Ordering phenomena of spin trimers accompanied by a large geometrical Hall effect

The wave function of conduction electrons moving in the background of a noncoplanar spin structure can gain a quantal phase-the Berry phase- A s if the electrons were moving in a strong fictitious magnetic field. Such an emergent magnetic field effect is approximately proportional to the solid angle subtended by the spin moments on three neighboring spin sites, termed the scalar spin chirality. The entire spin chirality of the crystal, unless macroscopically canceled, causes the geometrical Hall effect of real-space Berry-phase origin, whereas the intrinsic anomalous Hall effect in a conventional metallic ferromagnet is of momentum-space Berry-phase origin induced by relativistic spin-orbit coupling. Here, we report the ordering phenomena of the spin-trimer scalar spin chirality and the consequent large geometrical Hall effect in the breathing kagome lattice compound Dy3Ru4Al12, where the Dy3+ moments form noncoplanar spin trimers with local spin chirality. Using neutron diffraction, we show that the local spin chirality of the spin trimers as well as its ferroic/antiferroic orders can be switched by an external magnetic field, accompanying large changes in the geometrical Hall effect. Our finding reveals that systems composed of tunable spin trimers can be a fertile field to explore large emergent electromagnetic responses arising from real-space topological magnetic orders.