  A detailed study of the effect caused by the partial substitution of Ru by Ir
on the magnetic and superconducting properties of the ruthenocuprate
Ru(1-x)Ir(x)Sr2GdCu2o8; 0 <= x <= 0.10; is presented. The combined experimental
results of structural, electrical, and magnetic measurements indicate that Ir
substitutes Ru for x <= 0.10 with no significant structural distortions.
Ir-doping gradually suppresses both the magnetic and the superconducting
states. However, all samples were observed to attain the zero-resistance state
at temperatures >= 2 K up to the highest applied magnetic field of 18 T. The
resistive upper-critical field Hc2 as a function of temperature has been
determined for these polycrystalline samples. Values of Hc2(0) were found to be
\~ 52 T, and weakly dependent on the Ir concentration. We have also observed
that the superconducting transition width decreases and the slope of the
resistive transition increases with increasing Ir doping, a feature which is
much more pronounced at high applied magnetic fields. The double-peak structure
observed in the derivative of the resistive curves has been related to an
inhomogeneous nature of the physical grains which is enhanced due to the Ru
substitution by Ir. This indicates that the Josephson-junction-array (JJA)
model seems to be appropriated to describe the superconducting state in these
ruthenocuprates. The low temperature rho(T) data along with the determined
vortex thermal activation energy are consistent with a 2D vortex dynamics in
these materials.
