ก 2D Frustrated Magnetism vs. 3D Spin Order in the Chalcogenide Triangular Antiferromagnets NiGa2S4, FeGa2S4, and Fe2Ga2S5

Geometrically frustrated magnets have attracted great interest for the possible exotic ground states and phase transition phenomena. The two dimensional triangular lattice is one of the simplest forms of a geometrically frustrated lattice with a single magnetic ion in a unit cell.

NiGa2S4 is the first example of S = 1 two dimensional (2D) antiferromagnets on an exact triangular lattice [1]. Despite an antiferromagnetic coupling with the Weiss temperature QW = 80 K, no antiferromagnetic or canonical spin glass ordering has been observed down to 0.08 K [1,2]. Instead, recent nuclear-quadrupole-resonance and muon-spin relaxation measurements have revealed unconventional freezing phenomena that set in below Tf ~ 10 K and have a wide temperature critical fluctuation regime down to 2 K [2,3]. Very interestingly, below Tf~ 10 K, it develops a short-range noncollinear order with 2D gapless linearly dispersive excitations characterized by a T2 dependence of the specific heat [1]. In order to explain the origin and stability of the 2D coherent behavior in the frozen spin-disordered state, various theoretical proposals have been made. Experimentally, it is highly important to clarify the effect of spin size change, bilayering, and geometrical frustration, and to see if any spin order can be realized in the related compounds.

Fig. 1: Crystal structure of the unit slab for (a) single layered AGa2S4(A = Ni, Fe) and (b) bilayered Fe2Ga2S5

Thus, we have made the single crystal study of the isostructural single-layer triangular antiferromagnets, NiGa2S4 and FeGa2S4, and the homologous bilayer triangular antiferromagnet Fe2Ga2S5 (see Figure 1) [4]. We have succeeded in growing single crystals of NiGa2S4 for the first time and found properties fully consistent with those for polycrystalline samples. Interestingly, the magnetic properties of FeGa2S4 bear strong resemblances to those of NiGa2S4 despite the fact that Fe2+ has twice larger S = 2 spin than S = 1 for Ni2+. Both compounds have basically Heisenberg spins, and form frozen disordered state (see Figure 2). In addition, the specific heat is insensitive to the field and shows a T2 dependence, which scales to that of NiGa2S4 (see Figure 2(b)). The similarities strongly suggest that the 2D coherent behavior in the frozen spin-disordered state has the same underlying mechanism. In contrast, a clear antiferromagnetic transition is observed for the bilayer system Fe2Ga2S5, whose dominant antiferromagnetic bonds most likely form an unfrustrated honeycomb lattice (see Figures 2(a) and (c)). These results suggest that the geometrical frustration of the single-layer triangular lattices stabilizes the spin-disordered state, probably associated with 2D antiferromagnetic ordering [4].

Fig. 2 (a) Four sublattices of the buckled honeycomb lattice formed by the Fe-S-Fe straight bonds in the bilayer of Fe2Ga2S5. (b), Scaling in the T2 dependence in the magnetic part of the specific heat CM for NiGa2S4(S = 1;QW = 80 K) and FeGa2S4(S = 2;QW = 160 K) at 0 T in full logarithmic scale (c) ab-plane and c-axis components of the susceptibility c for FeGa2S4 and Fe2Ga2S5 under B = 0.1 T. Inset c(T) and d c /dT of Fe2Ga2S5. The vertical lines indicate the Néel point TN
[1] S. Nakatsuji, Y. Nambu, H. Tonomura, O. Sakai, S. Jonas, C. Broholm, H. Tsunetsugu, Y. Qiu, and Y. Maeno, Science 309, 1697-1700 (2005).
[2] H. Takeya et al., Physical Review B 77 054429/1-13 (2008).
[3] D.E. MacLaughlin, Y. Nambu, S. Nakatsuji, R. H. Heffner, L. Shu, O.O. Bernal, and K. Ishida, Physical Review B 78, 220403/1-4 (2008).
[4] S. Nakatsuji, H. Tonomura, K. Onuma, Y. Nambu, O. Sakai, Y. Maeno, R. T. Macaluso, and Julia Y. Chan, Physical Review Letters 99, 157203/1-4 (2007).

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