Fabrication Of β-FeSi₂ Nanocubes With Strong Facet-induced Room-temperature Ferromagnetism

Development of Si-based spintronics needs applicable Si-based magnetic semi-conductor materials. β-FeSi2 is a narrow bandgap semiconductor with an orthorhom-bic structure, which is a particularly interesting material due to its many attractive properties desirable for optoelectronic and photoltaic applications. For single crystal-line bulk β-FeSi2, it was reported to have diamagnetism at low temperarure. Hall ef-fect measurements of β-FeSi2 polycrystalline films reveal that ferromagnetism is at-tained only at temperature below 100 K and there is no evidence of a ferromagnetic phase transition. Recently, β-FeSi2 nanowires have been found to exhibit a ferromag-netic behavior even at room temperature. In this thesis, we aim to find the relationship between magnetism and special crystal facets and size of β-FeSi2. The obtained main conclusion is described as follows:In our experiment, we used chemical vapor deposition (CVD) to synthesize crystalline semiconducting β-FeSi2 nanocubes with average size about 300 nm. Strong room-temperature ferromagnetism is observed from the β-FeSi2 nanocubes larger than 150 nm with both {100} and {011} facets due to electron-electron repulsion of the surface states within the gap. We also use first-principles calculation to understand the origin of the facet- and size-dependent ferromagnetism. We studied the spin configu- rations on both {011} and {100} surfaces of β-FeSi2 and theoretically investigated the conditions for the ferromagnetism in the nanocubes.The results described here suggest a promising means to induce strong magnet-ism from nonmagnetic semiconductors by simply exposing specific crystal facets. Coexistence of surface ferromagnetism, polarized surface band within the gap, and semiconductor characteristics in the same nanocube provides a feasible mean to elec-trically and optically control magnetism and conversely magnetically control the elec-trical and optical properties. The semiconducting P-FeSi2 nanocubes may be used as an elementary magnetic storage units and magneto-optic components in future sili-con-based spintronics applications.