Interfacing Topological Insulators with Ferromagnets in Magnetic Heterostructures

Anthony Richardella (Penn State University)

Topological Insulators (TI) are characterized by conducting surface states with a Dirac-like dispersion protected by time reversal symmetry. A magnetic perturbation that breaks this symmetry, such as placing a ferromagnet in proximity with a TI, can lead to a wide range of unusual effects such as a half integer quantum Hall conductance, magnetic monopoles, or an inverse spin-galvanic effect, among others. Such structures are challenging to create however due to the difficulty in finding insulating magnetic materials that are compatible with topological materials. Several approaches will be discussed, including bulk magnetic doping of TIs, the epitaxial growth of Bi2Se3 on the ferromagnetic semiconductor Ga1-xMnxAs and growth of the ferromagnetic insulator GdN on Bi2Se3. In GaMnAs, the Mn concentration can be tuned from a highly resistive state near the metal-insulator transition, up to a highly doped semiconductor, with a TC well in excess of 100 K, allowing a wide range of regimes between the two layers. GdN is an insulating ferromagnet that, importantly, can be grown at room temperature because the TI cannot withstand high temperatures and that can be used as a gate dielectric to help tune the Fermi energy. The characterization of these heterostructures reveals several unique challenges associated with each but also common signatures in magnetotransport arising from interfacing magnetic elements with the surface states. Structures like these open up the exciting possibility of a range of hybrid spintronic/Topological Insulator structures. Funded by ONR and DARPA.

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