Interrogating and manipulating exchange pathways in the magnetic topological insulator MnBi2Te4

Magnetic order in topological materials results in phases that exhibit quantized behavior such as the topological magnetoelectric effect, quantum anomalous Hall effect, and more. Central to the phenomenology of these materials are exchange interactions – among localized spins constituting the magnetic order, as well as between localized spins and the topological bands. I will present findings from our recent work^1,2 employing optical spectroscopy to interrogate and manipulate these exchange couplings in MnBi₂Te₄, a quasi-two-dimensional antiferromagnetic topological insulator. In the first part of the talk, we consider the weak exchange coupling between localized Mn spins across the van der Waals gap, an interaction that is key to achieving long-range magnetic order¹. Using magneto-Raman spectroscopy, we discover that optical phonons strongly modulate this interlayer coupling in MnBi₂Te₄. This effect is found to be selective to A_1g phonons that simultaneously alter both the interlayer distance and the intralayer superexchange bond angles. In the second part, we quantify the exchange coupling between the Mn localized spins and the Bi-Te topological bands². This is achieved by employing a pump-probe approach, where we optically excite the topological bands, and study the consequent ultrafast dynamics of the localized spins using a multimodal probe. We find that this exchange coupling is two orders of magnitude larger than the primary in-plane superexchange interaction. I will discuss the implications of our findings for the exchange gap in the surface states, and outline the opportunities they create for ultrafast light-driven coherent control of magnetism and topology in MnBi₂Te₄.

1. Padmanabhan, H. et al. Nature Communications 13, 1929 (2022).

2. Padmanabhan, H. et al. Advanced Materials 2202841 (2022).