Effect of MgB2 addition on thermal fluxon motions of two-dimensional pancake vortices in Bi-2223 superconducting ceramics

Abstract

This study reports the variations of magnetoresistivity performance and pinning ability for the bulk Bi1.8Pb0.4Sr2(MgB2)xCa2.2Cu3.0Oy ceramic materials with the MgB2 addition (0 ≤ x ≤ 0.5) in the crystal structure by magnetotransport measurements performed in the applied magnetic fields (0–7T). According to the results, the maximum critical transition temperature values (Tcoffset and Tconset) are about 114.1 K and 121.3 K for the best sample exposed to x = 0.05 content level of MgB2 addition while the minimum Tcoffset and Tconset are obtained to be 109.4 K and 118.6 K for the pure sample at zero applied magnetic field, respectively. Besides, the highest Tcoffset value of 69.6 K is observed for the former sample at 7 T external field strength. This is attributed to the fact that the existence of optimum MgB2 in the Bi-2223 crystal structure results in the increased nucleation center formations so that the binding between the dopants and adjacent layers can pin the fluxon motions of two-dimensional (2D) pancake vortices at higher magnetic field such a value of 7 T. On the other hand, the main reason of decrement in the Tcoffset parameters stems from the pair-breaking mechanism based on the changed spin states of cooper-pair electrons. Moreover, the flux pinning energy parameters estimated from thermally activated flux creep model tend to augment considerably with the increment of MgB2 content level up to x = 0.05. Additionally, the coherence length and penetration depth parameters as well as the irreversibility and upper critical fields confirm the positive effect of optimum MgB2 addition on the elastic moduli of vortex lattice, Josephson coupling length, magnetic performance and especially pinning ability throughout the Bi-2223 crystal structure.