Scaling law and irreversibility fields in MgB2 superconductors

Theoretical analysis of the irreversibility fields B i was obtained from the flux creep theory based on a depinning mechanism caused by thermally activated flux creep and predicted to vary according to power of (1 - (T/T c)2). The measured B i, however, increases more rapidly at low temperatures. This enhancement from the power law in low temperatures and high fields has been ascribed to the different pinning mechanism. The irreversibility fields B i of MgB2 superconductors are estimated from the onsets of imaginary parts of AC susceptibilities and shown to agree well with the numerical estimation of the original flux creep equation with the upper critical field term. The magnetization characteristics are also successfully obtained from the pinning parameters