Magnetotransport properties of doped ${\mathrm{RuSr}}_2{\mathrm{GdCu}}_2{\mathrm{O}}_8$

${\mathrm{RuSr}}_2{\mathrm{GdCu}}_2{\mathrm{O}}_8,$ in which magnetic order and superconductivity coexist with $T_{\mathrm{mag}}\gg T_c,$ is a complex material which poses new and important questions to our understanding of the interplay between magnetic and superconducting order. Resistivity, Hall-effect, and thermopower measurements on sintered ceramic ${\mathrm{RuSr}}_2{\mathrm{GdCu}}_2{\mathrm{O}}_8$ are presented, together with results on a broad range of substituted analogs. The Hall effect and thermopower both show anomalous decreases below $T_{\mathrm{mag}},$ which may be explained within a simple two-band model by a transition from localized to more itinerant behavior in the ${\mathrm{RuO}}_2$ layer at $T_{\mathrm{mag}}.$