Observation of the Magnon Hall Effect

Observing the Magnon Hall Effect The anomalous thermal Hall effect occurs when transverse heat transport is generated in the absence of an external magnetic field. The transport may be mediated by free carriers, phonons, or spin waves (magnons). Onose et al. (p. 297) observed this effect in an insulating ferromagnet and were able to rule out free carriers and phonons as the mediators of the thermal transport. Instead, the observations are consistent with a theory ascribing its origin to magnon propagation. This thermal magnon Hall effect is caused by the spin-orbit interaction, which acts in a fashion similar to the magnetic field in the conventional Hall effect. Its observation in an insulator may enable the elimination of losses in spintronic applications. Spin excitations in an insulating ferromagnet exhibit an anomalous thermal Hall effect. The Hall effect usually occurs in conductors when the Lorentz force acts on a charge current in the presence of a perpendicular magnetic field. Neutral quasi-particles such as phonons and spins can, however, carry heat current and potentially exhibit the thermal Hall effect without resorting to the Lorentz force. We report experimental evidence for the anomalous thermal Hall effect caused by spin excitations (magnons) in an insulating ferromagnet with a pyrochlore lattice structure. Our theoretical analysis indicates that the propagation of the spin waves is influenced by the Dzyaloshinskii-Moriya spin-orbit interaction, which plays the role of the vector potential, much as in the intrinsic anomalous Hall effect in metallic ferromagnets.