Effect of magnetic ordering on the optical properties of transition-metal halides: Ni${\mathrm{Cl}}_2$, Ni${\mathrm{Br}}_2$, Cr${\mathrm{Cl}}_3$, and Cr${\mathrm{Br}}_3$

New reflectance and thermoreflectance spectra, in the (3-9)-eV region of ferromagnetic Cr${\mathrm{Br}}_3$ and antiferromagnetic Cr${\mathrm{Cl}}_3$, Ni${\mathrm{Cl}}_2$, and Ni${\mathrm{Br}}_2$, insulators are reported. Changes in the optical spectra near the magnetic transition temperature have been observed. In particular, below $T_c$, the main optical structures of Cr${\mathrm{Br}}_3$ are split into two components $\simeq 0.1$ eV apart. This finding is shown to agree with the Kerr-effect data reported in the literature. While the first charge-transfer transitions ($A_1$) begins to split above $T_c$, the splitting of the higher-energy bands follows the macroscopic magnetization law. This different behavior is attributed to a different localization of the electronic states. Antiferromagnetic crystals do not show any band splitting. Their complex phenomenology is discussed in some detail.