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

TL;DR

This study investigates the spin dynamics and freezing behavior in the quasi-two-dimensional antiferromagnet NiGa$_{2}$S$_{4}$ using Ga-NMR, NQR, and muon spin measurements, revealing intrinsic spin freezing and slow spin fluctuations.

Contribution

It provides the first detailed analysis of spin freezing and dynamics in NiGa$_{2}$S$_{4}$ using multiple local probes, highlighting the presence of inhomogeneous magnetic states and slow spin fluctuations.

Findings

Detection of two distinct Ga signals possibly due to stacking faults.

Observation of spin freezing at temperature T_f with divergent relaxation rates.

Evidence of inhomogeneous magnetic states with slow spin fluctuations below 2 K.

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 $T_{\rm f}$, 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 $T_{\rm f}$, 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 $T_{\rm f}$, where the NQR signal was not observed, suggests that the Ni spins do not freeze immediately below $T_{\rm f}$, but keep fluctuating down to 2 K with the MHz frequency range.