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We have performed $^{69,71}$Ga nuclear magnetic resonance (NMR) and nuclearquadrupole resonance (NQR) and muon spin rotation/resonance on the quasitwo-dimensional antiferromagnet (AFM) NiGa$_2$S$_4$, in order to investigateits spin dynamics and magnetic state at low temperatures. Although there existsonly one crystallographic site for Ga in NiGa$_2$S$_4$, we found two distinctGa signals by NMR and NQR. The origin of the two Ga signals is not fullyunderstood, but possibly due to stacking faults along the c axis which induceadditional broad Ga NMR and NQR signals with different local symmetries. Wefound the novel spin freezing occurring at $T_{\rm f}$, at which the specificheat shows a maximum, from a clear divergent behavior of the nuclearspin-lattice relaxation rate $1/T_{1}$ and nuclear spin-spin relaxation rate$1/T_{2}$ measured by Ga-NQR as well as the muon spin relaxation rate$\lambda$. The main sharp NQR peaks exhibit a stronger tendency of divergence,compared with the weak broader spectral peaks, indicating that the spinfreezing is intrinsic in NiGa$_2$S$_4$. The behavior of these relaxation ratesstrongly suggests that the Ni spin fluctuations slow down towards $T_{\rm f}$,and the temperature range of the divergence is anomalously wider than that in aconventional magnetic ordering. A broad structureless spectrum andmulti-component $T_1$ were observed below 2 K, indicating that a staticmagnetic state with incommensurate magnetic correlations or inhomogeneouslydistributed moments is realized at low temperatures. However, the widetemperature region between 2 K and $T_{\rm f}$, where the NQR signal was notobserved, suggests that the Ni spins do not freeze immediately below $T_{\rmf}$, but keep fluctuating down to 2 K with the MHz frequency range.

Spin dynamics and spin freezing behavior in the two-dimensional antiferromagnet NiGa$_{2}$S$_{4}$ revealed by Ga-NMR, NQR and $\mu$SR measurements