Magnetic properties of the quasi-two-dimensional antiferromagnet Ni0.7Al2S3.7

Abstract

We report the magnetic properties of the new layered antiferromagnet Ni${}_{0.7}$Al${}_{2}$S${}_{3.7}$. This compound is isostructural to NiGa${}_{2}$S${}_{4}$, which is the unique low spin ($S=1$) two-dimensional (2D) antiferromagnet on the exact triangular lattice. No magnetic long-range order (LRO) was observed in Ni${}_{0.7}$Al${}_{2}$S${}_{3.7}$ down to 0.4 K, as in NiGa${}_{2}$S${}_{4}$. Instead, a clear spin freezing is observed at ${T}_{\mathrm{f}}\ensuremath{\sim}4$ K, which is one order magnitude smaller than the Weiss temperature $|{\ensuremath{\theta}}_{\mathrm{W}}|\ensuremath{\sim}55$ K. In contrast with the field independent frustrated magnetism of the pure NiGa${}_{2}$S${}_{4}$, both the susceptibility and specific heat are found to be strongly field dependent, indicating disorder effects due to vacancies at the Ni and S sites. However, under a field of 9 T, Ni${}_{0.7}$Al${}_{2}$S${}_{3.7}$ shows a ${T}^{2}$-dependent magnetic specific heat that scales with $|{\ensuremath{\theta}}_{\mathrm{W}}|$, similarly to NiGa${}_{2}$S${}_{4}$. This implies an emergence of a 2D linearly dispersive mode without a magnetic LRO. Electron spin resonance (ESR) measurements reveal a systematic broadening of the resonance spectra on cooling with ${T}^{\ensuremath{-}2.5}$, suggesting that Ni spins develop 2D antiferromagnetic correlation with decreasing $T$ toward $T=0$. Moreover, Ni${}_{0.7}$Al${}_{2}$S${}_{3.7}$ exhibits crossover from a high temperature isotropic to a low temperature easy-plane anisotropic state across ${T}_{\mathrm{A}}\ensuremath{\sim}70$ K. This scale ${T}_{\mathrm{A}}$ is higher than $|{\ensuremath{\theta}}_{\mathrm{W}}|$, and is too large to be attributed either to antiferromagnetic correlation or to single ion anisotropy of Ni${}^{2+}$ that is found less than 0.1 K from the ESR experiment. We discuss that ferronematic correlation is a possible origin of the magnetic anisotropy.