Tricritical-Point Phase Diagram in FeCl2

Detailed measurements of the magnetization and sublattice magnetization of Fe${\mathrm{Cl}}_2$ in a magnetic field have been performed by use of polarized- and unpolarized-neutron-diffraction techniques. The phase diagram so determined is found to bear a close resemblance to that of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-${}_{}{}^4{}_{}{}^{}\mathrm{He}$ mixtures near the tricritical point although there are a number of important differences which seem to require, at the minimum, an extension of present theories of tricritical phenomena.