Manipulating Charge-to-Spin Conversion via Insertion Layer Control at the Interface of Topological Insulator and Ferromagnet

Abstract

Abstract Owing to their unique electronic properties and potential for charge-to-spin conversion (CSC), topological insulators (TIs) are emerging materials for spintronic applications, particularly in spin-orbit torque magnetoresistive random-access memory. Strong spin-orbit coupling and highly spin-polarized surface states in TIs are key parameters that explain their extremely high CSC efficiency at interfaces with ferromagnetic materials (FMs). Therefore, the performance of TI-based spintronic devices is strongly influenced by the interface between the TI and FM, where various interactions and effects occur, such as atomic intermixing, charge transfer, exchange coupling, and induced magnetism. This study focused on the influence of the insertion layer on the proximity effect occurring in a Co4Fe4B2/Bi2Se3 interface. Various insertion layers, including Au, MgO, and Se, were introduced to modulate the proximity effect from TI to FM and vice versa. X-ray photoelectron spectroscopy and transmission electron microscopy revealed that the Se insertion layer effectively suppresses the formation of an additional Bi layer, reducing intermixing against Co4Fe4B2. Electrical transport properties such as RXX and RXY under a vertical magnetic field show that the Se-inserted structure features the lowest anomalous Hall angle and exhibits a pristine topological surface state, indicating its potential for improving CSC efficiency. The Se-inserted structure exhibits the highest spin Hall angle among various heterostructures, according to results obtained from spin-torque ferromagnetic resonance. These findings highlight the importance of selecting an insertion layer and controlling the interface to optimize the spin-transport properties of TI-based spintronic devices and provide insights into the design of future spin devices.