By means of high-resolution ultrasonic velocity measurements, as a function of temperature and magnetic field, the nature of the different low temperatures magnetic phase transitions observed for the quasi-one-dimensional compound ${\mathrm{CsNiCl}}_3$ is established. Special attention has been devoted to the field-induced 120° phase transition above the multicritical point in the $H\mathrm{\text{−}}T$ phase diagram where the elastic constant $C_{44}$ reveals a steplike variation and hysteresis effects. These results represent the first experimental evidence that the 120° phase transition is weakly first order and contradict the popular notion of new universality classes for chiral systems.

• Received 1 June 2006

DOI:https://doi.org/10.1103/PhysRevLett.97.077202