Consistent surface and bulk magnetic properties of MnBi₆Te₁₀ observed by x-ray absorption spectroscopy

In order to understand how magnetic moments in a magnetic topological insulator (MTI) interact with the topological surface states (TSS) to open an exchange gap, it is crucial to experimentally distinguish the surface magnetic properties from those of the bulk. Here we achieve this for the layered van der Waals intrinsic MTI MnBi6Te10 by using x-ray absorption spectroscopy (XAS) in the surface-sensitive total electron yield (TEY) and bulk-sensitive fluorescence yield (FY) mode. The TEY data show substantial x-ray magnetic circular dichroism and x-ray magnetic linear dichroism, resulting from robust surface ferromagnetism with out-of-plane oriented Mn moments. We introduce an approach to decompose the XAS signal in a Fourier series, allowing us to extract the rather sharp resonant Mn edges, which overlap with a strong but only gradually fluctuating Te signal. We show that the experimental TEY and FY data, measured both with circular and linear light polarization, can all be fitted with a multiplet ligand field theory model with the same values for the model parameters. This reveals a close similarity between the surface and bulk electronic and magnetic properties.