Magnetoelectric effect in Co-Cl boracite

The ferromagnetoelectric boracite CO₃B₇O₁₃Cl has been investigated on (111) cubic cut single crystal platelets in its low temperature monoclinic ferroelectric/ferroelastic/weakly ferromagnetic phase. These platelets were electroded and poled by a thermal process or by the application of a high enough electric field to get a "ferroelectric/ferroelastic nearly single domain" with polarization along the (111)_cub direction perpendicular to the platelets. Magnetoelectric measurements were performed on these platelets in the temperature range 1.5-40 K and up to an applied magnetic field of 10 kOe. The induced polarization, at a dc magnetic field of 0.5 kOe, which is proportional to the coefficient of the linear magnetoelectric effect, shows an anomaly at about 11.3 K. With further increase in temperature, a sharp change at nearly 12 K corresponding to the Néel temperature was observed. The coefficient α₃₂ of the linear magnetoelectric effect, measured quasi-statically at 4.2 K, is about 13 ps/m (∼3.9 × 10^-3 in Gaussian units). The ferromagnetic domain switching with the angular variation of magnetic field is consistent with the monoclinic Shubnikov point group m. The coercive field as a function of temperature measured magnetoelectrically shows an exponential behaviour.