Anisotropic spin fluctuations and anomalies of nuclear quadrupole interactions in the itinerant antiferromagnetNpCoGa5:Co59NMR andGa69,71NMR/NQR studies

Abstract

Nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) experiments have been performed on a single crystal of tetragonal $\mathrm{Np}\mathrm{Co}{\mathrm{Ga}}_{5}$, an itinerant antiferromagnet with a N\'eel temperature ${T}_{N}=47\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The antiferromagnetic phase is inverted to a field-induced ferromagnetic (FIF) phase with an applied field $({H}_{0})$ above ${H}_{m}$ $(T\ensuremath{\rightarrow}0)$ $\ensuremath{\sim}40\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$ oriented along the $c$ axis. NMR spectra have been measured above and below ${T}_{N}$ with ${H}_{0}\ensuremath{\Vert}c$ and $a$ axes and have been assigned to $^{69,71}\mathrm{Ga}$ nuclei on two crystallographically inequivalent $1c$ and $4i$ sites and to $^{59}\mathrm{Co}$ nuclei on the $1b$ site. Using second-order perturbation calculations, Knight shift $(K)$, electric field gradient (EFG), frequency $({\ensuremath{\nu}}_{\mathrm{Q}})$, and asymmetry parameter $(\ensuremath{\eta})$ of the EFG are deduced for each site. These parameters for the $^{69}\mathrm{Ga}(1c)$ and $^{69,71}\mathrm{Ga}(4i)$ 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 $(\ensuremath{\chi})$. Hyperfine tensors for each site are deduced from $K\ensuremath{-}\ensuremath{\chi}$ plots with temperature as an implicit parameter. Antiferromagnetic NMR spectra in zero field were also observed, finding an internal field of $\ensuremath{\sim}20\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$ at the $\mathrm{Ga}(4i)$ site at the lowest temperature. The ordered moment can be estimated from this to be $0.81\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{B}∕\mathrm{Np}$. The nuclear quadrupolar parameters (${\ensuremath{\nu}}_{\mathrm{Q}}$ and $\ensuremath{\eta}$) 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 ${H}_{0}\ensuremath{\Vert}c$, ${T}_{1}\ensuremath{\sim}\text{constant}$ behavior suggests localized $5f$ character for $T>100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and itinerant $(1∕{T}_{1}\ensuremath{\propto}T)$ behavior for ${T}_{N}<T<\ensuremath{\sim}100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in $\mathrm{Np}\mathrm{Co}{\mathrm{Ga}}_{5}$. ${T}_{2}$ measurements in the case of ${H}_{0}\ensuremath{\Vert}c$ clearly define a phase crossover between PM and FIF phases. A sharp anisotropy for spin fluctuations in $\mathrm{Np}\mathrm{Co}{\mathrm{Ga}}_{5}$ has also been demonstrated.