Effects of Fe and Co substitution on phase formation and superconducting properties of Bi2223 bulk materials

The pellet samples with the nominal compositions of Bi1.8Pb0.4Sr2.0Ca2.1Cu3 .0-XMXO10+delta(M=Fe and Co, x=0 similar to0.10) 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 exami ned. 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% O-2. Furthermore, the formation rate gr eatly decreases with increasing the Fe and Co substitution in 20% O-2, and shows less dependency on the amount of the substitution in 4% O-2. The crit ical temperature, Tc decreases linearly with increasing the Fe and Co subst itution. This suggests that Fe and Co atoms are substituted for Cu site. Th e magnetization hysteresis width, DeltaM and the irreversibility field, H-i rr which are calculated from magnetic hysteresis curves at 77 K show the ma ximum at a Fe substitutive amount of 0.02. For the Co substitution, on the contrary, both DeltaM and H-irr decrease greatly with increasing the substi tution 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 som e pinning centers are introduced into the Bi2223 crystals by the Fe substit ution. Our present experiments did not elucidate the nature of the pinning centers.