Epitaxial growth and magneto-transport properties of kagome metal FeGe thin films

Antiferromagnetic kagome metal FeGe has attracted tremendous attention in condensed matter physics due to the charge density wave (CDW) being well below its magnetic transition temperature. Up to now, numerous works on kagome FeGe have been based on single crystal bulk, but its thin film form has still not been reported. Here, we achieved epitaxial growth of FeGe thin films on Al2O3 substrates using molecular beam epitaxy. Structural characterization with x-ray diffraction, atomic force microscopy, and high-resolution scanning transmission electron microscopy reveals single phase with flat surface of kagome FeGe thin films. Moreover, a Néel temperature of 397 K and a rapid variation of Hall coefficient and magnetoresistance around 100 K, which might be related to the CDW, were revealed via transport measurements. The high quality kagome FeGe thin films are expected to provide a versatile platform to study the mechanism of CDW and explore the application of FeGe in antiferromagnetic spintronics.

💡 Research Summary

The authors report the first successful epitaxial growth of antiferromagnetic kagome metal FeGe in thin‑film form using molecular‑beam epitaxy (MBE) on Al₂O₃ (0001) substrates. A 2 nm Fe buffer layer was introduced before depositing a 17 nm FeGe layer, followed by a two‑step annealing process (100 °C rapid cool, then 390 °C for 2 h). Structural analysis by X‑ray diffraction (XRD), atomic‑force microscopy (AFM) and high‑resolution scanning transmission electron microscopy (STEM) demonstrates a single‑phase, hexagonal kagome lattice with excellent crystallinity and a surface roughness of only ~0.55 nm when the Fe buffer is used, compared with ~2.35 nm without it. The XRD patterns show only the (0002) and sixfold symmetric (2‑1‑10) reflections, confirming epitaxial alignment, while STEM images reveal the expected atomic spacing (s₁ ≈ 2.48 Å, s₂ ≈ 2.87 Å) of the kagome net.

Transport measurements reveal a Néel temperature (T_N) of 397 K, slightly lower than the bulk value (≈410 K), likely due to finite‑size and strain effects in the film. The longitudinal resistivity ρ_xx(T) displays metallic behavior across the whole temperature range (2–400 K). A clear kink in dρ/dT appears at ~100 K, accompanied by a rapid jump in the ordinary Hall coefficient (R₀) and carrier density, suggesting the onset of a charge‑density‑wave (CDW) transition well below T_N. The authors fit ρ_xx(T) in three temperature regimes (100–380 K, 60–100 K, 10–60 K) using a combination of temperature‑independent residual resistivity (ρ₀), electron‑phonon scattering (∝ T) and electron‑electron scattering (∝ T²). The high‑temperature regime is dominated by electron‑phonon scattering, whereas below 100 K the electron‑electron term grows rapidly, indicating that CDW formation enhances electron‑electron interactions.

Hall measurements show both ordinary and anomalous contributions (ρ_H = R₀B + ρ_AHE). The anomalous Hall signal is primarily due to the ferromagnetic Fe buffer layer, but the ordinary Hall coefficient mirrors the bulk FeGe values and exhibits the same ~100 K anomaly. Magnetoresistance (MR) measurements under out‑of‑plane fields up to ±6 T reveal positive MR at low fields, with a pronounced change around 100 K, further supporting a CDW‑related modification of the electronic structure. Low‑field MR recorded with a separate setup shows sharp peaks for both in‑plane and out‑of‑plane configurations; the coercive fields (B_C) for in‑plane switching are smaller than those for out‑of‑plane, consistent with the known easy‑axis anisotropy of FeGe. The temperature dependence of the in‑plane coercive field shows a gradual increase below ~30 K, aligning with previously reported spin‑reorientation transitions in bulk FeGe.

Overall, the work establishes a reliable growth protocol for high‑quality FeGe thin films, demonstrates that the CDW transition survives in the two‑dimensional limit, and provides a versatile platform for manipulating CDW and antiferromagnetic order via strain, electric fields, or optical excitation. These findings open pathways toward antiferromagnetic spintronic devices—such as ultrafast memory elements and spin‑logic components—where the interplay between CDW and magnetic order can be harnessed for functional control.