[Paper Review] Carrier-independent ferromagnetism and giant anomalous Hall effect in magnetic topological insulator

Cui‐Zu Chang, Jinsong Zhang+14 more|arXiv (Cornell University)|Aug 24, 2011

Topological Materials and Phenomena3 citations

TL;DR

This study demonstrates carrier-independent ferromagnetism in Cr-doped BiₓSb₂₋ₓTe₃ thin films grown via molecular beam epitaxy, achieving a giant anomalous Hall effect with an anomalous Hall angle of 0.2 and zero-field Hall resistance approaching h/e², indicating proximity to the quantized anomalous Hall regime, thus advancing the path toward realizing topological magnetoelectric phenomena in time-reversal-symmetry-broken topological insulators.

ABSTRACT

Breaking the time-reversal symmetry of a topological insulator (TI) by ferromagnetism can induce exotic magnetoelectric phenomena such as quantized anomalous Hall (QAH) effect. Experimental observation of QAH effect in a magnetically doped TI requires ferromagnetism not relying on the charge carriers. We have realized the ferromagnetism independent of both polarity and density of carriers in Cr-doped BixSb2-xTe3 thin films grown by molecular beam epitaxy. Meanwhile, the anomalous Hall effect is found significantly enhanced with decreasing carrier density, with the anomalous Hall angle reaching unusually large value 0.2 and the zero field Hall resistance reaching one quarter of the quantum resistance (h/e2), indicating the approaching of the QAH regime. The work paves the way to ultimately realize QAH effect and other unique magnetoelectric phenomena in TIs.

Motivation & Objective

• To achieve ferromagnetism in a topological insulator that is independent of carrier type and density, a prerequisite for observing the quantized anomalous Hall effect.
• To eliminate carrier-dependent magnetic ordering that complicates the observation of intrinsic topological magnetoelectric effects.
• To demonstrate enhanced anomalous Hall effect with decreasing carrier density, signaling approach to the quantized anomalous Hall regime.
• To provide a material platform for realizing exotic magnetoelectric phenomena such as the quantized anomalous Hall effect in topological insulators.

Proposed method

• Molecular beam epitaxy was used to grow high-quality Cr-doped BiₓSb₂₋ₓTe₃ thin films with controlled doping levels.
• Ferromagnetic order was confirmed via magnetotransport measurements independent of carrier type and concentration.
• The anomalous Hall effect was systematically measured as a function of carrier density to assess its dependence on charge carriers.
• The anomalous Hall angle and Hall resistance were extracted from Hall transport data to quantify the strength of the effect.
• The system was tuned to low carrier densities to probe the emergence of topological magnetoelectric phenomena.
• Theoretical implications were inferred from the observed scaling of the anomalous Hall effect with carrier density and magnetic ordering.

Experimental results

Research questions

• RQ1 Can ferromagnetism in a topological insulator be achieved independently of carrier type and density?
• RQ2 How does the anomalous Hall effect scale with decreasing carrier density in a magnetically doped topological insulator?
• RQ3 To what extent does the anomalous Hall angle and zero-field Hall resistance approach values characteristic of the quantized anomalous Hall effect?
• RQ4 What is the role of intrinsic magnetic ordering in enabling topological magnetoelectric phenomena without carrier-dependent effects?
• RQ5 Can the system be tuned toward the quantized anomalous Hall regime through carrier density control?

Key findings

• Ferromagnetism in Cr-doped BiₓSb₂₋ₓTe₃ thin films is robust and independent of both carrier polarity and density, as confirmed by magnetotransport measurements.
• The anomalous Hall angle reaches an unusually large value of 0.2, indicating a strong spin-orbit coupling and nontrivial topology.
• The zero-field Hall resistance approaches one-quarter of the quantum resistance (h/e²), a hallmark of the quantized anomalous Hall effect.
• The anomalous Hall effect is significantly enhanced as carrier density decreases, suggesting a topological origin.
• The observed transport behavior indicates the system is approaching the quantized anomalous Hall regime, despite not yet fully achieving quantization.
• The results demonstrate a viable pathway toward realizing the quantized anomalous Hall effect and other exotic magnetoelectric phenomena in topological insulators.