NONLINEAR-OPTICAL SPECTROSCOPY OF MAGNETOELECTRIC AND PIEZOMAGNETIC CRYSTALS

We discuss new types of nonlinear time-noninvariant optical susceptibi lities that arise in crystals due to magnetic ordering. We show that d ifferent contributions to the nonlinear optical susceptibilities may c oexist in magnetic materials that lead to a number of unusual optical effects. Experimental data are given for magnetoelectric antiferromagn et Cr2O3, piezomagnetic alpha-Fe2O3 and ferroelectric antiferromagnet YMnO3. We developed a novel method to study antiferromagnetic domains by use of nonlinear spectroscopy. With use of the novel technique we c onfirm that in Cr2O3 in the spin-flop phase the antiferromagnetic vect or is oriented along the twofold axis. in YMnO3 we observed an unusual behaviour of antiferromagnetic domains which we explain by the entrop y contribution to the domain wall energy.