Magnetic properties of the quasi-two-dimensional antiferromagnet Ni${}_{0.7}$Al${}_{2}$S${}_{3.7}$

Magnetic properties of the quasi-two-dimensional antiferromagnet Ni $_{0.7}$ Al $_{2}$ S $_{3.7}$

Tomoya Higo, Rieko Ishii, Melissa C. Menard, Julia Y. Chan, Hironori Yamaguchi, Masayuki Hagiwara, and Satoru Nakatsuji

Phys. Rev. B 84, 054422 – Published 8 August 2011

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 $T_{f} \sim 4$ K, which is one order magnitude smaller than the Weiss temperature $| θ_{W} | \sim 55$ 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 $| θ_{W} |$ , 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 $T^{- 2.5}$ , 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 $T_{A} \sim 70$ K. This scale $T_{A}$ is higher than $| θ_{W} |$ , 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.

Received 8 March 2011

DOI:https://doi.org/10.1103/PhysRevB.84.054422

Authors & Affiliations

Tomoya Higo¹, Rieko Ishii^1,*, Melissa C. Menard², Julia Y. Chan², Hironori Yamaguchi^3,†, Masayuki Hagiwara³, and Satoru Nakatsuji^1,‡

¹Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
²Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803,USA
³KYOKUGEN, Osaka University, Machikaneyama 1-3, Toyonaka 560-8531, Japan

^*Present address: KYOKUGEN, Osaka University, Machikaneyama 1-3, Toyonaka 560-8531, Japan.
^†Present address: Department of Physical Science, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan.
^‡satoru@issp.u-tokyo.ac.jp

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Vol. 84, Iss. 5 — 1 August 2011

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Images

Figure 1
(a) Crystal structure of Ni $_{0.7}$ Al $_{2}$ S $_{3.7}$ . (b) NiS $_{2}$ layer in Ni $_{0.7}$ Al $_{2}$ S $_{3.7}$ viewed along the $c$ -axis, which consists of edge-sharing NiS $_{6}$ octahedra. Empty Ni sites and the associated NiS $_{6}$ octahedra are shown by blank circle and by broken line, respectively. (c) Schematic network formed by the third nearest interaction $J_{3}$ (pink, blue, green, and yellow lines) on a triangular Ni lattice of Ni $_{0.7}$ Al $_{2}$ S $_{3.7}$ .Reuse & Permissions
Figure 2
(a) Temperature dependence of the susceptibility $χ \equiv$ $M$ / $B$ of Ni $_{0.7}$ Al $_{2}$ S $_{3.7}$ under 0.01 T and 7 T for $B ∥ a b$ and $B ∥ c$ . There is a hysteresis between FC and ZFC measurement below 4 K at 0.01 T. (b) Temperature dependence of the inverse susceptibility $χ^{- 1}$ of Ni $_{0.7}$ Al $_{2}$ S $_{3.7}$ at 7 T for $B ∥ a b$ (red circle) and $B ∥ c$ (blue square). Solid lines indicate the fit to the Curie-Weiss law. (c) Temperature dependence of the anisotropy ratio $χ_{a b}$ / $χ_{c}$ obtained for Ni $_{0.7}$ Al $_{2}$ S $_{3.7}$ under 0.01 T and 7 T and NiGa $_{2}$ S $_{4}$ under 7 T.[34] Inset: $χ_{a b}$ / $χ_{c}$ vs $T$ in logarithmic scale. A clear enhancement of $χ_{a b}$ / $χ_{c}$ below 70 K, as indicated by a black arrow, is observed in Ni $_{0.7}$ Al $_{2}$ S $_{3.7}$ .Reuse & Permissions
Figure 3
Temperature dependence of (a) the total specific heat $C_{P}$ / $T$ at 0 T (red circle) and 9 T (green triangle), and (b) the magnetic specific heat $C_{M}$ / $T$ (left axis) at 0 T (red open circle) and 9 T (green open triangle), and the magnetic entropy $S_{M}$ at 0 T (red solid circle) and 9 T (green solid triangle) for $B ∥ c$ . The horizontal line indicates the theoretical total entropy for $S$ $=$ 1, $S_{M}$ $=$ $R$ ln 3.Reuse & Permissions
Figure 4
(a) Temperature dependence of the magnetic specific heat under 0 T (red circle) and 9 T (green triangle) below 30 K. (b) $C_{M}$ $| θ_{W} | / T$ as a function of $T$ / $| θ_{W} |$ in Ni $_{0.7}$ Al $_{2}$ S $_{3.7}$ at 0 T (red circle) and 9 T (green triangle), NiGa $_{2}$ S $_{4}$ at 0 T (purple diamond) and Ni $_{0.7}$ Zn $_{0.3}$ Ga $_{2}$ S $_{4}$ at 0 T (orange square) under $B ∥ c$ .Reuse & Permissions
Figure 5
Temperature dependence of the ESR absorption spectra of Ni $_{0.7}$ Al $_{2}$ S $_{3.7}$ at 584.8 GHz for $H ∥ c$ . The arrows indicate the resonance fields. The sharp small signals come from the ESR standard sample of DPPH for correction of the magnetic field. The inset shows the temperature dependence of the full width at half maximum (FWHM) of the ESR spectra. The error bars with about 5 $%$ of the FWHM values are put on the data. The solid line represents the result of a fit of the experimental data to Eq. (4).Reuse & Permissions
Figure 6
(a) Frequency dependence of the ESR absorption spectra of Ni $_{0.7}$ Al $_{2}$ S $_{3.7}$ at 1.3 K for $B ∥ c$ . The large and small arrows indicate the resonance fields of Ni $_{0.7}$ Al $_{2}$ S $_{3.7}$ and the ESR standard DPPH, respectively. (b) Frequency-field plot of the resonance fields at 1.3 K for $B ∥ c$ . The open and closed circles denote the resonance fields of NiGa $_{2}$ S $_{4}$ and Ni $_{0.7}$ Al $_{2}$ S $_{3.7}$ , respectively. The dotted, solid, and broken lines represent calculated ESR modes with different values of easy-plane anisotropy $D$ / $k_{B}$ $=$ 0.8 K, 0.1 K, and 0 K, respectively.Reuse & Permissions

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