We report the absorption of a diffusive spin current in a bulk-carrier compensated topological insulator: Sn 0.02 -Bi 1.08 Sb 0.9 Te 2 S (Sn-BSTS). By injecting spins into a Sn-BSTS crystal from a ferromagnetic metal using spin pumping, we found that the magnetic damping of the ferromagnetic layer is enhanced due to the absorption of the spin current in the surface and bulk states of the topological insulator. We found that the damping enhancement depends critically on temperature, which allows us to disentangle the spin absorption in the surface and bulk states, owing to the nearly perfect carrier compensation in the bulk part at low temperature. Our results show that the absorption of the spin current is dominated by the surface state of the Sn-BSTS crystal at low temperature, whereas the spin absorption in the surface state is comparable to that in the bulk state near room temperature. The coexistence of the spin absorption in the surface and bulk states has been demonstrated in two different systems, where the surface state is coupled with the dynamical magnetization through the diffusive spin current or exchange coupling. These results will be essential for quantitative understanding of the spin absorption and spin-charge conversion due to the spin-momentum locked surface state of topological insulators.
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