Scaling Law and Irreversibility Fields in High Temperature Superconductors

Theoretical analysis of the irreversibility fields Birr was obtained from the flux creep theory based on a depinning mechanism caused by thermally activated flux creep. Near the critical temperature Tc, the calculated Birr depends on power of ( 1-(T/Tc)2). The measured Birr, however, increases more rapidly than the power law at low temperature. This enhancement from the power law in low temperatures and high fields has been ascribed to a different pinning mechanism. The experimental temperature dependence of Birr is obtained from the AC susceptibility measurements in Hg-based composites of Agx(HgBa1.9Bi0.1Ca2Cu3O8+δ)1-x(x = 0.1 and 0.3) superconductors. AC and DC magnetizations were measured using a SQUID magnetometer and a PPMS susceptometer at temperature range 5-150 K under magnetic fields up to 14 T. The irreversibility fields Birr are estimated from the peaks of imaginary parts of AC susceptibilities and shown to agree well with the numerical estimation of the original equation. The magnetization characteristics are also successfully obtained from the pinning parameters