High-Field ESR and Magnetization of the Triangular Lattice Antiferromagnet NiGa2S4

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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 NiGa2S4. 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 $T_{\text{v}}=8.5$ 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 $H\parallel c$ and $H\perp c$, although the magnetization for $H\perp c$ 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 $D/k_{\text{B}}=0.8$ K is the same order of $T_{\text{V}}(\xi/a)^{-2}=0.3--0.4$ K where $\xi$ and $a$ are the correlation length and the lattice constant of the triangular plane in NiGa2S4, respectively. The relation $D/k_{\text{B}}\leq T_{\text{V}}(\xi/a)^{-2}$ 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 $D\neq 0$ in a classical nearest-neighbor spin model owing to the large $\xi$. Accordingly, these results suggest the occurrence of a $Z_{2}$-vortex-induced topological transition at $T_{\text{v}}$ and indicate quantum effects beyond the descriptions based on the above classical spin models, for example, those due to quadrupolar correlations.
Physical Society of Japanの論文
2010-05-15

High-Field ESR and Magnetization of the Triangular Lattice Antiferromagnet NiGa2S4

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