Optical studies of the magnetic phase diagram of FeCl2

The magnetic phase diagram of Fe${\mathrm{Cl}}_2$ in fields along [0001] has been determined anew with greater accuracy using magneto-optical-rotation and light-scattering techniques. The Néel point and the tricritical point have been found to be 23.7 and 21.60 K, respectively. In the rotation-temperature plane the phase boundaries approach the tricritical point nearly linearly. A careful comparison of these data with those of Griffin et al. and those of Birgeneau et al. leads to the conclusion that all three are in essential agreement near the tricritical point, a result consistent with tricritical theory.