Spin dynamics and spin freezing behavior in the two-dimensional antiferromagnet NiGa2S4 revealed by Ga-NMR, NQR and μSR measurements

We have performed (69,71)Ga nuclear magnetic resonance (NMR), nuclear quadrupole resonance (NQR), and muon spin rotation and resonance on the quasi-two-dimensional antiferromagnet NiGa(2)S(4), in order to investigate its spin dynamics and magnetic state at low temperatures. Although there exists only one crystallographic site for Ga in NiGa(2)S(4), we found two distinct Ga signals by NMR and NQR. The origin of the two Ga signals is not fully understood, but possibly due to stacking faults along the c axis which induce additional broad Ga NMR and NQR signals with different local symmetries. We found the spin freezing occurring at T(f), at which the specific heat shows a maximum, from a clear divergent behavior of the nuclear spin-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 spin freezing is intrinsic in NiGa(2)S(4). The behavior of these relaxation rates strongly suggests that the Ni spin fluctuations slow down towards T(f), and the temperature range of the divergence is anomalously wider than that in a conventional magnetic ordering. A broad structureless spectrum and multicomponent T(1) were observed below 2 K, indicating that a static magnetic state with incommensurate magnetic correlations or inhomogeneously distributed moments is realized at low temperatures. However, the wide temperature region between 2 K and T(f), where the NQR signal was not observed, suggests that the Ni spins do not freeze immediately below T(f), but keep fluctuating down to 2 K with the MHz frequency range. Below 0.5 K, all components of 1/T(1) follow a T(3) behavior. We also found that 1/T(1) and 1/T(2) show the same temperature dependence above T(f) but different temperature dependence below 0.8 K. These results suggest that the spin dynamics is isotropic above T(f), which is characteristic of the Heisenberg spin system, and becomes anisotropic below 0.8 K.