Thermally generated spin current in the topological insulator Bi ₂ Se ₃

Abstract

We present measurements of thermally generated transverse spin currents in the topological insulator Bi 2 Se 3 , thereby completing measurements of interconversions among the full triad of thermal gradients, charge currents, and spin currents. We accomplish this by comparing the spin Nernst magneto-thermopower to the spin Hall magnetoresistance for bilayers of Bi 2 Se 3 /CoFeB. We find that Bi 2 Se 3 does generate substantial thermally driven spin currents. A lower bound for the ratio of spin current density to thermal gradient is J s ∇ x T = (4.9 ± 0.9) × 10 6 ( ℏ 2 e ) A m − 2 K μ m − 1 , and a lower bound for the magnitude of the spin Nernst ratio is −0.61 ± 0.11. The spin Nernst ratio for Bi 2 Se 3 is the largest among all materials measured to date, two to three times larger compared to previous measurements for the heavy metals Pt and W. Strong thermally generated spin currents in Bi 2 Se 3 can be understood via Mott relations to be due to an overall large spin Hall conductivity and its dependence on electron energy.