Flat Films of Exotic Material Could Unlock New Electronic Devices

Researchers are increasingly focused on the fascinating properties of kagome metals, and a new study details the successful epitaxial growth of antiferromagnetic FeGe thin films. Xiaoyue Song, Yanshen Chen, and Yongcheng Deng, all from the State Key Laboratory of Semiconductor Physics and Chip Technologies, alongside Fei Wang and Guodong Wei et al., demonstrate a method for producing high-quality FeGe films on Al2O3 substrates using molecular beam epitaxy. This work represents a significant advance as previous investigations have largely relied on bulk single crystals, and the ability to create thin films opens avenues for exploring charge density waves (CDW) and their relationship to magnetic transitions, potentially enabling novel antiferromagnetic spintronic devices. Transport measurements reveal a Néel temperature of 397 K and intriguing variations in Hall coefficient and magnetoresistance around 100 K, suggesting a strong link to CDW behaviour within these films.

This achievement addresses a significant gap in materials science, as previous investigations of FeGe relied exclusively on bulk single crystals. The resulting thin films exhibit a flat surface and maintain the characteristic kagome lattice structure confirmed by x-ray diffraction, atomic force microscopy, and high-resolution scanning transmission electron microscopy.

Transport measurements reveal a Néel temperature of 397 K, indicating the material’s antiferromagnetic ordering, alongside a notable variation in Hall coefficient and magnetoresistance around 100 K. This observed behaviour is strongly suggestive of the charge density wave (CDW) previously reported in bulk FeGe, presenting a new avenue for studying this intriguing phenomenon.
The ability to fabricate FeGe in thin film form opens possibilities for manipulating its properties through external stimuli such as strain, electrical fields, or light. These films are expected to serve as a versatile platform for detailed investigations into the interplay between CDW states and antiferromagnetism.

The out-of-plane antiferromagnetic arrangement of FeGe, coupled with its high Néel temperature, positions it as a promising candidate for next-generation spintronic devices. The research details a three-step growth process, beginning with a 2nm FeGe or Fe seed layer deposited at 460°C, followed by rapid cooling and deposition of a 15nm FeGe layer at 100°C, and finally a 2-hour annealing step at 390°C to enhance crystallinity.

Characterisation techniques included XRD, AFM, cross-sectional STEM, and magneto-transport measurements performed using a physical property measurement system and a low-field magneto-electrical measurement system. The development of these FeGe thin films promises to accelerate research into CDW mechanisms and unlock the potential of antiferromagnetic spintronics.

Epitaxial film growth, structural and transport characterisation, and piecewise resistivity analysis

Molecular beam epitaxy was employed to grow epitaxial FeGe thin films on Al2O3 substrates. Structural characterisation involved x-ray diffraction, atomic force microscopy, and high-resolution scanning transmission electron microscopy, confirming the single-phase formation and flat surface morphology of the kagome FeGe films.

Transport measurements were then conducted to investigate the magnetic and electronic properties of the fabricated thin films. These measurements revealed a Néel temperature of 397 K and a notable variation in both the Hall coefficient and magnetoresistance around 100 K, potentially linked to the emergence of a charge density wave.

A piecewise fitting procedure was applied to the temperature dependence of resistivity, dividing the data into three distinct regimes. The resistivity, denoted as qxx, was fitted using q = q0 + aT for 100, 380 K, q = q0 + a1T + a2T2 for 60, 100 K, and q = q0 + b1T + b2T2 for 10, 60 K. This analysis identified three dominant scattering mechanisms: defect scattering, electron, phonon scattering, and electron, electron scattering.

The parameter q0 represents the temperature-independent residual resistivity, while a1T and a2T2 describe the contributions from electron, phonon and electron, electron scattering, respectively. Further investigation focused on Fe(2nm)/FeGe(17nm) thin films, utilising a standard Hall effect measurement setup to determine the Hall resistivity as a function of out-of-plane magnetic field.

The ordinary Hall coefficient, R0, was extracted from the qH vs B curves at fields exceeding 3 T, allowing for the calculation of the carrier density, n, as a function of temperature. Values of R0 and n were found to be comparable to those reported for kagome FeGe single crystals. Magnetoresistance measurements were performed, revealing a sharp change around 100 K, again suggesting a connection to the charge density wave. Low-field magnetoresistance measurements identified critical magnetic fields, BC-IP-P and BC-OP-P, corresponding to peaks in the MR curves at 2 K for both in-plane and out-of-plane conditions.

Kagome FeGe thin film growth, structural characterisation and low temperature resistivity behaviour

Epitaxial FeGe thin films were successfully grown on Al2O3 substrates via molecular beam epitaxy. Structural analysis using x-ray diffraction, atomic force microscopy, and high-resolution scanning transmission electron microscopy confirmed the single-phase formation of kagome FeGe thin films with a flat surface.

Atomic-resolution STEM imaging revealed the expected hexagonal periodicity, matching the projected atomic arrangement of the material. A 2nm Fe buffer layer significantly enhanced film flatness, reducing surface roughness from 2.35nm to 0.55nm. Transport measurements established a Néel temperature of 397 K for the FeGe thin films, slightly lower than the 410 K observed in single crystal bulk material.

The longitudinal resistivity exhibited a gradual reduction with decreasing temperature, and a kink in the derivative of resistivity at 100 K suggests a connection to the charge density wave transition. Fitting the temperature dependence of resistivity revealed three distinct regimes, governed by defect scattering, electron-phonon scattering, and electron-electron scattering.

In the 100, 380 K range, electron-phonon scattering dominated, while electron-electron scattering became increasingly significant below 100 K, with a coefficient of 9.458x 10^-4 Ω cm/K^2, approximately three times larger than the electron-phonon scattering coefficient of 3.494x 10^-4 Ω cm/K^2. A rapid variation in the Hall coefficient and carrier density was observed around 100 K, with values comparable to those reported for kagome FeGe single crystals. Magnetoresistance measurements demonstrated a clear field dependence, indicating potential for spintronic applications.

Kagome FeGe thin film growth and indications of charge density wave behaviour

Epitaxial growth of kagome FeGe thin films has been successfully demonstrated using molecular beam epitaxy. Structural analysis via x-ray diffraction, atomic force microscopy, and high-resolution scanning transmission electron microscopy confirmed the single-phase formation and flat surface morphology of the films.

Transport measurements revealed a Néel temperature of 397 K and notable variations in the Hall coefficient and magnetoresistance around 100 K, potentially linked to the emergence of a charge density wave. The creation of high-quality FeGe thin films establishes a promising platform for investigating the underlying mechanisms of charge density wave formation and exploring potential applications in antiferromagnetic spintronics.

A two-nanometre thick iron buffer layer proved beneficial for hexagonal FeGe formation and enhanced film flatness. While the Néel temperature is slightly lower than that observed in bulk single crystals, the thin film format allows for manipulation of structural variations through strain, offering new avenues for research.

The authors acknowledge that the influence of the iron buffer layer on magneto-transport properties requires further investigation. Future work could employ techniques such as scanning tunneling microscopy or angular-resolved photoemission spectroscopy to examine the surface properties and gain a deeper understanding of the charge density wave mechanism. This research was supported by several grants from the National Key R&D Program of China and the National Natural Science Foundation of China, among others.