Crystal structures of ternary ruthenium ferrites SrM _2±xRu_4∓xO₁₁ with M = Fe, Co and magnetic and transport properties of Al-doped single crystals

Single crystals of SrM_2±xRu_4∓xO ₁₁ with M = Fe, Co with maximum width of 2 mm and a thickness around 0.05 mm were grown from a chloride flux. Sample compositions were determined from refinements of X-ray diffraction data and microprobe analysis. The hexagonal crystal structure (space group P6₃/mmc, Z= 2) consists of basal-plane layers of edge-sharing octahedra (M,Ru)O₆, which are connected along [001] by face-sharing pairs of octahedra (M,Ru) ₂O₉ and trigonal bipyramids MO₅. A significant homogeneity range is generated by variable occupation of the octahedral sites by 3d and 4d elements. Aluminum is also found to substitute for transition elements in the octahedral sites. The title compounds are narrow-gap semiconductors and soft ferromagnets with Curie temperatures that can substantially exceed room-temperature, depending on composition. Aluminum doping significantly modifies the transport properties, but magnetic properties such as the Curie temperature, magnetization and anisotropy are less affected. Hall coefficient measurements reveal the predominant charge carriers are holes. The structural, electronic and magnetic properties suggest that these solid solutions are promising candidates for spin-injection devices.