## Abstract

We report the experimental and analytical results of electron spin resonance (ESR) and magnetization at high magnetic fields of up to about 68 T of the quasi two-dimensional triangular lattice antiferromagnet NiGa _{2}S_{4}. From the temperature evolution of the ESR absorption linewidth, we find a distinct disturbance of the development of the spin correlation by Z_{2}-vortices between 23 and 8.5 K. Below T_{v} = 8.5 K, spin-wave calculations based on a 57° spiral spin order well explain the frequency dependence of the ESR resonance fields and high-field magnetization processes for H ∥ c and H ⊥ c, although the magnetization for H ⊥ c at high fields is different from the calculated one. Furthermore, we explain the field-independent specific heat with a T^{2} dependence by the same spin-wave calculation, but the magnitude of specific heat is much less than the observed one. The single ion anisotropy constant D/k_{B} = 0.8K is the same order of T_{V}(ξ/a)^{-2} = 0.3-0.4K where ξ and a are the correlation length and the lattice constant of the triangular plane in NiGa_{2}S_{4}, respectively. The relation D/k_{B} ≤ T_{V}(ξ/a)^{-2} recently derived by Kawamura et al. [H. Kawamura et al.: J. Phys. Soc. Jpn. 79 (2010) 023701] is a necessary condition for the realization of a Z_{2}-vortex- induced topological transition, which would be destroyed by an extremely small D ≠ 0 in a classical nearest-neighbor spin model owing to the large ξ. Accordingly, these results suggest the occurrence of a Z_{2}-vortex- induced topological transition at T_{v} and indicate quantum effects beyond the descriptions based on the above classical spin models, for example, those due to quadrupolar correlations.

Original language | English |
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Article number | 054710 |

Journal | Journal of the Physical Society of Japan |

Volume | 79 |

Issue number | 5 |

DOIs | |

Publication status | Published - 2010 May |

## Keywords

- Frustrate magnetism
- High field magnetization
- High-field ESR
- NiGaS
- Triangular lattice

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