MgB_2超伝導体の粒径と交流帯磁率特性(電気工学科)

Abstract

AC susceptibility x of MgB_2 superconductor particles with several particle sizes d were studied. MgB_2 showed symmetric and large magnetization and their susceptibility follows well with the theoretical results of the critical state model. In smaller size samples the values of x became smaller than the calculated value of the critical state model and in some cases double peaks in the imaginary x" and shoulders in the real x' appear. Double layer model is proposed in which the superconductor is composed of two regions with different superconducting characteristics, the surface and the residual inner layer. The magnetization and susceptibility are numerically computed following the critical state model and they show a variety of characteristics including the double peaks and the shoulders. The double layer model is successfully compared with the measurement of MgB_2. The depth of surface layer gets larger in the small size MgB_2 reflecting an improper composition condition.