AC Magnetic Susceptibility of Y3Ba5Cu8O18 Substituted by Nd3+ and Ca2+ Ions

Effect of Nd3+ and Ca2+ ion substitutions in Y3Ba5Cu8O18 (Y-358) was studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), electrical resistivity, and ac magnetic susceptibility measurements. Superconducting samples of type Y3 − xNdxBa5 − yCayCu8O18 (with 0 ≤ x, y ≤ 0.4) were synthesized by the standard solid-state reaction technique. XRD patterns revealed that all the samples were mainly composed of Y-358 phase with orthorhombic structure. SEM micrographs showed that Nd3+ and Ca2+ ion substitutions had the ability to alter the grain morphology of Y-358 to irregular randomly oriented grains with smaller average grain sizes. The temperature-dependent electrical resistivity curves indicated that the superconducting transition temperature Tc was decreased from 96.51 to 87.41 K as the substitution contents were increased from 0 to 0.4. Detailed AC magnetic susceptibility measurements were done using a quantum design physical property measurement system (PPMS) at different dc fields (B = 0.5–20 kG) and frequency of 1 kHz. A comparison between electrical resistivity and AC magnetic susceptibility data was performed. It was found that Tcs obtained from AC magnetic susceptibility were much lower than those obtained from the resistivity measurements, for all studied samples. This was discussed in terms of filamentary paths that could produce sharp pronounced drops in diamagnetism. Generally, the depression in Tc was attributed to the decrease in relative volume fraction of Y-358 or due to trapping of mobile holes or other mechanism connected with oxygen vacancy disorder. Werthamer–Helfand–Hohenberg (WHH) theory was employed to study the temperature dependence of the upper critical magnetic field Bc2(T) for the samples. The relative importance of both orbital and paramagnetic effects in the suppression of superconductivity was described with the aid of Maki parameter (κM). The estimated values of the upper critical magnetic field at 0 K, Bc2(0), were found to decrease from 37.3 to 13.5 T as Nd3+ and Ca2+ contents were increased from 0 to 0.4. This decrement was attributed to the weak electron–phonon coupling and to the orbital as well as spin pair breaking mechanisms. Furthermore, Ginzburg–Landau coherence length ξGL of Y3 − xNdxBa5 − yCayCu8O18 was found to increase with the increase of the Nd3+ and Ca2+ contents. Therefore, Nd3+ and Ca2+ ions play an important role in the pair-breaking mechanism and in suppressing superconductivity of the Y-358 phase.