Critical state model determined from flux-creep activation energy in SmBa2Cu3Oy superconductor

Time dependent magnetization was measured on two forms (polycrystal and aligned grains) of high-Tc superconductor SmBa2Cu3Oy. The flux-creep activation energy obtained in the aligned grains showed inverse dependence on current density as expected from collective flux-creep theory. However, that obtained from a polycrystal, which can be called intergranular flux-creep activation energy, showed linear dependence on current density as suggested by Kim-Anderson model. This proves that Kim's critical state model is more suitable to the intergranular system, where flux lines should be in fixed positions and have fixed sizes, than the flux bundle system. In the critical state, where the pinning force is balanced with Lorentz force, the collective pinning theory is proved to be related to the generalized critical state model, which needs to be modified by surface-barrier effect when it is applied to M−H curve fitting of aligned grains.