MgB_2とHg1223超伝導体における磁化とスケーリング則(電気工学科,工学部創設40周年記念号)

Abstract

From the shape of magnetization curves, the important superconduction characteristics can be evaluated. The critical current densities, the irreversibility fields and the losses of the superconductors can be estimated from hysteretic width, disappearance of the width and the area of the hysteresis loop, respectively. Magnetic field and temperature dependence of the pinning strength F_p in type-II superconductors is known to be well formalized by the scaling law of the pinning force as F_p = AH^^m_<c2>(B/B^<c2>)^γ(1 - B/B_^c2>)^δ, where H_<c2> is the upper critical field, γ, δ and m are the pinning parameters. In high temperature superconductors, the scaling law fails to describe the pinning properties since the irreversibility field B_<irr> is much smaller than the upper critical field B_<c2>. Conversion of field dependence term in the scaling law is accomplished by replacing the B_<c2> by the B_<irr>. The magnetization curves differentially computed using the scaled J_c with the pinning parameters of γ, δ and m show various shapes and can be fitted well to the observed curves of MgB_2 and Ag-Hg1223 superconductors in the wide range of temperature and magnetic field.