Contrasting spin-polarization regimes in Co nanowires studied by density functional theory

Abstract

In electronic structure calculations of magnetic nanostructures, a commonly used definition of the degree of spin polarization (DSP) employs only the density of states (DOS) at the Fermi level. However, the Fermi velocity can also play a crucial role in the DSP in ballistic and diffusive transports. We illustrate this by investigating the spin-dependent electronic structure of freestanding cylindrical hcp Co nanowires with diameters of up to $11.6\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. Using density functional theory, the energy bands and DOS are calculated. For all but the monowire, we find that there is stark disagreement between the DOS, ballistic, and diffusive DSPs: they are highly negative, small, and highly positive, respectively. This is shown to result from the nature of $d$ and $s$ bands together with hybridization effects and bond formation.