Prof. Shuichi Murakami (Tokyo Institute of Technology)
"Magnon transport in ferromagnets and Berry phase"
Various types of intrinsic Hall effects in solids such as spin Hall effect are caused by the Berry curvature in momentum space, which characterizes the geometrical and topological properties of the electronic bands. Because the Berry curvature is based on the wave nature of electrons, we can expect similar phenomena in other waves as well. In my talk we take magnons (spin waves) in ferromagnets as an example. In ferromagnetic insulators, magnons are low-energy excitations, and form band structure. Therefore, the Berry curvature from the magnon bands also show Hall effects such as thermal Hall effect. While the thermal Hall effect of magnons have been discussed by other groups, we found correction termsto the previously known results. In addition, the magnon wavepacket should undergo a "cyclotron-like" motion, caused by the Berry curvature, and should also have an anomalous velocity along the edge. We show how these motions of the magnon wavepacket are related with the correction of the thermal Hall effect. These theoretical results apply not only to quantum-mechanical exchange magnons but also to magnetostatic spin waves, e.g. in YIG. We show our numerical results on some ferromagnets. Reference: R. Matsumoto, S. Murakami, Phys. Rev. Lett. 106, 197202 (2011)