Robust A-type order and spin-flop transition on the surface of the antiferromagnetic topological insulator MnBi$_2$Te$_4$

TL;DR

This study provides microscopic evidence of persistent uniaxial A-type antiferromagnetic order on the surface of MnBi₂Te₄, revealing termination-dependent magnetic phenomena and surface spin-flop transitions that are crucial for understanding its topological properties.

Contribution

It demonstrates the existence of robust surface A-type antiferromagnetic order and termination-dependent spin-flop transitions in MnBi₂Te₄, advancing understanding of surface magnetism in topological insulators.

Findings

Surface A-type order persists at the microscopic level.

Magnetic contrast varies with surface termination and can be screened.

Surface spin-flop transitions are observed and are termination-dependent.

Abstract

Here we present microscopic evidence of the persistence of uniaxial A-type antiferromagnetic order to the surface layers of MnBi$_2$Te$_4$ single crystals using magnetic force microscopy. Our results reveal termination-dependent magnetic contrast across both surface step edges and domain walls, which can be screened by thin layers of soft magnetism. The robust surface A-type order is further corroborated by the observation of termination-dependent surface spin-flop transitions, which have been theoretically proposed decades ago. Our results not only provide key ingredients for understanding the electronic properties of the antiferromagnetic topological insulator MnBi$_2$Te$_4$, but also open a new paradigm for exploring intrinsic surface metamagnetic transitions in natural antiferromagnets.