Study of proximity induced magnetism and magnetic coupling in nanocrystalline ferromagnet-antiferromagnet-topological insulator heterostructures

Magnetic topological insulators (MTIs) have been at the centre of condensed matter research due to the wealth of quantum effects they harbour. One example is the quantum anomalous Hall effect (QAHE), resembling the quantum Hall effect without the requirement for large applied magnetic fields, and, in principle, cryogenic cooling. While transition metal doped MTIs were central to observing the QAHE, magnetic doping is detrimental to other material parameters, pushing the operating temperature below 1 K. A promising alternative is proximity coupling of TIs with magnetic materials. However, in TI/ferro¬magnet (FM) bilayers, the QAHE could not be demon¬strated due to stray fields disrupting the chiral edge states. Our approach uses an antiferromagnet (AF) sharing a common interface with a TI, giving the considerable advantage of no magnetic stray fields. However, the lack of a net magnetisation makes achieving a monodomain state as well as controlling/identifying this state challenging. While controlling the AF state in NiO typically requires large magnetic fields, we have used x-ray magnetic linear dichroism (XMLD) to show the re-orientation of the Néel vector in AF NiO using low fields of 1 T by interfacing it to a FM Co layer. Here, we plan to investigate a heterostructure of Co/NiO/Cr-doped Sb2Te3 as a function of NiO thicknesses. The addition of Co allows for a more monodomain and controllable AF state in the NiO, enhancing the PIM in the TI, while the middle NiO layer prevents the FM stray fields from damaging the TI surface states. The goal is to negate the issues with FM/TI and AF/TI interfaces and allow for PIM in the nc-TI to lay the foundation for realising the QAHE using this new materials approach and to achieve high-temperature dissipationless transport in topological materials.