Magnetic orders in copper hydroxide n-alkylsulfonate layered compounds
Introduction
Magnetic interactions in molecule-based materials have been the subject of very active investigation in the past decade [1], [2], [3]. Molecule-based magnetism exhibited by inorganic/organic hybrid layered structures are noteworthy especially because the inorganic layers provide the pathway for cooperative magnetism, and the organic molecules act as a structural “template” controlling the interlayer spacing. A typical example is the series of layered hydroxide compounds, M2(OH)3X·zH2O (MII=Co, Ni, and Cu; X=inorganic or organic anion) [4], [5], [6], [7], [8], [9], [10]. These systems have positively charged magnetic layers, each with a triangular network of metal ions that are interleaved by an anion X−. In the Botallackite structure [11], there are two crystallographically distinct copper sites. One of them, Cu(1), is coordinated by four equatorial OH− groups and two anion X− groups in it trans positions. The other one, Cu(2), is coordinated by five OH− groups and one anion X− group.
A remarkable feature of this series is that the intercalation of large organic species (n-alkyl sulfates or n-alkyl carboxylates, dicarboxylates [12], [13], [14], [15], [16], n-alkylsulfonates [17]) gives rise to modulation of the interlayer spacing over very large distances. This feature enables tuning of the magnetic properties of the material, which depend to a large extent on the interlayer separation. The magnetic properties depend closely on the nature of the anion function and chain packing. For example, the n-alkyl carboxylate derivatives with bilayered structures show anomalous behaviors going from antiferromagnetic (AFM) to ferromagnetic (FM) states, depending on the layer spacing; in contrast, the analogous n-alkyl sulfate derivatives with monolayered structures are all antiferromagnets.
Another remarkable feature of the systems is that the stoichiometry and the crystal packing are influenced by the lattice water content. It has been known that both n-alkyl sulfate and n-alkyl carboxylate derivatives with long chains have two different structural varieties. For n-alkyl carboxylate derivatives, the hydrous α-phase is AFM, but the anhydrous β-phase is FM. Recently, Kurmoo et al. [17] synthesized metal hydroxy dodecylsulfonates, M2(OH)3(C12H25SO3)·H2O (M=Cu, Ni, and Co). The copper compound exhibits short-range AFM interaction whereas the nickel compound is a ferromagnet, and the cobalt compound is a ferrimagnet.
In the present study, we have synthesized novel basic cupric(II) salts with a linear alkanesulfonate (LAS) group, Cu2(OH)3(CnH2n+1SO3), with n=8 and 10 (hereafter referred to as CuLAS-n), and the magnetic properties of the new anhydrous series of compounds will be presented.
Section snippets
Experimental
The parent compound, Cu2(OH)3NO3, was obtained following a previous report [18]. The inorganic/organic hybrid compounds, CuLAS-n, were obtained by exchange reaction, starting from Cu2(OH)3NO3; the parent material (0.2 g) and the corresponding sodium n-alkanesulfonate (0.1 mol) were dispersed in distilled water (100 ml). The mixture was stirred in air-free round flask for 48 h at room temperature. Then the shiny blue powder was filtered, washed with distilled water and ethanol, and dried in vacuum
Results and discussion
Fig. 1 shows the temperature dependence of the dc magnetic susceptibility of the CuLAS-8 system. The magnetic susceptibility, χM, increases with decreasing temperature, reaching a maximum of 0.035 cm3 mol−1 at around 13 K, sharply decreasing at lower temperatures. The susceptibility at the lowest temperature, 5 K, is close to 2/3 of the maximum value, which is the case for a powder sample of a three-dimensional antiferromagnet [20]. The Neel temperature, TN, of the n=8 material was determined to be
Conclusion
It was shown in this work that the newly synthesized layered copper hydroxy n-alkylsulfonate anhydrous compounds possess a long-range magnetic order, in contrast to the hydrated derivatives exhibiting no sign of a long-range order. Both the CuLAS-8 and CuLAS-10 materials were found to have an AFM long-range order with TN of 10 and 11 K, respectively, nearly independent of the chain length. By fitting our magnetic susceptibility data to Okazaki et al.’s model, we were able to obtain two coupling
Acknowledgements
This work was supported by the Korea Research Foundation (BK 21 program) and the Korea Ministry of Science and Technology (National Research Laboratory program).
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