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

Topological insulators (TIs) are promising for efficient spin-orbit torque (SOT) switching of ferromagnets due to spin-momentum locking of their surface states. However, bulk-conducting channels limit their full potential for low-power operations. Here we synthesize bulk-insulating Bi2Se3 on (BiIn)2Se3/In2Se3 buffer layers by molecular beam epitaxy and compare their SOT efficiency to bulk-conducting Bi2Se3 using Kerr rotation and second harmonic Hall measurements. For bulk-insulating Bi2Se3, we find a 4-fold reduction in critical current density to switch 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 current flowing predominantly through the top TI layer, where the generated spins are in close proximity to the NiFe interface. In addition, our heterostructures are grown in situ, where the clean interface can facilitate strong hybridization and the formation of "descendent states" with spin-momentum locking, leading to enhanced SOT efficiency.