Topography of antiferromagnetic domains using second harmonic generation with an external reference

A new method of visualizing antiferromagnetic domains is proposed. It is based on the interference between a time-noninvariant contribution to the optical second-harmonic signal from the magnetically ordered crystal with an externally generated time-invariant second-harmonic signal from a reference source. The superposition of the two separately generated light waves leads to a pronounced polarization dependence of the second-harmonic signal which depends on the order parameter of the antiferromagnetic crystal. The new technique is applied to Cr2O3, in which the time-invariant second-harmonic reference can be generated either intrinsically by the crystal or externally by use of the new technique, so that both methods may be compared. The technique is then used to visualize the domain structure of YMnO3, an antiferromagnetic crystal which does not provide the time-invariant reference wave intrinsically. The experiments demonstrate the wide applicability of the method.