The Practical Material Challenges Involved in using the Topological Insulator BiSb in a Spin Transfer Device

The Topological Insulator (TI) BiSb, grown with a (012) film texture using molecular beam epitaxy, and coupled with a FM layer has been shown to have a very high Spin Hall Angle (SHA) > 2 and high electrical conductivity (>10 5 Ohm -1 m -1 ) in thin film stacks 1 . This makes the TI material ideally suited for spin transfer devices requiring several orders of magnitude less power consumption to operate than other traditional devices 2 . However, the practical implementation of this TI material into devices using PVD requires you overcome many severe material challenges. The BiSb material is very soft, easily damaged in ion milling or by subsequent depositions, it has a very low melting point ~ 280C, and hence possesses a very large in-plane grain size as deposited at room temperature, ~6X the targeted thickness in devices. Thermal Migration of Sb out of the TI material is also a severe problem, which is exacerbated by film roughness and large grain sizes, and all of this must be solved while growing and maintaining a non-prismatic (012) texture of BiSb with low roughness, good interfacial TI properties, at reasonable operating temperatures > 180C in actual device stacks, non-epitaxially on SiOx or ceramic substrates. We discuss here how we addressed these challenges through appropriate selection of epitaxial and non-epitaxial materials for buffer layer(s) (bottom interface), and (top interface) interlayer(s), and by using a non-uniform thickness doping of the BiSb(X) material to stabilize and grow a BiSbX(012) textured layer with low roughness, good TI properties, at elevated temperatures in FM thin film stacks.