be 0.81 μB Np. The nuclear quadrupolar parameters (νQ and η) are found to exhibit an anomaly just below TN in the FIF phase. T1 and T2 have been measured for each site. For H0 c, T1 ∼constant behavior suggests localized 5f character for T>100 K and itinerant (1 T1 T) behavior for TN <T<∼100 K in NpCo Ga5. T2 measurements in the case of H0 c clearly define a phase crossover between PM and FIF phases. A sharp anisotropy for spin fluctuations in NpCo Ga5 has also been demonstrated.