Magneto-optical investigations of the flux distributions in high-Tc superconductors are reviewed. The various techniques (which are all based on the Faraday effect) are compared with each other concerning resolution and working range of temperature and external magnetic field. A short description of the historical development of the magneto-optical methods is given and the existing equipment is classified. Various aspects of flux visualization are presented and special observations made by means of magneto-optical techniques are shown. The capability of the magneto-optical techniques to observe dynamic processes allows observation of flux motion due to thermal activation or even quantum creep, and under the influence of transport currents. It is shown that magneto-optical techniques are a unique tool to show that flux patterns are extremely sensitive to the presence of defects.
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