Nonlinear optical spectroscopy of magnetoelectric and piezomagnetic crystals

We discuss new types of nonlinear time-noninvariant optical susceptibilities that arise in crystals due to magnetic ordering. We show that different contributions to the nonlinear optical susceptibilities may coexist in magnetic materials that lead to a number of unusual optical effects. Experimental data are given for magnetoelectric antiferromagnet 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 confirm that in Cr2O3 in the spin-flop phase the antiferromagnetic vector is oriented along the twofold axis. in YMnO3 we observed an unusual behaviour of antiferromagnetic domains which we explain by the entropy contribution to the domain wall energy.