Successive Magnetic Transitions of the Kagome Plane and Field-Driven Chirality in BaMn$_{2.49}$Ru$_{3.51}$O$_{11}$*

The magnetization of single-crystal BaMn$_{2.49}$Ru$_{3.51}$O$_{11}$ exhibits anomalies at temperatures T$_{1}$ = 183 K, T$_{2}$ = 171 K and T$_{3}$ = 128 K, which signal complex magnetic order induced by competing ferro- and antiferro-magnetic correlations, and magnetic frustration within the Kagome (hexagonal \textbf{ab-}) plane. The T$_{2}$- and T$_{3}$-anomalies and unconventional transverse magnetoresistance are observed only for magnetic field \textbf{H} applied in the Kagome plane. We conclude a topological Hall effect (THE) is generated by non-zero scalar chirality K$_{S}$ of spins canted out of the Kagome plane, but is suppressed in a collinear structure induced by only modest in-plane fields. BaMn$_{2.49}$Ru$_{3.51}$O$_{11}$ is a unique example of an unusually large and anisotropic THE in a magnetically ordered state. The THE is driven by nonzero scalar spin chirality and can be controlled by unusually modest applied fields $\mu _{o}H \quad <$ 1 T, implying low-field alteration of scalar spin chirality may provide a new way to control electronic properties in magnetic materials having requisite non-centrosymmetric structure.