Brillouin Scattering Investigation of the One - Magnetic Ordering in Cesium-Nickel Chloride.

Brillouin light scattering has been used to study the spin-phonon interaction in CsNiCl(,3) near the one -dimensional antiferromagnetic ordering temperature. The Brillouin spectra of CsNiCl(,3) were obtained at temperatures from 13 K to 300 K, using a three-pass plane parallel Fabry-Perot interferometer. Longitudinal acoustic phonons of frequency (TURN) 27GHz propagating along the c-axis and (TURN) 22 GHz phonons propagating in the basal plane were studied at 5145 A. From the measured Brillouin shifts, the temperature dependence of the velocity of phonons propagating along the c-axis and in the basal plane were obtained. We have observed an anomalous change in velocity of the c-axis phonons near 35 K, the temperature at which magnetic susceptibility measurements suggest that one-dimensional antiferromagnetic ordering occurs along the c-axis. The anomaly in the phonon velocity is attributed to the spin-phonon interaction in the critical regime. No anomaly was observed in the velocity of basal plane phonons, as expected. These measurements, when combined with the ultrasonic results of Mountfield and Rayne, and Almond and Rayne provide a comprehensive body of data with which the various theories of the spin-phonon interaction in one-dimensional magnetic insulators may be tested. The experimental results support the recent theory of the classical compessible Heisenberg chain developed by Fivez et al. and Fivez. The strength of the spin-phonon interaction is determined to be. (DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI). In contrast, neither the theory of Nagano and Okamoto nor the theory of Suzuki are supported by these observations.