Structural, Magnetic and Transport Properties of a New Class of Ferromagnetic Semiconductors/Metals: (Ba, Sr)M$_{2\pm x}$Ru$_{4-x}$O$_{11}$ (M = Fe, Co)

Single crystals (mm size) of (Ba, Sr)Fe$_{2+x}$Ru$_{4-x}$O$_{11}$ and (Ba, Sr)Co$_{2+x}$Ru$_{4-x}$O$_{11}$ were grown for the first time. X-ray refinements confirmed a hexagonal space group (P6$_{3}$/mmc, No. 194) with two crystallographic sites having mixed Ru and Fe/Co occupation, and one site occupied exclusively by the 3d species. Structural parameters and charge balance suggest oxidation states Co$^{2+}$ and mixed Ru$^{3+}$/Ru$^{5+}$ in the Co compound, and mixed Fe$^{2+}$/Fe$^{3+}$ and Ru$^{3+}$/Ru$^{5+}$ in the Fe compound. The physical properties of these single crystals are sensitive to site disorder among the transition metal ions. Magnetic and transport measurements show the Co-bearing barium ferrite is a ferromagnetic metal below 105 K. In contrast, Fe-bearing barium and strontium compounds exhibit long-range ferromagnetic order at temperatures above 400 K, and narrow-gap semiconducting properties that include a large anomalous Hall conductance, low resistivity, and high carrier concentration. These characteristics make the Fe-bearing materials promising new candidates for spintronic applications.