MgB_2超伝導体の帯磁率とスケーリング則(電気工学科,工学部創設40周年記念号)

Abstract

From the real part χ' and the imaginary part χ" of AC susceptibilities, the essential features of fluxoid motion in superconductors are obtained. The χ' indicates the flux penetration and χ" gives the lossy component. The peak of χ" gives an information of the irreversibility field B_<irr>. AC susceptibilities were measured on powdered new superconductor MgB_2 using a SQUID magnetometer and a PPMS susceptometer at temperature range 4.5 - 40 K under DC magnetic fields up to 14 T. Powdered MgB_2 were divided into three samples with different particle size d by sieves with several mesh sizes : 45 < d < 50 μm, 50 < d < 63 μm and 75 < d < 100 μm. The critical temperatures are 38.5 K in all samples determined by the temperature dependence of magnetization under zfc and fc conditions. The AC susceptibilities are characterized from the ratio of superposed AC field amplitude and the characteristic pinning field b_p which is proportional to the pinning strength F_p. The 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_<irr>)γ(1 - B/B?<irr>)^δ, where B^<irr> is the irreversibility field, γ, δ and m are the pinning parameters. The temperature and field dependence of χ' and χ"can be numerically estimated using the scaling law and are favorably compared with the observed data of MgB_2 superconductors.