Effect of doping with magnetic 3D-elements on the thermal fluctuations and critical parameters of CaLaBaCu3-x(Ni,Co)xO7-δ superconductors

Systematic measurements on conductivity fluctuation in the CaLaBaCu3-x(Ni,Co)xO7-δ system are reported. Samples with x=0, 0.03, 0.06, 0.09, 0.12, 0.15 and 0.18 were prepared by the standard solid-state reaction recipe. Results of resistivity measurements reveal a linear-like decreasing of the critical temperature Tc with progressive substitution of magnetic elements Ni and Co into the Cu crystallographic sites. From the fluctuation analysis, above and close to Tc, we found the occurrence of three- and two-dimensional Gaussian fluctuation regimes. Closer to Tc, a genuinely critical regime is observed. On the Ginzburg-Landau formalism, from the reduced temperature of the three-dimensional Gaussian region and the mean field critical temperature, we have experimentally obtained the Ginzburg number for the CaLaBaCu3-x(Ni,Co)xO7-δ material. Then, critical magnetic field, critical current density and the jump in the specific heat at the critical temperature are calculated. Critical parameters are strongly affected by the doping with magnetic ions