Flux visualization in high-T_c superconductors using a high-resolution magneto-optical microscope

We report on the first observations of magnetic flux structures in high-T_c superconductors using a newly designed low temperature polarization microscope. Magnetic flux in the superconductor is visualized by detecting the rotation of the polarization vector of light (Faraday effect) within a magneto-optically active EuSe layer which is evaporated onto the surface of the sample. The low temperature microscope as a whole is built as an insert into a commercial cryostat equipped with a superconducting coil, enabling us to achieve magnetic fields up to 7 T. The optical system (lenses, polarizers and translation table) is cooled together with the sample in order to minimize the distance between the sample and the objective. Key properties are a large numerical aperture, a high extinction ratio of the polarizers and a high sensitivity of the image-intensified camera system. This should allow in the near future the visualization of individual vortices using the Faraday effect in transmission in low magnetic fields.