  The Hall effect usually occurs when the Lorentz force acts on a charge
current in a conductor in the presence of perpendicular magnetic field. On the
other hand, neutral quasi-particles such as phonons and spins can carry heat
current and potentially show the 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 wave is influenced by the Dzyaloshinskii-Moriya
spin-orbit interaction, which plays the role of the vector potential as in the
intrinsic anomalous Hall effect in metallic ferromagnets.
