[Paper Review] Dimensional Crossover Induced Topological Hall Effect in a Magnetic Topological Insulator

Chang Liu, Yunyi Zang+6 more|arXiv (Cornell University)|Sep 24, 2017

Topological Materials and Phenomena4 citations

TL;DR

This study demonstrates a topological Hall effect (THE) in Mn-doped Bi2Te3 thin films that emerges at a 4-quintuple-layer (QL) thickness due to dimensional crossover, where surface state coupling stabilizes magnetic skyrmions. The THE vanishes at 3 QL, indicating a critical thickness-dependent topological transition driven by inter-surface magnetic coupling in ultrathin topological insulators.

ABSTRACT

We report transport studies of Mn-doped Bi2Te3 topological insulator (TI) films with accurately controlled thickness grown by molecular beam epitaxy. We find that films thicker than 5 quintuple-layer (QL) exhibit the usual anomalous Hall effect for magnetic TIs. When the thickness is reduced to 4 QL, however, characteristic features associated with the topological Hall effect (THE) emerge. More surprisingly, the THE vanishes again when the film thickness is further reduced to 3 QL. Theoretical calculations demonstrate that the coupling between the top and bottom surface states at the dimensional crossover regime stabilizes the magnetic skyrmion structure that is responsible for the THE.

Motivation & Objective

• To investigate the thickness-dependent electronic and magnetic response in molecular beam epitaxy-grown Mn-doped Bi2Te3 topological insulator films.
• To identify the emergence of the topological Hall effect (THE) in ultrathin films and understand its origin in the context of dimensional crossover.
• To explore the role of surface state coupling in stabilizing magnetic skyrmion textures responsible for THE.
• To clarify the thickness evolution of anomalous versus topological Hall effects in magnetic topological insulators.

Proposed method

• Molecular beam epitaxy (MBE) was used to grow Mn-doped Bi2Te3 films with atomic-layer precision control over thickness, spanning 3 to 5 quintuple layers (QL).
• Transport measurements, including Hall resistivity, were performed to extract anomalous Hall effect (AHE) and topological Hall effect (THE) contributions across different thicknesses.
• Theoretical modeling was employed to analyze the coupling between top and bottom surface states in the 4 QL regime, linking it to skyrmion formation.
• Theoretical calculations confirmed that inter-surface exchange coupling stabilizes chiral magnetic textures, enabling the observed THE.

Experimental results

Research questions

• RQ1 What is the thickness dependence of the Hall effect in Mn-doped Bi2Te3 topological insulator films?
• RQ2 Why does the topological Hall effect emerge specifically at 4 quintuple layers and not at 3 or 5 QL?
• RQ3 How does dimensional crossover between 2D surface states influence magnetic texture formation in ultrathin topological insulators?
• RQ4 What is the role of inter-surface coupling in stabilizing skyrmion-like spin textures responsible for THE?
• RQ5 Can the observed THE be attributed to a topological origin rather than conventional AHE?

Key findings

• The topological Hall effect (THE) emerges prominently in 4-QL Mn-doped Bi2Te3 films, indicating the presence of non-collinear spin textures.
• The THE vanishes in 3-QL films, indicating a critical thickness threshold for the emergence of topological spin textures.
• In 5-QL films, only the conventional anomalous Hall effect (AHE) is observed, confirming the absence of topological Hall response in thicker films.
• Theoretical analysis confirms that inter-surface coupling in the 4-QL regime stabilizes magnetic skyrmions, which are responsible for the observed THE.
• The dimensional crossover at 4 QL induces a transition from trivial AHE to non-trivial THE due to enhanced surface state hybridization and chiral spin ordering.