ANGULAR-DEPENDENCE OF THE H-T PHASE-DIAGRAM OF CSNICL3

The results of an ultrasonic study of the magnetic-field-temperature (H-T) phase diagram of hexagonal CsNiCl3 as a function of the angle between the magnetic field and the c axis are presented. Upon rotation of the field, the structure of the phase diagram is modified from four phases joined at a multicritical point for H parallel-to c to three phases without a multicritical point at other field directions. A previously unseen transition has been detected at intermediate field angles. The behavior of the spin-flop anomaly on the acoustic velocity is also investigated as a function of field angle and temperature. The amplitude of this anomaly has been found to exhibit a strong dependence on these two parameters. An analysis of these phase diagrams has been made using a nonlocal Landau free-energy approach