Direct observation of thermally activated flux motion in high-Tc superconductors

Abstract The time dependence of the local magnetic flux density B ( r , t ) and of the magnetic flux Φ( 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 Y 1 Ba 2 Cu 3 O 7− x films due to thermal activation. The radial flux density distribution shows a characteristic time dependence. The magnetic flux obeys Φ(t) = ∫ S B ( r , t) d f = n(t) × Φ 0 , and accordingly the number n ( t ) of flux lines each carrying a flux quantum Φ 0 into the sample increases following a typical ln ( t /τ)-law.