Circular photo-galvanic effect in magnetically-doped topological insulator thin films*

There is growing interest in inducing magnetic order in topological insulators (TIs) to pursue novel spintronic devices in the emerging field of ‘topological spintronics.’ Currently, there are two methods for achieving this: TI films can be doped with magnetic elements or interfaced with a magnetic material. It is important in this context to develop probe of the magnetism of a topological spintronic device using methods that directly involve the helical Dirac surface states. The circular-photo-galvanic effect (CPGE) in TI films provides a natural route for achieving this goal. CPGE involves the preferential optical excitation of charge carriers via transitions that depend on the helicity of the incident light. The effect has been thoroughly studied recently in non-magnetic TIs [Nat. Comm. 8, 1037 (2017)]. Here, we discuss how the CPGE changes with magnetic doping by studying the phenomenon in Cr-doped (Bi,Sb)₂Te₃ thin films as a function of temperature, chemical potential, and magnetic field. We compare these measurements to other magnetically dependent photocurrents in these samples. Ultimately, this will offer a new way to probe and control magnetism in future topological spintronic devices.