Abstract
The flux noise of a field-cooled wide bicrystal grain boundary junction (GBJ) in a YBa 2Cu 3O 7− δ thin film was directly measured at 77 K. For various cooling fields (−5 to +20 μT), random switching noise appeared when the applied field was changed above a threshold value after cooling. The threshold field was different for positive or negative direction of the field change and decreased linearly as the cooling field was raised. The relation suggests that the entry of vortices into the GBJ starts when the magnetic field that is enlarged by demagnetization effect at the edge of the GBJ reaches ± B c1 of the GBJ, where the demagnetization factors for the cooling field and for the field change depend on the dimensions of the sample. The same relation can be applied to predict a threshold field of a flux dam in a pickup coil of a high- T c dc superconducting quantum interference device magnetometer.
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