Unusual ferromagnetism in CoSi nanowires from internal and interfacial defects

The diamagnetic semimetal CoSi presents unanticipated ferromagnetism as CoSi/SiO2 nanowires (NWs). Using first-principles calculations, we offer physical insights into the origins of this unusual magnetism. Due to the distorted and dangling bonds near the NW 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 NWs. The transmission electron microscopy selected area electron diffraction analysis of the CoSi region suggested a superlattice structure existed in the cubic CoSi, and defects existing as ordered vacancies in the CoSi were present. The simulation's results for the Co moment in the CoSi NWs without these ordered vacancies, but incorporating the surface and internal spin moments, is only 0.1638 μ(B)/atom Co, which is a ∼80% shortfall compared to the experimental value of 0.8400 μ(B)/atom Co. When the effects of ordered vacancies are incorporated into the simulation, 0.7886 μ(B) per surface Co atom, a much better match with the experimental value (within ∼6%), indicating that the internal ordered vacancies in the CoSi NWs are the dominant mechanism of ferromagnetism.