Successive Phase Transitions in CsNiBr3Observed by133Cs NMR Including Critical Examination of the Local Field

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Abstract
The ordering process in the Ising-like Heisenberg antiferromagnet on the hexagonal lattice has been studied through the 133 Cs NMR in CsNiBr 3 . The observed local field at 133 Cs was 3.5 times as large as the calculated field from the point dipoles at the Ni 2+ sites. The limited applicability of the conventional point dipole model has been overcome by treating the symmetry property at the 133 Cs site. Under the applied field of 4.6 T, we have found that the Ni 2+ spin component parallel to the c -axis orders below T N1 =15.0 K, whereas the perpendicular component orders below T N2 =12.5 K to form the triangular three-sublattices in the c-plane: one spin is parallel to the c -axis and the other two are canted ± 38° from the c -axis. The comparison is made with the existing data of CsNiCl 3 with an emphasis on the dynamical behavior of the triangular spin arrangement.