(Ba, K)Fe2As2 超伝導体の高密度多結晶試料の合成とその臨界特性

Abstract

A bulk iron arsenide superconductor of (Ba, K)Fe2As2 was synthesized by a combined process of high temperature heat treatment and deformation using a Ta sheath. Observations by an optical microscope and a scanning electron microscope and the result of x-ray diffraction indicate that the sample has a polycrystalline dense structure with much less impurity phases compared to the samples prepared by conventional sintering process. Magneto-resistance and magnetization measurements have been carried out on the obtained (Ba, K)Fe2As2 sample in order to evaluate superconducting critical properties. The temperature dependence of upper critical field, -dHc2(T)/dT is~6.3 T/K, which corresponds to the value of single crystals. The temperature dependence of irreversibility fields, -dHirr(T)/dT, has also a steep slope at about 10 K below Hc2(T), suggesting that (Ba, K)Fe2As2 superconductors has good potential for magnetic applications at medium temperatures like MgB2. However, the global Jc at 5 K estimated from the magnetization hysteresis results of bulk and powder samples and the transport Jc at 20 K measured by a standard four probe method are as low as ~2×107 A/m2 and ~105 A/m2, respectively, in applied magnetic fields. We also estimated the local Jc from the magnetization hysteresis of powdered sample, which are the order of 109 A/m2 at 5 K and 108 A/m2 at 20 K. Those results indicate that current path is still largely limited in the bulk sample despite its high density and weak link at the grain boundary is most plausible origin of current limitation in the bulk sample.