Inelastic polarized neutron scattering from S = 1 antiferromagnet CsNiCl₃ in applied field

The authors report inelastic polarized neutron scattering measurements of the excitations in the S=1, quasi-one-dimensional antiferromagnet CsNiCl 3 with applied magnetic field. A field up to 5.9 T was applied perpendicular to the chain direction and the field dependence of spin excitation modes at wave vectors corresponding to the one-dimensional zone centre (i.e. ( xi , xi ,1)) was studied. Near the antiferromagnetic ordering point Q=( 1/3,1/3,1) two types of field dependence are observed. The behaviour of the higher energy mode suggests that it is the novel longitudinal mode recently proposed by Affleck (1989). The application of the field also reveals a near-degeneracy of the excitations observed at Q=(0,0,1) in zero field. This degeneracy is incompatible with the result of the classical spin wave theory based on the Heisenberg Hamiltonian with a single-site anisotropy.