DIRECT OBSERVATION OF THERMALLY ACTIVATED FLUX MOTION IN HIGH-TC SUPERCONDUCTORS

The time dependence of the local magnetic flux density B(r, t) and of the magnetic flux PHI(t) is observed directly and dynamically in epitaxially grown high temperature superconducting thin films using the high resolution magneto-optical Faraday effect. After zero field cooling and application of an external magnetic field, the magnetic flux penetrates spontaneously into the sample. Subsequently, at constant applied field the flux front further penetrates towards the center of the Y1Ba2Cu3O7-x films due to thermal activation. The radial flux density distribution shows a characteristic time dependence. The magnetic flux obeys PHI(t) = integral(s)B(r, t)df=n (t) X PHI0, and accordingly the number n(t) of flux lines each carrying a flux quantum PHI0 into the sample increases following a typical ln(t/tau)-law.