Epitaxial Growth and Anomalous Hall Effect in High-Quality Altermagnetic $α$-MnTe Thin Films

The recent identification of $α$-MnTe as a candidate altermagnet has attracted considerable interest, particularly for its potential application in magnetic random-access memory. However, the development of high-quality thin films - essential for practical implementation - has remained limited. Here, we report the epitaxial growth of centimeter-scale $α$-MnTe thin films on InP(111) substrates via molecular beam epitaxy (MBE). Through X-ray diffraction (XRD) analysis, we construct a MnTe phase diagram that provides clear guidance for stabilizing the pure $α$-MnTe phase, revealing that it is favored under high Te/Mn flux ratios and elevated growth temperatures. Cross-sectional electron microscopy confirms an atomically sharp film-substrate interface, consistent with a layer-by-layer epitaxial growth mode. Remarkably, these high-quality $α$-MnTe films exhibit a pronounced anomalous Hall effect (AHE) originating from Berry curvature, despite a net magnetic moment approaching zero - a signature of robust altermagnetic character. Our work establishes a viable route for synthesizing wafer-scale $α$-MnTe thin films and highlights their promise for altermagnet-based spintronics and magnetic sensing.

💡 Research Summary

This paper reports the successful epitaxial growth of centimeter‑scale α‑MnTe thin films on InP(111) substrates using molecular‑beam epitaxy (MBE) and demonstrates that these films exhibit a pronounced anomalous Hall effect (AHE) despite an almost vanishing net magnetic moment, confirming their altermagnetic nature. The authors systematically varied the Te/Mn flux ratio (4.86–14.64) and substrate temperature (250–500 °C) while keeping the Mn source temperature fixed at 750 °C. High‑resolution X‑ray diffraction (XRD) ω–2θ scans revealed that the phase composition is highly sensitive to these parameters: low flux ratios (< 6.96) and temperatures ≤ 300 °C favor the γ‑MnTe phase, whereas higher flux ratios (≥ 7) combined with temperatures ≥ 350 °C stabilize pure α‑MnTe. At 500 °C, a narrow window around a flux ratio of 5.8–6.0 still shows a faint γ‑MnTe peak, indicating that high temperature and sufficiently Te‑rich conditions are optimal for α‑MnTe crystallinity. By quantifying the relative intensities of the α‑MnTe (0002) and γ‑MnTe (111) reflections, the authors constructed a two‑dimensional phase diagram that serves as a practical guide for reproducible growth of phase‑pure α‑MnTe.

Reciprocal‑space mapping (RSM) on a 30 nm α‑MnTe film gave lattice constants a = 4.19 Å and c = 6.68 Å, about 1 % larger in‑plane than bulk, which the authors attribute to tensile strain arising from the mismatch in thermal expansion coefficients (α‑MnTe ≈ 1.6 × 10⁻⁵ K⁻¹ vs. InP ≈ 4.7 × 10⁻⁶ K⁻¹). Raman spectroscopy identified the A₁g (122 cm⁻¹) and E_TO (140 cm⁻¹) phonon modes and a two‑magnon (2M) scattering feature at 268 cm⁻¹, the latter being characteristic of altermagnets. X‑ray photoelectron spectroscopy confirmed the chemical states of Mn (2p₃/₂ = 641.6 eV) and Te (3d₅/₂ = 572.2 eV) while also detecting surface Te‑O bonds, indicating a thin native oxide layer.

Electrical transport measurements from 2 K to 300 K revealed a clear hysteretic Hall resistance that switches sign with a modest magnetic field (±0.5 T). The anomalous Hall conductivity reaches ~0.5 μΩ·cm despite SQUID magnetometry showing a net moment below 10⁻⁴ μ_B per Mn, i.e., essentially zero. This AHE is therefore attributed to Berry curvature intrinsic to the altermagnetic band structure rather than conventional ferromagnetic mechanisms. The authors discuss how the tensile strain may enhance spin splitting and Berry curvature, further amplifying the AHE.

Structural quality was verified by reflection high‑energy electron diffraction (RHEED) patterns showing layer‑by‑layer growth, and by cross‑sectional scanning transmission electron microscopy (STEM). STEM images display an atomically sharp α‑MnTe/InP interface without any buffer layer, and the atomic columns match density‑functional‑theory predictions, confirming epitaxial alignment and high crystallinity.

In summary, the work delivers (1) a comprehensive flux‑ratio/temperature phase diagram for MnTe, (2) evidence that tensile strain can tune the altermagnetic electronic structure, (3) the first observation of a robust Berry‑curvature‑driven AHE in phase‑pure α‑MnTe thin films with negligible net magnetization, and (4) a scalable MBE growth route that yields wafer‑scale, high‑quality altermagnetic films. These results open a realistic pathway toward integrating α‑MnTe into spin‑orbit‑torque memory, magnetic sensors, and other next‑generation spintronic devices that exploit the unique symmetry‑protected properties of altermagnets.