| Summary: | 碩士 === 國立清華大學 === 材料科學工程學系 === 102 === The diamagnetic semimetal CoSi presents unanticipated ferromagnetism as CoSi/SiO2 nanowires.[1] Using first-principles calculations, we offer physical insights into the origins of this unusual magnetism. Due to the distorted and dangling bonds near the nanowire surface with different bond lengths, the transition metal (Co) d-orbital electron spin up and spin down populations become asymmetric from the exchange interactions, providing the mechanism for some of the measured magnetization. However, the distorted and dangling bonds are clearly not the only factor contributing to the magnetization of the nanowires. The transmission electron microscopy selected area diffraction (SAD) analysis of the CoSi region suggested a superlattice structure existed in the cubic CoSi, and defects existing as ordered vacancies in the CoSi resulted in the observed SAD lower contrast image components. The simulation’s results for the Co moment in the CoSi nanowires without these ordered vacancies, but incorporating the surface and internal spin moments, is only 0.1638μB/atom, which is a ~80% shortfall compared to the experimental value of 0.8400μB/atom. When the effects of ordered vacancies are incorporated into the simulation, 0.8074μB per Co atom, a much better match with the experimental value (within ~3.881%) results, indicating that the internal ordered vacancies in the CoSi nanowires is a dominant mechanism of the ferromagnetism. Investigation on the density of states (DOS) of Co atoms around the ordered vacancies shows that the Co atoms with lower coordination number induce more magnetization due to the unpaired electrons created by the break of Co-Si bonds, which cause the unbalance between spin up and spin down states. But in some cases, the magnetization of Co in the structure without ordered vacancies is higher than the Co in the structure with ordered vacancies. In such case, the bond length of Co in the structure without ordered vacancies falls in a range that leads to stronger interacting spin-exchange through the overlap between magnetic orbitals.
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