Determination of spin direction in the spin-flop phase ofCr2O3

Despite a number of experimental investigations the determination of the orientation of spins in the spin-flop phase of antiferromagnetic (AFM) ${\mathrm{Cr}}_2$ ${\mathrm{O}}_3$ is still an open problem. We show that one has to take into account magnetic domains in order to clarify the magnetic structure. Domains can be distinguished optically by the interference of time-invariant and time-noninvariant contributions to the second harmonic. With use of this technique the domain structure in the spin-flop phase of ${\mathrm{Cr}}_2$ ${\mathrm{O}}_3$ is determined with high resolution in only a few minutes. Three images with light polarized parallel to the respective twofold axes ($y$ axes) of the crystal are sufficient to identify the six possible domains. It was found that the AFM vector is oriented along the $y$ axes of the crystal.