Structure and Magnetotransport Properties of High-T$_{C}$ Ferromagnetic Semiconductors, (Ba,Sr)\textit{M}$_{2\pm x}$Ru$_{4\pm x}$O$_{11}$ with \textit{M} = Mn, Fe, Co

We have grown single crystals of R-type ferrites, (Ba,Sr)M$_{2\pm x}$Ru$_{4\pm x}$O$_{11}$ (M = Mn, Fe, Co), with compositions determined from X-ray refinements and microprobe analysis. The hexagonal crystal structure (P6$_{3}$/mmc, Z = 2) consists of (001) layers of edge-sharing (M,Ru)O$_{6}$ octahedra connected within [001] by face-sharing pairs of (M,Ru)$_{2}$O$_{9}$ octahedra and MO$_{5 }$trigonal bipyramids. A significant homogeneity range is generated by variable occupation of octahedral sites by 3d and 4d elements. These compounds are soft ferromagnetic semiconductors with T$_{C}$'s that can substantially exceed room-temperature, depending on composition. The temperature-dependent, in-plane (current parallel to \textbf{ab-}plane) resistivity of SrFe$_{2.51}$Ru$_{3.42}$Al$_{0.07}$O$_{11}$ indicates semiconductivity, and exhibits activated behavior with narrow gap of $\Delta \quad \approx $ 30 meV for T $>$180 K. Hall measurements show the predominant charge carriers are holes; our results suggest these materials are promising for spintronic devices.