Unusual ferromagnetism in CoSi nanowires from internal and interfacial defects

Abstract

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.

Ab initio simulations of defect-based magnetism: the case of CoSi nanowires

“…Additionally, this plane-wave formalism produced the Hellmann-Feynman forces 12 that were used for the ab initio relaxation of the ionic positions until residual forces on individual atom were less than 0.5 eV ÅÀ1 . 6,7,14,15 This average dri force of less than 0.5 eV ÅÀ1 per atom (mainly around the interface) has been used before, 16 where those simulations provided a $90% agreement with experiment. In this work, details of magnetization for individual atoms are discussed using tighter and more exact convergence settings, which (in principle) should be more reliable when one judges agreement with experiment, and more accurate for individual atomic magnetizations.…”

Magnon modes as a joint effect of surface ferromagnetism and spin-orbite coupling in CoSi chiral topological semimetal

“…(i) Surface ferromagnetism 14,15 takes a role of a reference layer, where spin-wave exitations can be induced by spin-polarized current. Ordered vacancies 16 and distorted bonds near the surface 17 provide the mechanism for the surface ferromagnetism.…”

“…Despite the diamagnetic bulk, small CoSi single crystals demonstrate surface ferromagnetism in addition to the symmetry-induced spin-orbit coupling 14,15 . The transition metal (Co) d-orbital electron spin up and spin down populations become asymmetric from the exchange interactions near the CoSi surface, providing the mechanism for the observed surface magnetization 16,17 .…”

Silicide Thermoelectrics: Materials for Energy Harvesting

“…Further support to the topologically non‐trivial nature of CoSi comes from the observation of several peculiarities in magnetic and thermodynamic properties of CoSi, inconsistent with the conventional band structure of this compound …”