A Novel Class of High-TC Ferromagnetic Semiconductors

We have grown single crystals of novel ruthenates (Sr,Ba)(Fe,Co){sub 2+x}Ru{sub 4-x}O{sub 11} that exhibit long-range ferromagnetic order well above room temperature, accompanied by narrow-gap semiconducting properties that include a large anomalous Hall conductance, low resistivity, high carrier concentration and low coercive field, which are properties well suited to spintronic applications. X-ray diffraction, EDX, neutron diffraction and x-ray absorption measurements on single crystals firmly establish the 'R-Type' hexagonal ferrite structure (space group P6{sub 3}/mmc, No 194) and single-phase nature of all samples. The electronic structure and physical properties can be tuned by simple chemical substitution of two elements, M = Fe or Co, or by varying the relative concentration of 3d solutes and 4d Ru. Our magnetotransport, x-ray magnetic circular dichroism and magnetic moment data suggest the mechanism for FM order is quite different from that governing known dilute magnetic semiconductors