Decoupled CuO 2 and RuO 2 layers in superconducting and magnetically ordered RuSr 2 GdCu 2 O 8

Comprehensive measurements of dc and ac susceptibility, dc resistance, magnetoresistance, Hall resistivity, and microwave absorption and dispersion in fields up to 8 T have been carried out on ${\mathrm{RuSr}}_{2}{\mathrm{GdCu}}_{2}{\mathrm{O}}_{8}$ with the aim to establish the properties of ${\mathrm{RuO}}_{2}$ and ${\mathrm{CuO}}_{2}$ planes. At $\ensuremath{\sim}130 \mathrm{K},$ where the magnetic order develops in the ${\mathrm{RuO}}_{2}$ planes, one observes a change in the slope of dc resistance, change in the sign of magnetoresistance, and the appearance of an extraordinary Hall effect. These features indicate that the ${\mathrm{RuO}}_{2}$ planes are conducting. A detailed analysis of the ac susceptibility and microwave data on both ceramic and powder samples show that the penetration depth remains frequency dependent and larger than the London penetration depth even at low temperatures. We conclude that the conductivity in the ${\mathrm{RuO}}_{2}$ planes remains normal even when superconducting order is developed in the ${\mathrm{CuO}}_{2}$ planes below $\ensuremath{\sim}45 \mathrm{K}.$ Thus, experimental evidence is provided in support of theoretical models which base the coexistence of superconductivity and magnetic order on decoupled ${\mathrm{CuO}}_{2}$ and ${\mathrm{RuO}}_{2}$ planes.