Molecular beam epitaxy and crystal structure of majority a-plane-oriented and substrate-strained Mn₃Sn thin films grown directly on sapphire (0001)

The Kagome antiferromagnet Mn3Sn has garnered much attention due to the presence of exciting properties such as anomalous Hall and Nernst effects. This paper discusses the synthesis of crystalline Mn3Sn thin films, prepared on Al2O3 (0001) substrates at 453 +/- 5 degrees C using molecular beam epitaxy. The growth is monitored in situ using reflection high energy electron diffraction and measured ex situ using x-ray diffraction, Rutherford back-scattering, and cross-sectional scanning transmission electron microscopy. Our analysis shows the in-plane lattice constants of a(1, M) = 4.117 +/- 0.027 A and a(2, M) = 4.943 +/- 0.033 angstrom, which is a very unexpected result when compared to the bulk a-plane Mn3Sn. This indicates a strain in the film and makes it challenging to provide a straightforward explanation. In an effort to explain our results, we discuss two possible orientation relationships between the Mn3Sn films and the sapphire substrates. Samples prepared under these conditions appear to have smooth surfaces locally, but overall the film has a 3D island morphology. First-principles calculations provide atomic models of the Mn3Sn (11 (2) over bar0) lattice on Al2O3 (0001) high symmetry sites, indicating that the L3-R90 degrees is the most stable configuration. A detailed discussion of the experimental data and theoretical results, as well as strain effects, is provided.