Magnetic structure, magnetization, and magnetotransport properties of (Ba,Sr) M_2±x T_4∓x O₁₁ (M=Fe,Co; T=Ru,Ti)

Hexagonal R -type ferrites (Ba,Sr) M_2±x T _4∓x O₁₁ (M=Fe,Co; T=Ru,Ti) were studied via neutron and x-ray diffraction, magnetization, and electrical transport measurements. Magnetization data for single-crystal BaFe_3.26 Ti_2.74 O₁₁ reveal two magnetic transitions at T₁=250 K and T ₂ =84 K, which indicates complex magnetic order driven by competing interactions on a frustrated lattice with a noncentrosymmetric structure. Magnetization data for single-crystal BaCo_1.85 Ru_4.15 O₁₁ reveal soft ferromagnetic order at T_C =105 K with an easy direction perpendicular to the c axis. Neutron diffraction data for polycrystalline BaCo_1.68 Ru_4.32 O₁₁ indicate a nearly compensated arrangement of spins lying within the a-b plane with a possible canting out of the plane that yields nonzero scalar spin chirality. The transverse magnetoresistivity ρ_xy of single-crystal BaCo _1.85 Ru_4.15 O₁₁ for current J||H⊥c axis is typical of an anomalous Hall effect observed in ferromagnets, whereas for J||H⊥c axis, ρxy is a nonmonotonic function of the magnetic field, consistent with a topological Hall effect that depends upon scalar spin chirality. Neutron diffraction reveals transitions to ferrimagnetic order at T_C =262 (1) K for single-crystal SrFe_2.6 Ru_3.4 O₁₁, and at T_C =403 K for polycrystalline SrFe _2.96 Ru_3.04 O₁₁ (refined as a collinear structure at T=5 K).