Logarithmic contribution to the electrical resistivity in (Ru1-xIrx)Sr2GdCu2O8 compounds

A systematic study of magnetoresistance and dc magnetization was conducted in polycrystalline (Ru1-xIrx)Sr2GdCu2O8 [(Ru,Ir)-1212] compounds, for 0 <= x <= 0.15. We found that a deviation from linearity in the normal-state electrical resistivity (rho) curves for temperatures below the magnetic transition temperature T-M < 130 K can be properly described by a logarithmic term. The prefactor C(x, H) of this anomalous ln T contribution to rho(T) increases linearly with the Ir concentration, and diminishes rapidly with increasing applied magnetic field up to H approximate to 4 T, merging with the C(0,H) curve at higher magnetic fields. Correlation with magnetic susceptibility measurements supports a scenario of local perturbations in the orientation of Ru moments induced in the neighborhood of the Ir ions, therefore acting as scattering centers. The linear dependence of the prefactor C(x,H=0) and the superconducting transition temperature T-SC on x points to a common source for the resistivity anomaly and the reduction in T-SC, suggesting that the CuO2 and RuO2 layers are not decoupled.