Development of 3-dimensional mapping system for high-resolution local field distribution at low temperatures

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For the development of measurement systems for the local magnetic-field distributions at low temperatures, we use He-flow type cryostat, electromagnet cooled by liquid nitrogen, and a polarizing optical microscope, a CCD camera, and a video signal processor. By using magneto-optical technique, the local magnetic field is transformed to the light intensity and recorded to computers.For the 2-dimensional measurements, we succeeded in observing a critical state of high-temperature superconductors down to 4 K in the field range up to 2,500 Oe. The spacial resolution is 5,000 *, which is limited by the wave length of the light, and the field resolution was a few G.For the 3-dimensional measurements, the sample was moved against the indicator film by using a precision Z stage. In this case, however, we found that the temperature range was restricted down to 60 K beacuse of the thermal leak to the sample holder from the Z stage.Using this system, we can determine the current profile flowing in the sample under a magnetic field. In untwinned single crystals of Y- and Bi- based high temperature superconductors, we determined the anisotropy of the critical current densities in the superconducting planes (a and b axs) for the first time.