Topological magnon bands in ultra-thin film pyrochlore iridates and iron jarosites

We describe recent theoretical efforts of our group aimed at exploring two-dimensional magnetic systems described by local moment models that possess topological band structures of the low-energy spin fluctuations. We study two systems: (1) Ultra-thin film pyrochlore iridates grown along the [111] direction and (2) the iron jarosites. For the thin film pyrochlore iridates, we consider a bilayer and trilayer and find that in the trilayer case the ground state is the all-in–all-out (AIAO) state, whereas the bilayer has a deformed AIAO state. For range of parameters the lowest magnon band in the trilayer and bilayer case has a nonzero Chern number. We calculate the magnon Hall response for both geometries, finding a striking sign change as a function of temperature. For the iron jarosites, we study magnetic and topological properties of antiferromagnetic kagome spin systems in the presence of both in- and out-of-plane Dzyaloshinskii-Moriya interactions. The in-plane interactions stabilize a canted noncollinear “umbrella” magnetic configuration with finite scalar spin chirality. We derive expressions for the canting angle and use the resulting order as a starting point for a spin-wave analysis. We find topological magnon bands, characterized by nonzero Chern numbers. We calculate the magnon thermal Hall conductivity and propose the iron jarosites as a promising candidate system for observing the magnon thermal Hall effect in a noncollinear spin configuration. We also show that the thermal conductivity can be tuned by varying an applied magnetic field or the in-plane Dzyaloshinskii-Moriya strength. In contrast to previous studies of topological magnon bands, the effect is found to be large even in the limit of small canting.

P. Laurell and G. A. Fiete, Phys. Rev. Lett. 118, 177201 (2017)
P. Laurell and G. A. Fiete, Phys. Rev. B 98, 094419 (2018)