Lower critical field and intragrain critical current density in the ruthenate-cuprate RuSr${}_2$Gd${}_{1.5}$Ce${}_{0.5}$Cu${}_2$O${}_{10}$

The lower critical field of the grains, $H_{c1}$, and the intragrain critical current density, $J_c$, were determined for the superconducting ruthenate-cuprate RuSr${}_2$Gd${}_{1.5}$Ce${}_{0.5}$Cu${}_2$O${}_{10-\delta }$ [Ru-1222(Gd)] through a systematic study of the hysteresis in magnetoresistance loops. A reliable method, based on the effects of the magnetization of the grains on the net local field at the intergranular junctions is provided, circumventing the problem of the strong masking of the superconducting diamagnetic signal by the ferromagnetic background. The temperature dependency of $H_{c1}$ and $J_c$ both exhibit a smooth increase on cooling without saturation down to $T\mathrm{/}{T}_{SC}$ $\cong$ 0.2. The obtained $H_{c1}$ values vary between 150 and 1500 Oe in the 0.2 $\le$ $$ $\le$ 0.4 interval, for samples annealed in an oxygen flow; oxygenation under high pressure (50 atm) leads to a further increase. These values are much larger than the previously reported rough assessments (25-50 Oe), using conventional magnetization measurements. High $J_c$ values of $$ 10${}^7$ A/cm${}^2$, comparable to the high-T${}_c$ cuprates, were obtained. The $H_{c1}(T)$ and $J_c(T)$ dependencies are explained in the context of a magnetic phase separation scenario.