| Résumé | Compounds possessing the CsNiC13 structure [1] have attracted considerable interest recently. The crystal structure (space group Pb3/mmc) comprises chains of face-sharing NiC164− octahedra lying along the c axis separated by the large Cs+ ions. For those compounds exhibiting magnetic ordering the exchange interaction between divalent transition-metal ions along the chain direction can be much larger than that between chains, resulting in pronounced one-dimensional (1D) magnetic behaviour [2]. The cobalt salts CsCoCl3, CsCoBr3 and RbCoCl3 have proved to be excellent examples of a spin−1/2 1D Ising-like antiferromagnet. All three salts exhibit three-dimensional (3D) magnetic ordering at low temperatures [3–5], and their respective Néel temperatures are listed in Table 1. At low temperatures, below TNII, the spin arrangements in CsCoC13 and CsCoBr3 comprise that of a triangular (ab plane) Ising lattice with interchain antiferromagnetic nearest-neighbour coupling, J’, very weak ferromagnetic next-nearest-neighbour interchain interactions and strong Ising coupling, J, along the chains. Thus the chains are ordered ferrimagnetically within the ab plane. For temperatures TNII < T < TNI one-third of the chains are disordered while the other two-thirds remain antiferromagnetically coupled in the basal plane. Above TNI all of the chains are disordered but they retain significant spin correlations along the c axis for temperatures beyond 2TNI. |
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