Magnetoelectric properties of LiCoPO 4 : microscopic theory

The microscopic description of the magnetoelectric e!ect in LiCoPO 4 is given. The ‘one-iona approximation in which the magnetic order is presented by a Weiss molecular "eld approximation is taken as the basis of the model. Using the lowest possible order of perturbation theory the mechanism analogous to that of the changes due to an electric "eld of g-factor rather than a mechanism which involves changes of the quadratic axial term (‘D terma) in the spin-Hamiltonian is considered. Within the framework of the model it is shown that the odd part of the crystalline electric"eld and operator of the electric dipole transition can be represented by an e!ective operator with even parity that allows us a priori to take into account a parity selection rule. The expressions for the magnetoelectric tensor have been obtained. The theory may be applicable not only to the description of the static ME response but also to the description of the response of the system at optical frequencies. ( 1999 Elsevier Science B.V. All rights reserved.