Linear nonreciprocal dichroism in boracite Co$_{3}$B$_{7}$O$_{13}$I

Linear nonreciprocal dichroism that is odd in magnetic field ${\mathbf B}$ was observed in the transverse geometry and studied for cubic (symmetry class $T_d$) phase of boracite Co$_{3}$B$_{7}$O$_{13}$I. Nonreciprocal dichroism was observed in the range of absorption bands corresponding to the transitions of the Co$^{2+}$ ion in the energy range $\Delta E=1.2$–$3.2\,$eV. The sign and magnitude of nonreciprocal dichroism depend on the mutual orientation of the magnetic field and crystallographic axes. Nonreciprocal dichroism refers to the phenomena of magnetically induced spatial dispersion, and its anisotropy is typical for the manifestation of the second-order magnetoelectric susceptibility in the optical range.