Study of longitudinal fluctuations in CsNiF3 by means of inelastic polarized neutron scattering

The polarized neutron scattering technique has been long applied in the study of magnetic systems mainly for elastic scattering, e.g. to separate unambiguously the magnetic from the nuclear scattering. Although the 3-axis polarization analysis (PA) technique was introduced by Moon et al./l/, it became generally available only very recently when strong neutron sources and powerful polarizing devices had been developed /1/. The inelastic PA technique can now be applied for a detailed analysis of the spin dynamics by separating the transverse and longitudinal response. As a result of this new opportunity, many theoretical studies can be tested on a much sounder basis. Since the conventional neutron spectra are dominated by the transverse components, up to now the test for the longitudinal part has been done for classical spin chains, using dynamical simulation /3/. In this case the thermally activated spin-energy coupling was shown /4/ to be an essential mechanism to explain the features of the magnetization fluctuation spectra of isotropic magnetic chains in an applied field H, at intermediate temperatures T and wavevectors k. Here we present polarized neutron scattering experiments in the 1-D ferfomagnet CsNiF3, showing the relevance of the spin-energy coupling in this regime (28 kG < H < 70 kG, T = 25 K). This experimental situation differs from the soliton -- bearing regime ( continuum limit, H <10 kG, T < 12 K) and from the hydrodynamic regime (k -> O, possible second magnon mode). The classical approximation for the spin model is no longer valid for this real experiment. Therefore a quantum version of the previous theory /4/, based on the competition between the ordering effect of the field and the thermal disorder, induced by spin-energy coupling, is presented