[Paper Review] Delicate Ferromagnetism in MnBi$_6$Te$_{10}$

Chenhui Yan, Yanglin Zhu+10 more|arXiv (Cornell University)|Jul 16, 2021

Topological Materials and Phenomena36 references4 citations

TL;DR

This study demonstrates disorder-mediated ferromagnetism in MnBi₆Te₁₀, where Mn vacancies and Mn migration stabilize a tunable ferromagnetic ground state with a 15 meV gap at the Dirac point. Using ARPES, TEM, and magnetization measurements, it identifies a 13 K Curie temperature and confirms a topological surface state with a broken-symmetry gap in the FM phase, while the AFM phase remains gapless across all terminations.

ABSTRACT

Tailoring magnetic orders in topological insulators is critical to the realization of topological quantum phenomena. An outstanding challenge is to find a material where atomic defects lead to tunable magnetic orders while maintaining a nontrivial topology. Here, by combining magnetization measurements, angle-resolved photoemission spectroscopy, and transmission electron microscopy, we reveal disorder-enabled, tunable magnetic ground states in MnBi$_6$Te$_{10}$. In the ferromagnetic phase, an energy gap of 15 meV is resolved at the Dirac point on the MnBi$_2$Te$_4$ termination. In contrast, antiferromagnetic MnBi$_6$Te$_{10}$ exhibits gapless topological surface states on all terminations. Transmission electron microscopy and magnetization measurements reveal substantial Mn vacancies and Mn migration in ferromagnetic MnBi$_6$Te$_{10}$. We provide a conceptual framework where a cooperative interplay of these defects drives a delicate change of overall magnetic ground state energies, and leads to tunable magnetic topological orders. Our work provides a clear pathway for nanoscale defect-engineering towards the realization of topological quantum phases.

Motivation & Objective

• To identify and characterize a ferromagnetic topological insulator phase in MnBi₆Te₁₀ that maintains nontrivial topology.
• To investigate how atomic-scale defects—specifically Mn vacancies and Mn migration—tune interlayer magnetic interactions.
• To determine whether disorder can stabilize a ferromagnetic ground state without compromising spin-orbit coupling or topological protection.
• To establish a direct link between defect engineering and tunable magnetic topological phases in MnBi₂Te₄-derived materials.

Proposed method

• Performed high-resolution laser-based angle-resolved photoemission spectroscopy (ARPES) to map electronic band structures on different surface terminations of MnBi₆Te₁₀.
• Conducted transmission electron microscopy (TEM) with annular dark field (ADF-STEM) imaging to directly visualize Mn vacancies and Mn migration across layers.
• Measured zero-field-cooled (ZFC) and field-cooled (FC) magnetic susceptibilities to identify Curie temperature (T_C) and magnetic phase transitions.
• Acquired isothermal magnetization curves under c-axis and ab-plane magnetic fields to confirm ferromagnetic ordering and anisotropy.
• Used X-ray diffraction (XRD) to confirm structural integrity and phase purity of FM and AFM MnBi₆Te₁₀ samples.
• Applied a conceptual framework based on defect-mediated interplay between Mn vacancies and Mn migration to explain tunable magnetic ground states.

Experimental results

Research questions

• RQ1 Can atomic defects in MnBi₆Te₁₀ induce a stable ferromagnetic ground state while preserving nontrivial topology?
• RQ2 What is the role of Mn vacancies and Mn migration in altering the interlayer magnetic coupling in MnBi₆Te₁₀?
• RQ3 Does the ferromagnetic phase of MnBi₆Te₁₀ host a broken-symmetry energy gap at the Dirac point, as confirmed by ARPES?
• RQ4 How do the magnetic and electronic properties differ between ferromagnetic and antiferromagnetic MnBi₆Te₁₀ across various surface terminations?
• RQ5 Can the magnetic transition temperature (T_C) be tuned via defect engineering in MnBi₆Te₁₀?

Key findings

• A ferromagnetic ground state in MnBi₆Te₁₀ is stabilized at a Curie temperature of 13 K, the highest reported among MnBi₂Te₄-derived compounds.
• ARPES measurements reveal a 15 meV energy gap at the Dirac point on the MnBi₂Te₄ (MBT) termination in the ferromagnetic phase, coinciding with the Curie temperature.
• In contrast, the antiferromagnetic phase exhibits no measurable energy gap on any surface termination, confirming gapless topological surface states.
• ADF-STEM imaging directly visualizes Mn vacancies in the MBT layers and Mn migration from MBT to Bi₂Te₃ layers in the ferromagnetic phase.
• The magnetic susceptibility shows a sharp Λ-like peak at 10.2 K in the antiferromagnetic phase, indicating a long-range antiferromagnetic transition.
• A conceptual framework is proposed where the cooperative interplay of Mn vacancies and Mn migration reduces interlayer antiferromagnetic coupling, enabling tunable magnetic topological phases.