Chlorine NMR in Antiferromagnetic CsNiCl3

Abstract

Nuclear magnetic resonance studies have been performed on single crystals of the hexagonal antiferromagnet CsNiCl₃ (T_N=4.5 K).¹ The ³⁵Cl transitions occur at 15.16 and 10.43 MHz in zero external field at 1.3 K. The corresponding ³⁷Cl transitions are at 12.30 and 8.60 MHz. The NQR transitions in the paramagnetic state (8.0 K) occur at 8.60 and 6.74 MHz for ³⁵Cl and ³⁷Cl. A computer fit to these transitions, using values of e²qQ and η from data taken above T_N, shows that the internal magnetic field at all Cl sites is 22.5 kg. The fields lie in the plane of the three Cl atoms arranged in an equilateral triangle between the collinear Ni atoms and points toward and away from the center of the triangle in alternate planes. This configuration of internal fields was confirmed by rotation experiments about two axes in external fields of 500, 1000, and 1500 g. Dipole sum calculations and symmetry considerations show that these results are consistent with the collinear spin configuration and are not consistent with the noncollinear spin structures previously proposed by others. The temperature dependence of the magnetization has been obtained from the frequency shifts in zero field from 1.3 K to 3.4 K.