Significant development on pinning of vortices in Y-123 superconductor with homovalent Ba/Nd substitution

In this study, the magnetotransport measurements are performed in the field range of 0–7 T to determine the effect of Nd nanoparticles inserted in the Y-123 crystal structure on the superconducting characteristics and flux pinning mechanism of the YBa_2−yNd_yCu₃O₇ materials (0.000 ≤ y ≤ 0.500). Moreover, the experimental measurement results enable us to evaluate the crucial parameters as regards critical transition temperatures (\(T_{c}^{onset}\) and \(T_{c}^{offset}\)), room temperature resistivities (ρ₃₀₀), residual resistivity ratios (RRR), thermodynamic critical fields (μ₀H_c), lower critical fields (μ₀H_c1), upper critical fields (μ₀H_c2), critical fields (μ₀H_c3), irreversibility fields (μ₀H_irr), coherence lengths (ξ), penetration depths (λ) and Ginzburg–Landau (\(\kappa\)) parameters. All the values obtained show the considerable improvement with the increment of the Nd nanoparticles in the Y-123 crystal system up to the critical value of y = 0.250 beyond which the prompt degradation begins in the crystal structure due to new induced artificial dislocations and permanent disorders in the Cu–O₂ consecutively stacked layers. Hence, the excess Nd inclusions lead to the thermal motion of the vortices easily. This fact is completely confirmed by the magnetotransport measurements performed in the external magnetic fields. Namely, the optimum level of Nd dopant seriously increases the coupled regions as a result of the degradation of pancake positional fluctuations, and pinning of two-dimensional pancake vortices enhances with the recoupling of the adjacent layers. However in case of the maximum Nd inclusions, the flattened pancakes with more interplane interaction energy never travel from layer to layer due to the alteration of the coupled vortices into the discrete pancakelike (recoupling linelike) nature. In other words, the excess Nd nanoparticles inserted in the Y-123 crystal system reduce significantly the interlayer Josephson coupling length, elastic moduli of the vortex lattice and magnetic field carrying capacity. Furthermore, the flux pinning energy (U₀) is calculated from thermally activated flux flow model. The activation energy computed increases regularly with the increment of Nd decorations until y = 0.250 after which the value significantly decreases towards to the global minimum point. However, the pinning energy values always decrease with the increase of the external magnetic field strength. In case of external magnetic fields of 7 T the maximum activation energy is found to be about 477 K. As for the Y-123 material doped with maximum Nd additives of y = 0.500, the flux pinning energy values are not calculated at any applied magnetic field which is >0.1 T due to the cooper pair-breaking in Ba-site Nd substituted Y-123 systems. At the same time, the evaluated parameters follow the same trend: firstly enhancement up to the critical Nd dopant value of y = 0.250 and then the rapid regression towards the global minimum point. Finally, the optimum Nd impurity level is found to be y = 0.250 in the Y-123 system at even higher applied magnetic field strengths.