A polarised neutron study of linear and non-linear spin fluctuations in CsNiF3

Inelastic neutron scattering and neutron polarisation analysis have been used to study the low-temperature spin dynamics of the one-dimensional (1d) ferromagnet CsNiF3. Measurements were made in the temperature range 2<or=T<or=12K with a magnetic field of 10 kG applied perpendicular to the chain direction. At the lowest temperatures the neutron polarisation induced by single-magnon scattering differs significantly from that expected for an isotropic Heisenberg system, but is in excellent agreement with a recent quantum calculation for a 1d, easy-plane Heisenberg magnet. The experiment demonstrates that the cross-over to isotropic spin dynamics is not complete at the temperature and magnetic fields for which solitons have been reported in CsNiF3. The authors suggest that this fact may be responsible for the agreement between experiment and Mikeska's (1978) sine-Gordon theory of in-plane spin dynamics.

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