Bi2223相の生成と超伝導特性に及ぼすFe, Coの置換効果

Abstract

The pellet samples with the nominal compositions of Bi1.8Pb0.4Sr2.0Ca2.1Cu3.0−XMXO10+δ (M=Fe and Co, x=0∼0.10) were prepared by the solid state reaction method, and the effect of Fe and Co substitution for Cu on the formation of Bi2223 phase and superconducting properties has been examined. The formation rate of Bi2223 phase strongly depends on the atmosphere (oxygen partial pressure) of the heat treatment, increasing with decreasing oxygen partial pressure down to 4%O2. Furthermore, the formation rate greatly decreases with increasing the Fe and Co substitution in 20%O2, and shows less dependency on the amount of the substitution in 4%O2. The critical temperature, Tc decreases linearly with increasing the Fe and Co substitution. This suggests that Fe and Co atoms are substituted for Cu site. The magnetization hysteresis width, ΔM and the irreversibility field, Hirr which are calculated from magnetic hysteresis curves at 77 K show the maximum at a Fe substitutive amount of 0.02. For the Co substitution, on the contrary, both ΔM and Hirr decrease greatly with increasing the substitution amount. The grain size of the samples scarcely depends on the amount of Fe and Co, showing almost the same size. These results suggest that some pinning centers are introduced into the Bi2223 crystals by the Fe substitution. Our present experiments did not elucidate the nature of the pinning centers.