High Field ESR and Magnetization of the Triangular Lattice Antiferromagnet NiGa2S4

TL;DR

This study investigates the magnetic properties of NiGa2S4 using high-field ESR and magnetization measurements, revealing evidence of a topological transition possibly driven by Z2 vortices and quantum effects beyond classical models.

Contribution

It provides experimental evidence and analytical modeling of spin dynamics and topological transitions in a triangular lattice antiferromagnet, highlighting the role of Z2 vortices and quantum effects.

Findings

Identification of Z2-vortex influence on spin correlations

Spin-wave calculations explain high-field magnetization and ESR data

Observation of a T^2 specific heat with discrepancies in magnitude

Abstract

We report the experimental and the analytical results of electron spin resonance (ESR) and magnetization in high magnetic fields 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 disturbing of the development of the spin correlation by $Z_2$-vortices between 23 K and 8.5 K. Below $T_{\rm{v}}=8.5$ K, spin-wave calculations based on a 57$^{\circ}$ spiral spin order well explains 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 $T^2$-dependence by the same spin-wave calculation, but the magnitude of the specific heat is much less than the observed one. Accordingly, these results suggest the occurrence of a $Z_2$ vortex-induced topological transition at $T_{\rm{v}}$ and may indicate quantum effects beyond the descriptions based on the above classical spin models.