FIELD-DEPENDENCE OF MAGNETIC-RELAXATION IN BIPBSRCACUO SUPERCONDUCTORS

Magnetic relaxation in ceramic cylindrical samples of BiPbSrCaCuO high T-c superconductors has been studied as a function of applied magneti c field for a constant decay rate of that field strength. We have anal yzed the variation of the ac susceptibility versus time just after rem oving the magnetic field. Data have been obtained by means of an induc tive technique from 1 to 3600 s, with magnetic fields that range from 10 to 100 Oe, and by switching off the field at a constant rate of 25 Oe/s. The field dependence of the thermally activated parameters has b een obtained within the framework of the Anderson-Kim theory and using the so-called box distribution of energies. Anderson-Kim considered a logarithmic decay of the magnetic flux governed by an effective energ y U=U-0-FVX, where U-0 is the pinning energy, V is the activation volu me, X the pinning length, and F the driving force density. In this cas e we have found a field dependence of the form H--n for the pinning en ergy, with n close to 2 and temperature dependent. However, we have ob served that the box distribution, which assumes the existence of a con tinuous energy spectrum, shows no field dependence for the effective e nergy. (C) 1996 American Institute of Physics.