Antiferromagnetic FeTe2 1T−phase formation at the Sb2Te3

Bilayer topological insulator/ferromagnet (TI/FM) heterostructures are promising for spintronic applications due to their low switching energy and therefore power efficiency. Until recently, the reactivity of TIs with FM films was overlooked in the spin-orbit-torque literature, even though there are reports that it is energetically favorable for TIs to react with transition metals and form interfacial layers. In this study, we fabricated a TI/FM heterostructure comprised of molecular beam epitaxy grown ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ and dc sputtered ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$. Broadband ferromagnetic resonance revealed spin-pumping evident by the significant enhancement in Gilbert damping, which is likely a signature of the topological surface states or the presence of large spin-orbit coupling in the adjacent ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$. With low-temperature magnetometry, an exchange bias is observed that indicates an exchange interaction between an antiferromagnet (AFM) and an adjacent FM. Cross-section high-angle annular dark-field scanning transmission electron microscopy characterization of the ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}\text{\ensuremath{-}}\mathrm{N}{\mathrm{i}}_{80}{\mathrm{Fe}}_{20}$ bilayer revealed a complex interface showing diffusion of Fe and Ni into the ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ film yielding the formation of a $\mathrm{Fe}{\mathrm{Te}}_{2} 1T\text{\ensuremath{-}}\mathrm{type}$ structural phase. Furthermore, density functional theory calculations revealed that the $\mathrm{Fe}{\mathrm{Te}}_{2} 1T\text{\ensuremath{-}}\mathrm{phase}$ has an AFM ground state. Due to experimental limitations in the electron-energy-loss spectroscopy measurements, the precise chemistry of the interfacial phase could not be determined, therefore it is possible that the $\mathrm{Fe}{\mathrm{Te}}_{2} 1T$ and/or an intermixed $({\mathrm{Fe}}_{1--x}{\mathrm{Ni}}_{x}){\mathrm{Te}}_{2} 1T$ is the AFM interfacial phase contributing to exchange bias in the system. This work emphasizes the chemical complexity of TI/FM interfaces that host novel, metastable magnetic topological phases and require more in-depth studies of other similar interfaces.