Prajwal M. Laxmeesha

Kagome metals are an intriguing class of quantum materials as the presence of both flat bands and Dirac points provides access to functional properties present in strongly correlated and topological materials. To fully harness these electronic features, the ability to tune the Fermi level relative to the band positions is needed. Here, we explore the structural, electronic, and magnetic impacts of substitutional alloying within ferromagnetic kagome metal Fe3Sn2 in thin films grown by molecular beam epitaxy. Transition metals, Mn and Co, are chosen as substitutes for Fe to reduce or increase the d-band electron count, thereby moving the Fermi level accordingly. We find that Co is not incorporated into the Fe3Sn2 structure but instead results in a two-phase Fe–Co and (Fe,Co)Sn composite. In contrast, Fe3−xMnxSn2 films are realized with x of up to 1.0, retaining crystalline quality comparable with the parent phase. The incorporation of Mn repositions the flat bands relative to the Fermi level in a manner consistent with hole-doping, as revealed by hard x-ray photoemission and density functional theory. The Fe3−xMnxSn2 films retain room temperature ferromagnetism, with x-ray magnetic circular dichroism measurements confirming that the Fe and Mn moments are ferromagnetically aligned. The ability to hole-dope this magnetic kagome metal provides a platform for tuning properties, such as anomalous Hall and Nernst responses.

Quantum materials hold great promise to meet the growing demand for computer processing power and storage by coupling the spin degree-of-freedom and charge transport properties of electrons for use in spintronic devices. Of these, kagome metals -- materials with a trihexagonal sublattice -- offers a unique platform to explore electron correlation and band topology within the same materials. To leverage upon these characteristics and realize next-generation integrated non-volatile memory and storage, it is crucial to synthesize high-quality heterostructures and control the functional properties of the materials that make them up. In this work, I showcase the progress I've made in thin film synthesis of multiple kagome materials-based heterostructures such as FeSn, Fe₃Sn₂ and CoSn via molecular beam epitaxy (MBE) on insulating oxide substrates, with a special focus on antiferromagnetic FeSn which exhibits strong exchange bias when coupled with an adjacent ferromagnet, confirmed using magnetometry. Using a combination of high-resolution x-ray diffraction, reflection high-energy electron diffraction, and electron microscopy I demonstrate that the FeSn films are flat, continuous and epitaxial. Furthermore, I will discuss doping strategies that allow for the tuning of electronic and magnetic properties such as alloying and interfacial charge transfer. Specifically in Fe₃Sn₂ and FeSn, I demonstrate successful hole doping achieved by alloying the two materials with Mn, leading to Fe_[3-x]Mn_xSn₂ and Fe_[1-x]Mn_xSn random alloys and examine their magnetic properties. Through the use of synchrotron-based X-ray magnetic circular dichroism (XMCD) I show the means to directly probe elemental contribution to magnetism in Fe_[3-x]Mn_xSn₂, disentangling both spin and orbital moments, and using neutron diffraction, I show suppressed Curie temperature in Fe_[1-x]Mn_xSn. I also report on the synthesis of all-kagome (Fe₃Sn₂)_m/(CoSn)_n superlattices, an alternative route to electron dope these materials, and using polarized neutron reflectometry, I test the hypothesis that ferromagnetism in Fe₃Sn₂ persists even when the film thickness is less than a single unit cell. These advanced characterization techniques provide exceptional insight into magnetic behavior globally across complex heterostructures and at localized interfaces, enabling a thorough understanding of the underlying magnetic phenomena.

Binary kagome compounds TmXn (T = Mn, Fe, Co; X = Sn, Ge; m:n = 3:1, 3:2, 1:1) have garnered recent interest owing to the presence of both topological band crossings and flatbands arising from the geometry of the metal-site kagome lattice. To exploit these electronic features for potential applications in spintronics, the growth of high-quality heterostructures is required. Here, we report the synthesis of Fe/FeSn and Co/FeSn bilayers on Al2O3 substrates using molecular beam epitaxy to realize heterointerfaces between elemental ferromagnetic metals and antiferromagnetic kagome metals. Structural characterization using high-resolution x-ray diffraction, reflection high-energy electron diffraction, and electron microscopy reveals that the FeSn films are flat and epitaxial. Rutherford backscattering spectroscopy was used to confirm the stoichiometric window where the FeSn phase is stabilized, while transport and magnetometry measurements were conducted to verify metallicity and magnetic ordering in the films. Exchange bias was observed, confirming the presence of antiferromagnetic order in the FeSn layers, paving the way for future studies of magnetism in kagome heterostructures and potential integration of these materials into devices.