  Ferromagnetic resonance is used to reveal features of the buried electronic
band structure at interfaces between ferromagnetic metals and topological
insulators. By monitoring the evolution of magnetic damping, the application of
this method to a hybrid structure consisting of a ferromagnetic layer and a 3D
topological insulator reveals a clear fingerprint of the Dirac point and
exhibits additional features of the interfacial band structure not otherwise
observable. The underlying spin-pumping mechanism is discussed in the framework
of dissipation of angular momentum by topological surface states (TSSs). Tuning
of the Fermi level within the TSS was verified both by varying the
stoichiometry of the topological insulator layer and by electrostatic
backgating and the damping values obtained in both cases show a remarkable
agreement. The high energy resolution of this method additionally allows us to
resolve the energetic shift of the local Dirac points generated by local
variations of the electrostatic potential. Calculations based on the chiral
tunneling process naturally occurring in TSS agree well with the experimental
results.
