Epitaxial Growth of Frustrated Kagome Lattice Fe-Sn Thin Films

Since the first experimental observation of magnetic skyrmions in chiral itinerant-electron magnet MnSi in 2009 [1], magnetic topological excitations have emerged as a promising candidate for spintronics research and applications. Skyrmions can potentially serve as carrier of information and can be created, manipulated, and annihilated using electric or magnetic fields. This makes skyrmions promising as information carriers for future compact, high density, low-energy consumption, ultrafast, devices. [2]<br/>Though skyrmions can exist in a wide range of materials, in recent years topological materials with kagome crystal lattice have gained significant popularity due to the observation of skyrmionic magnetic bubbles at room temperature in the frustrated kagome ferromagnet Fe₃Sn₂[3]. Fe₃Sn₂ is a non-collinear frustrated ferromagnet with a Curie temperature of 640 K and it can host skyrmions in a wide temperature range. It has a centrosymmetric crystal structure which consists of offset Kagome bilayers of Fe₃Sn and a honeycomb Sn₂spacer layer separating the kagome bilayers. The stacking sequence of Fe₃Sn and Sn₂spacer layer can be varied to explore different topological phases. Antiferromagnetic kagome metal FeSn is one such example that has a single layer of Kagome Fe₃Sn and a layer of honeycomb Sn₂. FeSn can host DIRAC fermions and flat bands [4].<br/>For both FeSn and Fe₃Sn₂, most of the previous experiments were done on bulk crystals and only recently a few attempts have been made to synthesize Fe-Sn thin films [5]. Epitaxial thin films of FeSn and Fe₃Sn₂will give access to the fundamental physics underlying the intrinsic properties of kagome materials. Epitaxial films allow for the manipulation of lattice parameters and interfaces of the Fe-Sn layers, thus allowing us to manipulate and explore the topological phases in a wider range. In this work, Fe-Sn thin films are grown on a Si (111) substrate using molecular beam epitaxy. Films are grown in a substrate temperature range of 400-650^oC to study the effect of substrate temperature on crystal quality and surface uniformity of the deposited films. The structural characterization of the deposited films is done by in-situ reflection high energy electron diffraction (RHEED) and X-ray diffraction (XRD). We find that the crystalline quality of the films improves with increasing substrate temperature. At approx. 650^oC, 2D growth is achieved, evident from the streaky nature of the in-situ RHEED patterns. This was accompanied by high intensity peaks in the XRD measurements. The peaks furthermore indicate the coexistence of FeSn and Fe₃Sn₂ phases in these layers. The surface morphology of the deposited films is studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The SEM images reveal the formation of big crystallites on the surface of the high temperature grown films. The presence of larger crystallites is also visible in AFM data and the average roughness decreased with increasing substrate temperature. This indicates the improvement of surface uniformity with increasing substrate temperature. Future investigations on the growth side will focus on achieving single phase and single crystalline film growth and investigation of strain and interface properties. Furthermore, we will focus more on the magnetic properties using for example magnetic force microscopy (MFM) and magneto-optic Kerr effect (MOKE) microscopy.<br/>References<br/>1. Mühlbauer, S. <i>et al.</i> <i>Science</i> <b>323</b>, 915–919 (2009).<br/>2. Zhang, X. <i>et al.</i> <i>J. Phys. Condens. Matter</i> <b>32</b>, 143001 (2020).<br/>3. Hou, Z. <i>et al.</i> <i>Adv. Mater.</i> <b>29</b>, 1701144 (2017).<br/>4. Kang, M., Ye, L., Fang, S. <i>et al.</i><i>Nat. Mater.</i> <b>19, </b>163–169 (2020)<br/>5. Inoue, H., Han, M., Ye, L., Suzuki, T. & Checkelsky, J. G. <i>Appl. Phys. Lett.</i> <b>115</b>, 072403 (2019).