Directional dichroism in Co$_{3}$B$_{7}$O$_{13}$I

Directional dichroism in a well-known magnetoelectric multiferroic boracite Co$_{3}$B$_{7}$O$_{13}$I has been investigated. In the ferroelectric and weak ferromagnetic phase below 37K, we have found that the optical absorption of intra $d$-$d$ transitions at Co$^{2+}$ exhibits a fairly large directional dichroism, which shows up as a change in absorption by the application of a magnetic field in Voigt configuration. Magnetic-field dependence of the optical absorption shows a hysteresis like the magnetization curve, which indicates that the change in optical absorption should originate from the rotation of the magnetic moments of Co$^{2+}$.