Materials consisting of two-dimensional kagome lattices (left panel) provides a fascinating platform for studying physics at the junction of non-trivial band topology and magnetism. In the real space, kagome magnets exhibit non-trivial magnetic textures (e.g., skyrmions) and spin frustration. In the momentum space, kagome materials feature Dirac cones, and flat bands (the band structure of a 2D kagome lattice calculated from tight-binding model is shown in the right panel). The Dirac cones can give rise to Berry curvature which leads to intrinsic Hall effects. The flat bands are ideal platforms for realizing strongly correlated electronic states.
Our group uses molecular beam epitaxy (MBE) to synthesize thin films of kagome magnets, and studies the structural, magnetic, and electronic properties of these thin films. A combination of structural characterizations, magnetic measurements, transport measurements, and angle-resolved photoemission spectroscopy (ARPES) studies provide new insights into these materials.
Highlights
Epitaxial Growth and Domain Structure Imaging of Kagome Ferromagnet Fe3Sn2 Thin Films
Fe3Sn2 is a metal composed of magnetic kagome layers embedded in the (111) planes in the material. Interestingly, this is a candidate magnetic Weyl semimetal and massive Dirac cones have been observed previously by ARPES. A family of Fe-Sn kagome compounds is possible ranging from the 3D to the 2D limit:
- Fe3Sn – (3D limit) consisting of repeated Fe3Sn kagome atomic layers
- Fe3Sn2 – consisting of repeated two Fe3Sn kagome layers and one Sn2 spacer layer (pictured above)
- FeSn – (2D limit) consisting of repeated one Fe3Sn kagome layer and one Sn2 spacer layer
Furthermore, other metallic elements (e.g. Mn) could replace the Fe for even greater variation of the electronic and magnetic properties (e.g. antiferromagnetic for Mn). These could form the building blocks of kagome atomic layer heterostructures consisting of M3Sn (M = metal) kagome layers and Sn2 spacer layers, where designing the layer sequence will tune the magnetic, electronic, and topological properties of the material.
The first step toward this goal is to develop high-quality epitaxial growth of these materials at low temperatures, as achieved in our first study. Here, we showed that Fe3Sn2 could be deposited at lower temperatures using atomic layer molecular beam epitaxy (AL-MBE). This consists of the sequential deposition of Fe3Sn and Sn2 atomic layers by opening and closing the effusion cell shutters with appropriate timing. We measured the magnetic properties which show ferromagnetism with easy plane magnetic anisotropy. Additionally, we utilized magnetothermal imaging to map out the magnetic domain structure during magnetization reversal. For further information, see Cheng et al., “Epitaxial Growth and Domain Structure Imaging of Kagome Magnet Fe3Sn2” arXiv:2105.12203 (2021).
Topological Flat Bands in Epitaxial Thin Films of Kagome Material CoSn
Materials with flat bands are ideal systems for studying the physics of strongly correlated electronic states due to their smaller bandwidth W compared to the Coulomb repulsion U. Correlated electronics states could be realized when the flat bands are tuned to the Fermi level since their high density of states may produce an energy instability where the many-body ground states (e.g. superconductivity, ferromagnetism, Mott insulator, etc.) are energetically favored over single-particle ground states. Kagome-structured CoSn is a promising material for studying flat bands because they host flat bands several hundreds meV below the Fermi level. Here we synthesize epitaxial CoSn thin films directly on insulating MgO(111) and 4H-SiC(0001) substrates. The band structures of CoSn thin films are visualized by ARPES measurements (see right panel for ARPES spectrum). This is the first direct observation of flat bands in epitaxial thin films of kagome materials. We have also developed a semiclassical transport theory to quantitatively explain the band-dependent transport properties of CoSn. For further information, see Cheng, et al. “Epitaxial Kagome Thin Films as a Platform for Topological Flat Bands.” Nano Letters 23, 7107-7113 (2023).
Kagome Ferrimagnet RMn6Sn6 (R = Rare Earth) with Tunable Magnetic Properties
Kagome ferrimagnet RMn6Sn6 (R = rare earth) is a highly tunable system for investigating the interplay between magnetic ordering and band topology. In RMn6Sn6, Mn atoms form Mn3 kagome layers that are separated from each other (see left column for crystal structure of RMn6Sn6). The spins of rare earth element R sublattice is antiferromagnetic coupled to the Mn sublattice. The magnetic anisotropy and band structure are dependent on the selection of R. Our group has synthesized ErMn6Sn6 and TbMn6Sn6 thin films using atomic-layer MBE.
| Science | Topological Kagome Magnets