Theory of the Spin Seebeck Effect at a Topological-Insulator/Ferromagnetic-Insulator Interface

TL;DR

This paper develops a theoretical model for the spin-Seebeck effect at a topological insulator/ferromagnetic insulator interface, explaining voltage signals via TI surface states scattering off magnons, aligning with recent experimental results.

Contribution

It introduces a Dirac model-based theory for the spin-Seebeck effect at TI/FI interfaces, generalizable to any 2D conductor exchange-coupled to a ferromagnetic insulator.

Findings

Calculated spin-Seebeck voltages match experimental data for Bi2Te3 and yttrium iron garnet.

The theory predicts voltage signals depend on surface-state carrier density.

Surface states scattering off magnons induce measurable voltages in TI/FI systems.

Abstract

The spin-Seebeck effect refers to voltage signals induced in metals by thermally driven spin currents in adjacent magnetic systems. We present a theory of the spin-Seebeck signal in the case where the conductor that supports the voltage signal is the topologically protected two-dimensional surface-state system at the interface between a ferromagnetic insulator (FI) and a topological insulator (TI). Our theory uses a Dirac model for the TI surface-states and assumes Heisenberg exchange coupling between the TI quasiparticles and the FI magnetization. The spin-Seebeck voltage is induced by the TI surface states scattering off the nonequilibrium magnon population at the surface of the semi-infinite thermally driven FI. Our theory is readily generalized to spin-Seebeck voltages in any two-dimensional conductor that is exchange-coupled to the surface of a FI. Surface-state carrier-density-dependent signal strengths calculated using Bi$_2$Te$_3$ and yttrium iron garnet material parameters are consistent with recent experiments.