Microscopic characterization of the magnetic properties of the itinerant antiferromagnet La2Ni7 by La139 NMR/NQR measurements

$¹³⁹\mathrm{La}$ nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements have been performed to investigate the magnetic properties of the itinerant magnet ${\mathrm{La}}₂{\mathrm{Ni}}₇$, which shows a series of antiferromagnetic (AFM) phase transitions at ${T}_{\mathrm{N}1}=61$ K, ${T}_{\mathrm{N}2}=56$ K, and ${T}_{\mathrm{N}3}=42$ K under zero magnetic field. Two distinct La NMR signals were observed due to the two crystallographically inequivalent La sites in ${\mathrm{La}}₂{\mathrm{Ni}}₇$ (La1 and La2 in the ${\mathrm{La}}₂{\mathrm{Ni}}₄$ and the ${\mathrm{LaNi}}₅$ subunits of the ${\mathrm{La}}₂{\mathrm{Ni}}₇$ unit cell, respectively). From the $¹³⁹\mathrm{La}$ NQR spectrum in the AFM state below ${T}_{\mathrm{N}3}$, the AFM state was revealed to be a commensurate state where Ni-ordered moments align along the crystalline $c$ axis. Owing to the two different La sites, we were able to estimate the average values of the Ni-ordered moments ($0.09--0.10$ ${\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{Ni}$ and $0.17 {\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{Ni}$ around La1 and La2, respectively) from $¹³⁹\mathrm{La}$ NMR spectrum measurements in the AFM state below ${T}_{\mathrm{N}3}$, suggesting a nonuniform distribution of the Ni-ordered moments in the AFM state. In contrast, a more uniform distribution of the Ni-ordered moments in the saturated paramagnetic state induced by the application of high magnetic fields is observed. The temperature dependence of the sublattice magnetization measured by the internal magnetic induction at the La2 site in the AFM state was reproduced by a local-moment model better than the self-consistent renormalization (SCR) theory for weak itinerant antiferromagnets. Given the small Ni-ordered moments in the magnetically ordered state, our results suggest that ${\mathrm{La}}₂{\mathrm{Ni}}₇$ has characteristics of both itinerant nature and localized nature in its magnetism. With this in mind, it is noteworthy that the temperature dependence of nuclear spin-relaxation rates ($1/{T}₁$) in the paramagnetic state above ${T}_{\mathrm{N}1}$ measured at zero magnetic field can be explained qualitatively by both the SCR theory and the local-moment model.