Voltage-driven Magnetization Switching via Dirac Magnetic Anisotropy and Spin--orbit Torque in Topological-insulator-based Magnetic Heterostructures

Electric-field control of magnetization dynamics is fundamentally and technologically important for future spintronic devices. Here, based on electric-field control of both magnetic anisotropy and spin--orbit torque, two distinct methods are presented for switching the magnetization in topological insulator (TI)/magnetic-TI hybrid systems. The magnetic anisotropy energy in magnetic TIs is formulated analytically as a function of the Fermi energy, and it is confirmed that the out-of-plane magnetization is always favored for the partially occupied surface band. Also proposed is a transistor-like device with the functionality of a nonvolatile magnetic memory that uses voltage-driven writing and the (quantum) anomalous Hall effect for readout. For the magnetization reversal, by using parameters of Cr-doped Bi_{1-x}Sb_{x})_{2}Te_{3}, the estimated source-drain current density and gate voltage are of the orders of $10^4$--$10^5$~A/cm$^2$ and 0.1~V, respectively, below 20~K and the writing requires no external magnetic field. Also discussed is the possibility of magnetization switching by the proposed method in TI/ferromagnetic-insulator bilayers with the magnetic proximity effect.