Epitaxial Kagome Thin Films as a Platform for Topological Flat Bands.

Systems with flat bands are ideal for studying strongly correlated electronic states and related phenomena. Among them, kagome-structured metals such as CoSn have been recognized as promising candidates due to the proximity between the flat bands and the Fermi level. A key next step will be to realize epitaxial kagome thin films with flat bands to enable tuning of the flat bands across the Fermi level via electrostatic gating or strain. Here, we report the band structures of epitaxial CoSn thin films grown directly on the insulating substrates. Flat bands are observed by using synchrotron-based angle-resolved photoemission spectroscopy (ARPES). The band structure is consistent with density functional theory (DFT) calculations, and the transport properties are quantitatively explained by the band structure and semiclassical transport theory. Our work paves the way to realize flat band-induced phenomena through fine-tuning of flat bands in kagome materials.

Magneto-Optical Detection of the Orbital Hall Effect in Chromium.

The orbital Hall effect has been theoretically predicted but its direct observation is a challenge. Here, we report the magneto-optical detection of current-induced orbital accumulation at the surface of a light 3d transition metal, Cr. The orbital polarization is in-plane, transverse to the current direction, and scales linearly with current density, consistent with the orbital Hall effect. Comparing the thickness-dependent magneto-optical measurements with ab initio calculations, we estimate an orbital diffusion length in Cr of 6.6±0.6 nm.

Spin-Orbit Torque in Bilayers of Kagome Ferromagnet Fe3Sn2 and Pt.

Spin-orbit torque phenomena enable efficient manipulation of the magnetization in ferromagnet/heavy metal bilayer systems for prospective magnetic memory and logic applications. Kagome magnets are of particular interest for spin-orbit torque due to the interplay of magnetic order and the nontrivial band topology (e.g., flat bands and Dirac and Weyl points). Here we demonstrate spin-orbit torque and quantify its efficiency in a bilayer system of topological kagome ferromagnet Fe3Sn2 and platinum. We use two different techniques, one based on the quasistatic magneto-optic Kerr effect (MOKE) and another based on time-resolved MOKE, to quantify spin-orbit torque. Both techniques give a consistent value of the effective spin Hall angle of the Fe3Sn2/Pt system. Our work may lead to further advances in spintronics based on topological kagome magnets.