EFFECTS OF CRITICAL-CURRENT DENSITY, EQUILIBRIUM MAGNETIZATION AND SURFACE-BARRIER ON MAGNETIZATION OF HIGH-TEMPERATURE SUPERCONDUCTORS

An extended critical state model which includes the effects of bulk critical current density, equilibrium magnetization and surface barrier is developed for the magnetization of superconductors. The equilibrium magnetization and surface barrier are modelled by an applied field dependent surface supercurrent density j(s)(H), whose presence changes the boundary field of the bulk. The volume supercurrents flow with a density equal to the internal field dependent critical current density J(c)(H(i)). The magnetization M is produced by both supercurrents. For the M(H) curve computation, exponential-type Jc(H(i)) and j(s)(H) values are used for the general case of an infinite sample of rectangular cross-section. A comparison between the experimental magnetization curves of a sintered YBa2Cu3O7 superconductor at 76 K and the model fit shows that j(s)(H) is null for the coupling matrix, whereas a non-zero j(s)(H) is needed for the grains. The model fit for the irreversible magnetization of the grains is improved by including a surface barrier to the entry and exit of flux.