Ultrasonic study of the quasi-one-dimensional spin fluctuations in CsNiCl3 in the presence of a magnetic field.

High-resolution ultrasonic-velocity and -attenuation data are used to characterize the paramagnetic phase of CsNiCl 3 as a function of temperature, frequency, and magnetic field (both amplitude and orientation). The interpretation of the results is based on a classical treatment of a compressible spin chain. Qualitative agreement is obtained but the experimental data put into question the validity of the classical approach for spin fluctuations in this system