Magnetic properties of the noncollinear antiferromagnet KNiCl3

Neutron-scattering measurements are reported on the magnetic structure of the noncollinear quasi-one-dimensional antiferromagnet KNiCl3. At 4.5 K there is a commensurate triangular structure. From the magnetization measurements a magnetic phase diagram for a field applied within the basal plane was detrmined. At low temperatures there is a phase transistion at field (2.3±0.1) T to a collinear magnetic structure. From measurements of the spin-wave dispersion along the chain direction the intrachain exchange coupling J=(0.310±0.0006) THz and the single-ion anisotropyD=(0.13±0.01) THz were deduced after taking into account a correction for quantum fluctuations.