I-V curves and intergranular flux creep activation energy in the magnetic superconductor RuSr2GdCu2O8

A systematic study of I-V characteristic curves for RuSr2GdCu2O8 [Ru-(1212)] is presented, with magnetic fields up to 3 T and 5 K<T<30 K, in the region of the superconducting transition. The activation energy E_{a}(H,T) for flux line depinning was determined by fitting the nonlinear region of the curves using the flux creep model. E_{a}(H,T) was found to vary linearly with temperature, while a power-law dependence on the magnetic field was observed up to H=0.1 T, where an abrupt reduction in its decreasing rate occurs. The extrapolated value, E_{a}(0,0)=50 meV, is twice the reported value for YBa2Cu3O7, but the critical current density J_{C}(0,0)=70 A/cm2 is about one order of magnitude lower. These results are explained as a consequence of the contribution of the magnetization in the grains to the effective field at the intergranular links and to a spin-flop transition of the Ru-sub-lattice.