  The intergranular pinning potential U and the critical current density Jc for
polycrystalline RuSr2Gd1.5Ce0.5Cu2O10 ruthenate-cuprate were determined at zero
magnetic field and temperature through the frequency shift in the peak of the
imaginary part of the ac magnetic susceptibility, X". A critical state model,
including a flux creep term, was found to accurately describe the X" behavior.
The obtained values, U(H=0,T=0) ~ 30 meV and Jc(H=0,T=0) ~ 110 A/cm^2 are about
two orders of magnitude and four times lower, respectively, in comparison with
the high-Tc cuprate YBa2Cu3O7. These results were ascribed to the effects of
the Ru magnetization on the connectivity of the weak-linked network, giving an
intrinsic local field at the junctions of ~ 15 Oe. The impact on Jc is less
intense because of the small average grain radius (~1 micrometer). The
intragranular London penetration length at T = 0, [lambda_L(0) ~ 2 micrometer],
was derived using a Kim-type expression for the field dependence of Jc. A
possible source for the large value of lambda_L in comparison to the high-Tc
cuprates is suggested to come from a strong intragrain granularity, due to
structural domains of coherent rotated RuO6 octahedra separated by antiphase
boundaries.
