On the Magnetoelectric Coupling in (Ni, Cu)B2O4

The mechanism of the magnetoelectric coupling of spins of Ni2+ and Cu2+ ions with the applied electric field in (Ni, Cu)B2O4, caused by the coupling of 3d electrons with the electric field and by the spin–orbit coupling, is analyzed. It is shown that the ordering of spins in the ab crystallographic plane of the crystal induces the electric polarization along the c axis of the crystal, and this polarization is mainly related to nickel ions. To obtain the electric polarization in the CuB2O4 antiferromagnet, it turns out to be necessary to apply the magnetic field in the ab plane.