Optical imaging of coexisting collinear and spiral spin phases in the magnetoelectric multiferroic MnWO4

Optical absorption spectra of a magnetoelectric multiferroic MnWO4 have been measured. It has been observed that the optical absorption with the light propagating along the b axis is suppressed by magnetic order. In particular, a spin-forbidden crystal-field transition shows a steep decrease in intensity upon the first-order phase transition from the spiral spin phase, AF2, to the low-temperature collinear spin phase, AF1. The large thermochroism makes it possible to visualize AF1 and AF2 domains, at the phase-coexistence state as a contrastive optical image. AF1 and the AF2 domains appear as stripes with a width of about 20-30 mu m, implying relatively strong magnetic exchange interaction along a Mn zigzag chain (c axis).