Field Dependence of Magnetization and dM/dH for Sm- and Gd-Doped Bi_1.7Pb_0.3Sr₂Ca_2-xRE_xCu₃O_10+y Compounds

We have performed magnetization measurements as a function of magnetic fields up to 4kOe on Bi_1.7Pb_0.3Sr₂Ca_2-xRE_xCu₃O_10+y (RE: Gd_0.01, Sm_0.03, at different fixed temperatures T< T_c in the range 15≤T≤75K. We have observed three important features. Firstly, the dM-dH curves of both the Sm- and Gd-doped samples are similar and each shows a minimum of M at a certain critical field H_c, above which there appears to be a monotonic decrease in magnitude. Secondly, the magnitude of the initial susceptibility dM/dH, related to the superconducting volume, decreases with increasing temperature. In other words, the decrease of dM/dH with T indicates that the superconducting regions reduce in size and the paramagnetic domains probably grow at the expense of superconductivity. The third feature is the fast decrease of hysteresis loops in both the Gd and Sm doped samples with increasing temperature, which implies existence of flux pinning centres. Thus the results show that the Ca²⁺→ Sm³⁺ and Gd³⁺ substitution decreases the hole carrier concentration and hence the volume fraction of the superconducting phase.