Estimation of the Penetration Depth and Study of the Magnetic Characteristics in Bi-Pb-Sr-Ca-Cu-O Superconductors by EPR Measurements

In this work, two polycrystalline samples were prepared using the solid-state reaction technique. The X-ray diffraction patterns revealed that the samples A and B are a mixture of the superconducting phases (Bi, Pb)-2212 (Bi_1.6Pb_0.4Sr₂CaCu₂O₈) and (Bi, Pb)-2223 (Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O₁₀). The \(T_c\) was measured using electron paramagnetic resonance (EPR) in absorption mode: 97 K for the sample A and 101 K for sample B. The EPR spectra were measured (derivative of the absorption) and showed a signal in the low-field region only present in the superconducting state. The study of this signal revealed the coexistence of antiferromagnetism and diamagnetism in the sample A, for \(T < T_{c}\). The same was observed in sample B after a light grinding. The effective penetration depth was estimated from the peak-to-peak linewidth of the EPR signal (at T = 92 K): \(\lambda \)(92 K) = 479 nm for sample A, and 440 nm for sample B. These results are in agreement with the values obtained using techniques such as \(\mu \)-spin rotation and magnetization measurements. With the aid of the two-fluid model and the BCS approximation \(\lambda \) was also estimated at 0 K. Using these values and the Ginzburg-Landau theory, the lower critical field was calculated at T = 92 K and 0 K.