Highly Efficient Spin-Orbit Torque Switching using Bulk-Insulating Topological Insulator Bi­₂Se₃

Topological insulators (TIs) have shown great potential for efficient spin-orbit torque (SOT) switching of adjacent ferromagnets due to spin-momentum locking of their surface states. However, a substantial bulk-conducting channel in many TIs limits their full potential for low-power operations.  Here we synthesize bulk-insulating Bi₂Se₃ on (BiIn)₂Se₃/In₂Se₃ buffer layers by molecular beam epitaxy (as confirmed by transport and angle-resolved photoemission spectroscopy (ARPES) measurements), and compare their SOT efficiency to bulk-conducting Bi₂Se₃ films using Kerr rotation and second harmonic Hall measurements.  For the bulk-insulating Bi₂Se₃, we find a 4-fold reduction in critical current density for switching of an adjacent NiFe layer and a 5-10-fold enhancement in SOT efficiency, unambiguously demonstrating that Fermi level tuning can significantly enhance switching performance.  This is attributed to the current flowing predominantly through the top TI surface state layer, where the generated spins are in close proximity to the NiFe interface.  In addition, our TI/FM heterostructures are grown entirely in situ without air exposure, where the clean interface can facilitate strong hybridization and the formation of interface states, or “descendent states” that exhibit spin-momentum locking, leading to enhanced SOT efficiency for both structures.