Sound propagation near the tricritical point of FeCl2

The magnetically induced phase transitions, near the tricritical point of FeCl₂ were studied by acoustic velocity measurements. Longitudinal waves propagating along the (100) trigonal axis exhibit a critical shift in the velocity along the second order λ-line and anomalous change at the first order phase transitions. The phase diagram in the plane of temperature and applied magnetic field is constructed near the tricritical point.