Digital magnetic moment of tetrahedrally bonded half-metallic nanoclusters

Abstract

We investigate the half metallicity and localized spin magnetic moments for individual nanoclusters of zinc-blende transition-metal pnictides and chalcogenides $TX$ ($T=\mathrm{V}$, Cr, and Mn; $X=\mathrm{N}$, P, As, Sb, S, Se, and Te) using first-principles molecular-orbital calculations. These nanoclusters, as well as the bulk, show half-metallic ferromagnetic behavior for a wide range of bond lengths; otherwise, an antiferromagnetic arrangement is stabilized. The total magnetic moment of an isolated half-metallic nanocluster ${T}_{k}{X}_{\ensuremath{\ell}}$ turns out to be $({Z}_{\mathit{tot}}\ensuremath{-}8)k+(4\ensuremath{-}\ensuremath{\Delta}Z)(k\ensuremath{-}\ensuremath{\ell})$ in units of ${\ensuremath{\mu}}_{B}$, where ${Z}_{\mathit{tot}}$ is the total number of valence electrons per formula unit, $\ensuremath{\Delta}Z=1$ for pnictides, and $\ensuremath{\Delta}Z=2$ for chalcogenides. This $\ensuremath{\ell}$ dependence results from anion dangling hybrids on the cluster surface. Induced antiparallel magnetic moments at anion sites are interpreted in terms of a bond-orbital model; the hybridization effect between cation $d$ states and anion $p$ states creates holes in the majority-spin states of the bond orbitals. When the nanocluster is embedded in a lattice-matched compound semiconductor with a common anion, the total moment approaches $({Z}_{\mathit{tot}}\ensuremath{-}8)k{\ensuremath{\mu}}_{B}$. Half metallicity is maintained at the boundary sites of the nanocluster without any sign of interface states, suggesting the Ohmic nature of the contact.