Successive Magnetic Ordering in a Triangular Lattice Antiferromagnet CsNiCl3
The hexagonal compound CsNiCl3 is one material typical of triangular lattice antiferromagnets of Heisenberg-type with small single-ion anisotropy (D/kB=-0.033 ~ -0.13K). The Ni2+ ions along the c-axis have the antiferromagnetic intrachain interaction (J/kB=16.6K) and the chains form the triangular lattice in the c-plane with a weak antiferromagnetic interchain interaction (J′/kB=0.28K). As early as in 1972, CLARK and MOULTON  observed two magnetic phase transitions at TN1=4.8K and TN2=4.4K from NMR experiment and proposed an interesting model for these successive transitions. In their model, the spin structure in the low temperature phase below TN2 is that the spins on the triangular lattice in the c-plane form a 120° spin structure in the ac-plane. In the intermediate phase between TN1 and TN2, only the S// component parallel to the c-axis is ordered, leaving the S⊥ component in the c-plane to be disordered (component disorder model), as has been supported by subsequent neutron diffraction measurements . However, a recent ESR experiment has shown that the model should be improved to include a dynamical character. In order to get more understanding, we have performed a detailed NMR experiment of 133Cs in the single crystals by a coherent pulsed-NMR method , The most interesting result is that the 120° spin structure remains to have a freedom of rotation even below TN2.
KeywordsAngular Dependence Intermediate Phase Spin Structure Triangular Lattice Magnetic Phase Transition