Magnetic Properties of FeCl2in Zero Field. II. Long-Range Order

In Paper I it has been shown that the critical behavior of Fe${\mathrm{Cl}}_2$ in zero field should be that appropriate to the three-dimensional $S=1\mathrm{}$ Ising model. This paper reports a detailed study of the critical behavior of the sublattice magnetization below $T_N$ together with a qualitative survey of the wave-vector-dependent susceptibility above $T_N$. The sublattice magnetization is found to follow the power law $\frac{M_T}{M_0}=D{(1-\frac{T}{T_N})}^{\beta }$, with $D=1.\mathrm{}47\pm 0.02$, $T_N=23.\mathrm{}553\pm 0.01$ °K, $\beta =0.\mathrm{}29\pm 0.01$, over the range 0.1 to < 0.001 in reduced temperature. The exponent $\beta$ is in reasonable agreement with the rigid-lattice Ising value of 0.313 ± 0.003. The regular behavior of $M_T$ together with the unusual metamagnetic properties of Fe${\mathrm{Cl}}_2$ make this an ideal system for a study of the critical behavior of the order parameter around a tricritical point. The wave-vector-dependent susceptibility ${\chi }^{\alpha \alpha }\left(\overrightarrow{\mathrm{Q}}\right)$ above $T_N$ is found to exhibit typical behavior for a three-dimensional anisotropic antiferromagnet. Only ${\chi }^{\mathrm{zz}}\left(\overrightarrow{\mathrm{\tau }}\right)$, the component along the anisotropy axis, diverges at the phase transition. Furthermore ${\chi }^{\mathrm{zz}}\left(\overrightarrow{\mathrm{Q}}\right)$ is found to be fully three dimensional; that is, it has the form of a peak rather than a ridge, in spite of the fact that the spin waves are essentially two dimensional in form.