Fe₃₋ₓMnₓSn₂ Kagome Metal Films Demonstrate Tunable Electronic Properties via Alloying

Kagome metals represent a fascinating frontier in materials science, offering the potential for novel electronic and magnetic behaviours due to their unique band structure, and researchers are actively seeking ways to control their properties. Prajwal Laxmeesha, Rajesh Dutta, and Rajeev Kumar Rai, alongside colleagues from Drexel University, University of Pennsylvania, Temple University, and Brown University, investigate how introducing other elements into the kagome metal Fe3Sn2 affects its characteristics. Their work demonstrates that substituting iron with manganese successfully creates new materials while maintaining structural integrity, and crucially, shifts the electronic bands in a way that effectively adds ‘holes’, positive charge carriers, to the system. This ability to tune the electronic structure, combined with the retention of room temperature ferromagnetism, establishes a promising platform for manipulating properties like the anomalous Hall and Nernst effects, potentially leading to advances in spintronic devices and other applications.

Kagome Metal Fe3Sn2, Magnetic and Electronic Properties

Research focuses on the Kagome metal Fe3Sn2, and related materials, combining materials synthesis, detailed characterization, and theoretical calculations to understand its fundamental magnetic and electronic behavior. Scientists investigate its potential for applications in spintronics and topological electronics, exploring its unique properties through the growth of thin films using techniques like molecular beam epitaxy and atomic layer epitaxy, carefully controlling film quality, stoichiometry, and interface properties, and examining heterostructures with other materials. A central focus is understanding the electronic band structure of Fe3Sn2, particularly the presence of Dirac and Weyl fermions, Van Hove singularities, flat bands, and nodal lines, believed to be responsible for its unique electronic properties. Theoretical calculations, employing methods like density functional theory, are used to predict and interpret experimental results, while the magnetic properties of Fe3Sn2 are investigated to determine the magnetic moments of iron atoms, the ratio of orbital to spin moment, and the magnetic anisotropy, with the goal of exploring its potential for spintronic applications like magnetic sensors, memory devices, and spin-orbit torque devices. Researchers employ a wide range of characterization techniques, including angle-resolved photoemission spectroscopy to map the electronic band structure, x-ray magnetic circular dichroism to probe element-specific magnetic moments and orbital character, x-ray absorption spectroscopy to study electronic structure and chemical bonding, x-ray diffraction to determine crystal structure and film quality, scanning tunneling microscopy and spectroscopy to image surfaces and probe local electronic density of states, and transport measurements to characterize electronic transport properties. The combination of topological electronic structure and magnetism is a central theme, with scientists striving to understand how these properties interact and can be exploited for technological applications, with particular attention to orbital magnetism, which plays a crucial role in determining magnetic anisotropy and spin-orbit coupling, important for spintronic applications.

Compositional Tuning of Kagome Metal Thin Films

Scientists have pioneered a method for tuning the properties of Kagome metals through precise compositional control, successfully growing thin films of Fe3-xMnxSn2, systematically substituting manganese for iron up to a composition of x = 1. 0, using molecular beam epitaxy under ultra-high vacuum conditions. Precise control over deposition rates and shutter times, monitored using a quartz crystal microbalance and calibrated with Rutherford backscattering spectrometry and energy dispersive x-ray spectroscopy, ensures the desired stoichiometry and film thickness. To maintain crystalline quality, films are grown with a thin, 4nm capping layer of calcium fluoride, deposited congruently during the process, and film growth is monitored in real-time using reflection high-energy electron diffraction to assess and maintain structural integrity.

Following deposition, films undergo thorough characterization using advanced techniques, including x-ray diffraction and reflectivity to quantify lattice parameters, thickness, and interface roughness, and high-angle annular dark-field scanning transmission electron microscopy to provide detailed structural information. Polarized neutron reflectometry is used to investigate magnetic properties, confirming ferromagnetic alignment between iron and manganese moments. X-ray absorption spectroscopy and magnetic circular dichroism, analyzed with specialized software, are used to determine electronic structure and magnetic moments, while bulk-sensitive hard x-ray photoemission spectroscopy probes the valence band structure, with binding energy calibration validated against a gold standard. Theoretical calculations, performed using the Abinit software package, complement the experimental results, providing insights into the electronic band structure and density of states, demonstrating a powerful approach to tailoring the properties of Kagome metals for potential applications in areas such as anomalous Hall and Nernst effects and spin-orbit torque devices.

Manganese Doping Stabilizes Fe3Sn2 Flat Bands

Scientists successfully incorporated manganese into the Fe3Sn2 crystalline structure, realizing Fe3-xMnxSn2 films with compositions up to x = 1. 0 while maintaining structural integrity comparable to the parent compound. In contrast, attempts to incorporate cobalt resulted in phase separation, indicating cobalt does not form a solid solution within the Fe3Sn2 structure. Detailed analysis using hard x-ray photoemission spectroscopy and density functional theory calculations revealed that incorporating manganese repositions the flat bands relative to the Fermi level, consistent with hole-doping. This downward shift of the Fermi level brings it closer to the flat bands as the manganese concentration increases, demonstrating a means for electronic control within this Kagome ferromagnet.

The Fe3-xMnxSn2 films retain room temperature ferromagnetism across the entire range of manganese concentrations studied, up to x = 1. 0, and measurements using x-ray magnetic circular dichroism confirmed that both iron and manganese moments align ferromagnetically, demonstrating that hole-doping does not suppress the ferromagnetic order. These films exhibit a Curie temperature between 640 K and 660 K, consistent with the parent Fe3Sn2 compound, demonstrating that manganese alloying provides a viable strategy for tuning the functional properties of Fe3Sn2, potentially impacting phenomena such as anomalous Hall and Nernst effects, and its application in spin-orbit torque structures.

Manganese Alloying Tunes Kagome Metal Properties

This research demonstrates the successful alloying of manganese into Kagome metal films, offering a pathway to tune their electronic properties while maintaining ferromagnetism at room temperature. Scientists created single-phase films of Fe3-xMnxSn2 across a range of compositions, demonstrating that manganese substitution effectively shifts the Fermi level relative to the material’s unique flat bands, a control over the electronic structure confirmed by experimental measurements and corroborated by theoretical calculations, revealing a redistribution of orbital character within the flat bands upon manganese incorporation. Researchers found that cobalt alloying leads to phase separation rather than stable integration into the Kagome structure, thus limiting its usefulness for tuning the material’s properties. While the manganese-alloyed films exhibit a slight decrease in magnetization with increasing manganese concentration, they retain robust room temperature ferromagnetism, and further investigation is needed to fully understand the observed nonlinear evolution of the coercive field with increasing manganese content, suggesting a potential role for material disorder. This work establishes manganese alloying as a viable strategy for manipulating the flat band characteristics of this magnetic Kagome metal, opening possibilities for tailoring properties relevant to anomalous Hall and Nernst effects.