Spin Transfer Torque Generated by the Topological Insulator Bismuth Selenide

We measure large spin-transfer torques generated by in-plane currents in thin films of the topological insulator bismuth selenide at room temperature. We use spin-torque ferromagnetic resonance in Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$/Ni$_{\mathrm{81}}$Fe$_{\mathrm{19}}$ bilayers to determine that the spin-torque arising from the Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ and acting on the Ni$_{\mathrm{81}}$Fe$_{\mathrm{19}}$ layer possesses substantial vector components both in the sample plane and perpendicular to the plane. The out-of-plane torque is several times larger than expected from the Oersted field, and the efficiency of in-plane (anti-damping) spin torque generation per unit current density in the Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ is greater than has been observed in any other material.