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 bet ween the magnetic field and the c axis are presented. Upon rotation of the field, the structure of the phase diagram is modified from four p hases joined at a multicritical point for H parallel-to c to three pha ses without a multicritical point at other field directions. A previou sly 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 amplit ude of this anomaly has been found to exhibit a strong dependence on t hese two parameters. An analysis of these phase diagrams has been made using a nonlocal Landau free-energy approach