  We study the thermoelectric properties of a Kramers pair of helical edge
states of the quantum spin Hall effect coupled to a nanomagnet with a component
of the magnetization perpendicular to the direction of the spin-orbit
interaction of the host. We show that the transmission function of this
structure has the desired qualities for optimal thermoelectric performance in
the quantum coherent regime. For a single magnetic domain there is a power
generation close to the optimal bound. In a configuration with two magnetic
domains with different orientations, pronounced peaks in the transmission
functions and resonances lead to a high figure of merit. We provide estimates
for the fabrication of this device with HgTe quantum-well topological
insulators.
