CRITICAL FIELDS AND CRITICAL CURRENTS OF SUPERCONDUCTING DISKS IN TRANSVERSE MAGNETIC-FIELDS

The large nonuniform and field-dependent demagnetizing factors of superconducting disks in transverse magnetic fields complicates the determination of the lower critical field and critical current from magnetization. Correcting the applied field with a constant ellipsoidally approximated demagnetization correction D' can result in significant errors. In this study of the magnetization characteristics of lead and Nb-Ti disks with various aspect ratios a, we find an empirical relation D'(a) that describes the scaling of the applied-field value H(c)' (H(c1)'), at which flux penetration occurs, with respect to the intrinsic (lower) critical field H(c) (H(c1)). A model is described for determining H(c1) and J(c) for such a geometry. The results have important implications for various magnetic measurements in high-T(c) superconductors. The errors that can result in the measured values of H(c1) and J(c), in the inferred penetration depths, and in the effective-mass anisotropies, are discussed.