  Spin-momentum locking in protected surface states enables efficient
electrical detection of magnon decay at a
magnetic-insulator/topological-insulator heterojunction. Here we demonstrate
this property using the spin Seebeck effect, i.e. measuring the transverse
thermoelectric response to a temperature gradient across a thin film of yttrium
iron garnet, an insulating ferrimagnet, and forming a heterojunction with
(BixSb1-x)2Te3, a topological insulator. The non-equilibrium magnon population
established at the interface can decay in part by interactions of magnons with
electrons near the Fermi energy of the topological insulator. When this decay
channel is made active by tuning (BixSb1-x)2Te3 to a bulk insulator, a large
electromotive force emerges in the direction perpendicular to the in-plane
magnetization of yttrium iron garnet. The enhanced, tunable spin Seebeck effect
which occurs when the Fermi level lies in the bulk gap offers unique advantages
over the usual spin Seebeck effect in metals and therefore opens up exciting
possibilities in spintronics.
