Anomalous Hall effect in the high-temperature ferrimagnetic semiconductors BaFe_2±xRu_4±xO11

Field and temperature dependences of the magnetic and transport properties of single crystals of BaFe2+x Ru4-x O11 (x=0.5,0.9,1.4) R-type ferrites are reported. A wide homogeneity range of Ru/Fe compositions allows to tune the strength of the anomalous Hall effect (AHE), the width of the semiconducting gap (Δ), and the temperature of magnetic ordering (TC) within one structural family. These parameters are enhanced for higher Fe content. A large Hall resistivity ρxy =77μΩcm observed in single-crystal BaFe3.4 Ru2.6 O11 at 300 K is several orders of magnitude large than that observed for bulk magnetic materials, and comparable to the Hall resistivity of thin films of magnetite. The anomalous Hall coefficient Rs changes sign at a characteristic temperature T^- that depends on the Fe/Ru ratio, which we attribute to variations in the electronic band structure. The Rs vs longitudinal resistivity (ρxx) dependence of BaFe3.4 Ru2.6 O11 can be described in terms of negative skew and positive side-jump scattering contributions that differ above and below T^-. The very small side jump Δy estimated from the contribution to the AHE that is quadratic in ρxx suggests that the AHE is caused by a Karplus-Luttinger (Berry phase) mechanism.