Abstract
Electrical resistivity, dc magnetization, ac susceptibility, thermopower, and magnetoresistance measurements have been carried out on Ni-substituted $\mathrm{Gd}\mathrm{Ba}{\mathrm{Co}}_{2}{\mathrm{O}}_{5+\ensuremath{\delta}}$. Structural characterization of the samples was done by x-ray diffraction and results show a systematic variation in the lattice parameters with Ni substitution. Oxygen stoichiometry determined by iodometric titration suggests that oxygen content remains unaltered with Ni substitution. The metal to insulator transition $({T}_{\mathrm{MIT}})$, the paramagnetic to ferromagnetic (FM) transition $({T}_{C})$, and ferromagnetic to antiferromagnetic (AFM) transition $({T}_{N})$ are altered strongly by Ni substitution. Magnetization and magnetoresistance studies carried out enable determination of magnetic phase diagram which suggests the stabilization of an AFM ground state for low Ni fractions, FM state for high Ni fractions, and a mixed phase at intermediate Ni fractions.
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