Intrinsic Even-Odd Thickness-Driven Anomalous Hall in Epitaxial MnBi2Te4 Thin Films

We demonstrate precise control of magnetism in MnBi2Te4 thin films through careful synthesis by molecular beam epitaxy, achieving minimal defects and accurate layer thickness control. By optimizing Mn-Bi-Te ratios and growth temperatures, we minimize detrimental self-doping effects and accurately target integer-layer films. X-ray diffraction and reflectivity provide quantitative measures of film quality and thickness. When these macroscale probes of structure and thickness are integrated with magnetotransport measurements, a striking even-odd layer dependence of the anomalous Hall effect is revealed. Odd-layer films exhibit a large hysteresis up to the Néel temperature (~25K), consistent with non-compensated antiferromagnetism, while even-layer films show minimal response, as expected for an antiferromagnet. The sign of the anomalous Hall effect exhibits a sign reversal for intrinsic magnetism versus magnetism associated with defects. This work identifies critical factors for inducing pure, non-compensated ferromagnetism and reveals the character of the intrinsic anomalous Hall effect in MnBi2Te4, which together is a step towards realizing the zero-field quantum anomalous Hall effect in topological materials.

💡 Research Summary

In this work the authors present a comprehensive study of MnBi₂Te₄ thin films grown by molecular‑beam epitaxy (MBE) with a focus on achieving atomically precise layer control and minimizing structural defects. By carefully tuning the Mn:Bi flux ratio (kept between 2.0 and 2.6) and maintaining a Te flux at least five to ten times higher than the Bi flux, they suppress self‑doping and avoid the formation of antisite defects, vacancies, and unwanted quintuple‑layer (QL) intergrowths that are typical in this material system. The growth protocol consists of a low‑temperature nucleation of three septuple‑layer (SL) units followed by a higher‑temperature (225 °C) deposition of the remaining film, with an in‑situ Te capping layer to protect the surface.

Structural quality is evaluated using a combination of high‑resolution X‑ray diffraction (XRD) and X‑ray reflectivity (XRR). Simulations based on a kinematic diffraction model predict that QL intergrowths cause a characteristic splitting of the 006 Bragg peak; experimental scans confirm that when the Mn:Bi ratio is optimized the 006 peak becomes a single, sharp feature, indicating phase‑pure MnBi₂Te₄. XRR measurements, fitted with a two‑layer model (MnBi₂Te₄ + Te cap), provide sub‑angstrom accuracy of the film thickness, allowing the authors to target integer numbers of SLs (4, 5, 6, and 7) with an error of 0.1–0.2 SL. This macroscopic thickness verification matches the length scale of subsequent transport measurements.

Transport experiments are performed in a van‑der‑Pauw geometry with indium contacts, under magnetic fields up to 9 T applied perpendicular to the film plane, and temperatures down to 2 K. After antisymmetrizing the Hall resistance to remove mixing with the longitudinal component, the anomalous Hall resistance (R_xy^A) is extracted. A striking even‑odd effect emerges: odd‑layer films (e.g., 5 SL) display a large hysteretic anomalous Hall signal that persists up to the Néel temperature (~25 K), whereas even‑layer films (e.g., 4 SL) show virtually no hysteresis. This behavior directly reflects the intrinsic antiferromagnetic ordering of MnBi₂Te₄, where ferromagnetic septuple layers are stacked antiferromagnetically; an odd number of layers leaves a net uncompensated moment, while an even number yields complete cancellation.

Importantly, the sign of the anomalous Hall effect reverses when comparing defect‑induced ferromagnetism (positive sign) with the intrinsic, defect‑free magnetism (negative sign). This sign inversion provides a clear diagnostic for distinguishing extrinsic magnetic contributions from the desired intrinsic behavior. Moreover, the anomalous Hall signal in the high‑quality films follows the bulk antiferromagnetic transition rather than disappearing at the lower temperatures typical of defect‑driven ferromagnetism, confirming that the observed effect originates from the intrinsic magnetic order.

Overall, the study demonstrates that (i) precise MBE growth combined with real‑time XRD/XRR feedback can reliably produce integer‑layer MnBi₂Te₄ with minimal defects, (ii) the even‑odd thickness dependence of the anomalous Hall effect is a robust signature of non‑compensated ferromagnetism in odd‑layer films, and (iii) controlling these parameters is essential for realizing a zero‑field quantum anomalous Hall state in this material system. The work thus provides a clear pathway toward the long‑sought quantum anomalous Hall effect without external magnetic fields, by leveraging finite‑thickness engineering and defect suppression.