Distinguishing Surface and Bulk Electromagnetism via Their Dynamics in an Intrinsic Magnetic Topological Insulator

TL;DR

This study investigates the distinct dynamics of surface and bulk electromagnetism in an intrinsic magnetic topological insulator, revealing mechanisms for manipulating magnetism and topological states with light.

Contribution

It combines experimental techniques and theoretical modeling to elucidate surface and bulk magnetic interactions in MnBi2Te4, advancing understanding of topological magnetism.

Findings

Surface-rooted exchange gap quenching explained by 2D RKKY interactions

Bulk demagnetization overestimated by theoretical model

Light-induced demagnetization comparable to advanced magnetophotonic crystals

Abstract

The indirect exchange interaction between local magnetic moments via surface electrons has been long predicted to bolster the surface ferromagnetism in magnetic topological insulators (MTIs), which facilitates the quantum anomalous Hall effect. This unconventional effect is critical to determining the operating temperatures of future topotronic devices. However, the experimental confirmation of this mechanism remains elusive, especially in intrinsic MTIs. Here we combine time-resolved photoemission spectroscopy with time-resolved magneto-optical Kerr effect measurements to elucidate the unique electromagnetism at the surface of an intrinsic MTI MnBi2Te4. Theoretical modeling based on 2D Ruderman-Kittel-Kasuya-Yosida interactions captures the initial quenching of a surface-rooted exchange gap within a factor of two but over-estimates the bulk demagnetization by one order of magnitude. This mechanism directly explains the sizable gap in the quasi-2D electronic state and the nonzero residual magnetization in even-layer MnBi2Te4. Furthermore, it leads to efficient light-induced demagnetization comparable to state-of-the-art magnetophotonic crystals, promising an effective manipulation of magnetism and topological orders for future topotronics.