Spin dynamics and spin freezing behavior in the two-dimensional antiferromagnet NiGa${}_2$S${}_4$ revealed by Ga-NMR, NQR and $\mu$SR measurements

Abstract

We have performed ${}^{69,71}$Ga nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) and muon spin rotation/resonance on the quasi two-dimensional antiferromagnet (AFM) 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 novel spin freezing occurring at $T_{\mathrm{f}}$, 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 $T_{\mathrm{f}}$, and the temperature range of the divergence is anomalously wider than that in a conventional magnetic ordering. A broad structureless spectrum and multi-component $T_1$ 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 $T_{\mathrm{f}}$, where the NQR signal was not observed, suggests that the Ni spins do not freeze immediately below $T_{\mathrm{f}}$, but keep fluctuating down to 2 K with the MHz frequency range.

14 pages, 14 figures. To appear in Phys. Rev. B

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