Spin dynamics and spin freezing behavior in the two-dimensional antiferromagnetNiGa2S4revealed by Ga-NMR, NQR andμ

Abstract

We have performed 69,71 Ga nuclear magnetic resonance ͑NMR͒, nuclear quadrupole resonance ͑NQR͒, 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 . 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.

Spin Fluctuations from Hertz to Terahertz on a Triangular Lattice

“…Figure 4(b) gives the in-phase component χ ′ as a function of temperature for frequencies from 1 to 1000 Hz. While no significant frequency dependence is observed at the T * anomaly where the spin fluctuation rate is in the MHz range, frequency dependence is observed for T ≈ 3 K where the NQR signal is recovered [19]. Below 2.5(5) K, a quasi-static inhomogeneous internal field was inferred from the NQR experiments [19] while µSR is inhomogeneous at all temperatures [20,21].…”

“…While no significant frequency dependence is observed at the T * anomaly where the spin fluctuation rate is in the MHz range, frequency dependence is observed for T ≈ 3 K where the NQR signal is recovered [19]. Below 2.5(5) K, a quasi-static inhomogeneous internal field was inferred from the NQR experiments [19] while µSR is inhomogeneous at all temperatures [20,21]. These observations indicate a wide distribution of spin fluctuation rates, which is corroborated by the distinct time scales inferred from µSR, NQR and ac susceptibility data ( Fig.1(b)).…”

“…1(b)), there is a temperature regime where the time-averaged ISF is approximately constant possibly due to quantum tunneling. The time-averaged ISF however, increases upon cooling below T 0 , indicating slow dynamics through a larger fraction of the sample, which eventually results in recovery of the NQR signal for T ≤ 2.5(5) K [19].…”

“…Multiple experimental techniques underlie these data as described in the following. Gallium nuclear quadrupole resonance (NQR) experiments [19] and µSR measurements [20,21] reveal critical slowing down of Ni-spin fluctuations between the Weiss temperature |θ W | = 80 K and a characteristic temperature T * = 8.5 K, where nominally elastic diffuse magnetic neutron scattering develops. This is accompanied by a loss of the NQR signal between T * and 2.5(5) K indicating dynamic nuclear spin relaxation resulting from slow Ni spin fluctuations [20,21].…”

“…NQR data in (b) is after Ref. [19]. Above and below T * the muon spin relaxation rates are in the motionally-narrowed (∝ τ ) and adiabatic (∝ 1/τ ) limits, respectively [20,21] (see Supplementary Information for details).…”