Microscopic theory of the second-order magnetoelectric susceptibility in the optical region:: Case of boracite Co3B7O13I -: art. no. 045103

The microscopic theory of second-order magnetoelectric susceptibility in the optical region is presented for the case of optical transitions between states of Co2+ ions in boracite Co3B7O13I. Using many particle wave functions of the Co2+ ion in the crystal field of tetragonal symmetry D-2d and accounting for spin-orbit and Zeeman interactions we obtain an expression for the second-order magnetoelectric susceptibility at optical frequencies, which takes into account the population of all Kramers doublets of the ground state and is valid in a wide temperature range. Both the splitting of Kramers doublets of the ground and excited states and the magnetic field dependence of electric and magnetic dipole matrix elements contribute to this susceptibility. The experimental data on dispersion, temperature dependences, and the absolute value of nonreciprocal birefringence in Co3B7O13I can be adequately described in the considered model.