Modeling and Evaluation of Topological Insulator/Ferromagnet Heterostructure-Based Memory

Topological insulators (TIs) are unique materials that have insulating bulk but conducting surface states. In this paper, we propose a simulation framework for TI/ferromagnet (FM) heterostructures that can capture the electronic band structure of a TI while calculating the transport properties. Our model differs from TI/FM models proposed in the literature in a way that it can account for the 3-D band structure of TIs, the effects of exchange coupling and external magnetic field on the band structure. The proposed approach uses 2-D surface Hamiltonian for TIs that includes the quantum confinement effect calculated from a 3-D band diagram. We use this Hamiltonian with self-consistent non-equilibrium Green's functions (NEGF) formalism to determine the charge and spin transport in TI/FM heterostructures. Our calculations agree well with experimental data and capture the unique features of a TI/FM heterostructure, such as the spin Hall angle, spin conductivity, and so on. Next, we incorporate the results into Landau-Lifshitz-Gilbert-Slonczewski formulation to simulate the magnetization dynamics of an FM layer sitting on top of a TI. Finally, we evaluate the performance of three different TI/FM memory structures and show that the TI-based memories can be energy efficient, if the shunting current through the FM layer is reduced.