  The surface of a topological insulator hosts Dirac electronic states with the
spin-momentum locking, which constrains spin orientation perpendicular to
electron momentum. As a result, collective plasma excitations in the
interacting Dirac liquid manifest themselves as coupled charge- and spin-waves.
Here we demonstrate that the presence of the spin component enables effective
coupling between plasma waves and spin waves at interfaces between the surface
of a topological insulator and insulating magnet. Moreover, the helical nature
of spin-momentum locking textures provides the phase winding in the coupling
between the spin and plasma waves that makes the spectrum of hybridized
spin-plasma modes to be topologically nontrivial. We also show that such
topological modes lead to a large thermal Hall response.
