Large Unidirectional Magnetoresistance in Topological Insulator/Ferromagnet Bilayers

Strong spin-orbit coupling in topological insulators (TI) is expected to induce strong spin-orbit torque on proximal magnetic moments. This has motivated significant recent interest in topological insulator/ferromagnet (TI/FM) systems, with potential applications in spintronic memory and sensor technologies. We have thus carried out magnetotransport measurements on Bi₂Se₃/Co bilayers. Using standard four-probe methods, low-field, in-plane longitudinal and transverse (planar Hall) magnetoresistance (MR) has been measured, at room temperature, as well as angular-dependent MR at various field strengths. When current is passed predominantly through the Co layer, we observe typical AMR behavior. However, by promoting higher current near the TI/FM interface (verified by a two probe resistance value between that of Co and Bi₂Se₃), we observe unidirectional magnetoresistance orders of magnitude higher than Co alone, as well as an unusual cos(φ) angular dependence. The effects of current, sample angle, and sample thickness, along with comparison with standard Co/heavy metal systems will be discussed.