Behnaz Hadi-Sichani, Hamideh Shakeripour⁎, Hadi Salamati

https://doi.org/10.1016/j.physc.2018.02.018

The Ni- substituted Y_1-xNi_xBa₂Cu₃O_7-δ high temperature superconducting samples with 0 ≤ x < 0.01 were synthesized by the standard solid-state reaction. The temperature dependent resistivity of the samples was measured under magnetic fields in the range of zero to 1 Tesla, applied perpendicular to the current direction. To study of magnetoresistance is one of the most important ways to investigate the intergranular nature of superconducting materials. The resistive transition is made of two parts. The first- unaffected to applied magnetic field part which is near the onset of superconductivity. This region is due to superconductivity in grains. The second- broaden tail part which is due to the connectivity of the grains. At temperatures close to Tc 0, (ρ=0), under applied magnetic fields, weak links are affected and the vortices are penetrated and move inside the intergranular and then the tail part is broaden. This broadening part observed in the electrical resistivity, ρ(T), and  in the derivative of the electrical resistivity, dρ/dT, becomes too small or even absent in Ni doped samples. For pure sample, Tc 0 was around 90 K; by applying a magnetic field H=0.3 T it shifted to 40 K. This broadening is 91.4 K to 80 K for x=0.002 and 91.7 K to 85 K for x=0.004 samples. We found an optimal value of Ni doping concentration which improves the coupling of the grains. Then, vortices get strongly pinned. These observations suggest that the Ni substitution can reduce the weak links and increase the J_c values of these superconductors.