Investigation of MBE grown Topological Insulator/Ferromagnet stack for SOT devices

Spin-orbit torque magnetic random access memory (SOT-MRAM) devices are a new kind of spintronic based devices exhibiting tremendous properties, like a reduced read and write power, high integration capabilities and ultra-low switching time. These features make them considered as viable candidates for a broad range of applications, going from aerospace to cyber-security. The writing step in SOT MRAM is based on the conversion of a charge current injected in a first layer into a spin-polarized current via spin Hall effect, and on the transfer of this spin current into an adjacent ferromagnetic layer, exerting a spin-torque on its magnetization. This allows to considerably reduce the current density required for this switching step. However, this current is still high when common materials are used (around 10^8 A/cm^2 for heavy metals), due to their limited efficiency of charge current to spin current conversion (called spin Hall angle). A new kind of materials, called topological insulators, exhibits particularly high spin Hall angles, due to the presence of spin-polarized surface channels arising from a strong spin orbit coupling. A famous TI is bismuth selenide, for which reported spin Hall angle values ranges from 0.01 to 3.5 at room temperature. Whereas the in-situ growth of Bi2Se3 thin films is well harnessed, leading to ultra high quality thin films, the deposition of the FM layer is commonly performed by sputtering, leading to a low quality interface and reducing by this way the efficiency of the interfacial spin current transfer. The present work aims at overcoming this limitation by growing the FM film in situ to achieve high interfacial quality, leading to a larger device efficiency. The present work investigated the structural and transport properties of MBE grown stacks of sapphire/Bi2Se3/Co/Al2O3, which different cobalt layer thicknesses (4.3 nm, 6.9 nm, 11.3 nm and 14.8 nm). Surface topography, roughness and magnetic properties are investigated, followed by second harmonic Hall voltage measurement to derive SOT efficiencies.