Magnetic properties of high-Tc superconducting grains

A phenomenological model has been developed for interpreting the magnetic properties of hard type-II superconductors. By means of a volume and a surface supercurrent density, this model considers the effects of the bulk flux pinning, the thermal equilibrium magnetization, and the surface barrier. Based on this model, several difficult problems concerning the explanation of the hysteresis loop and susceptibility data of high-Tc superconducting grains are solved. Our work demonstrates the presence of a surface-barrier effect in high-Tc superconductors. Accordingly, some past work on Hc1 and Jc determination and ac loss mechanism should be reinterpreted. © 1992 The American Physical Society.