  Samples of superconducting composite (100-x)YBa2Cu3O7-{\delta} + xNi (x = 1,
2.5, 5, 7.5, 10 and 15 wt.%) were synthesized. Scanning electron microscope
(SEM) images revealed that the grain size of YBCO reduces gradually as Ni
content is increased. At the same time segregation of Ni as NiO in patches
forms throughout the composite. At low Ni wt.%, it was found that the
superconducting state properties improved and then dropped with higher Ni wt.%.
Segregated patches of NiO on different length scales were observed. The change
in the structural and superconducting state parameters of YBCO and the
variation of the magnetic critical current density with improved flux pinning
in some composites with Ni additives were ascribed to the Jahn-Teller (JT)
distortions and pinning of vortices due to antiferromagnetic NiO patches within
the samples.
