Size-Dependent Spintronic Properties of Dilute Magnetic Semiconductor Nanocrystals

The electronic structure and magnetic properties of Mn-doped Ge, GaAs, and ZnSe nanocrystals are investigated using real space ab initio pseudopotentials constructed within the local spin-density approximation. The ferromagnetic and half-metallicity trends found in the bulk are preserved in the nanocrystals. However, the Mn-related impurity states become much deeper in energy with decreasing nanocrystalline size, causing the ferromagnetic stabilization to be dominated by double exchange via localized holes rather than by a Zener-like mechanism.

Figure 1

Total valence charge density (a), (c), (e) and spin density (b), (d), (f) for passivated ${\mathrm{Ge}}_{81}\mathrm{Mn}$, ${\mathrm{Ga}}_{40}\mathrm{Mn}{\mathrm{As}}_{41}$, and ${\mathrm{Zn}}_{40}{\mathrm{MnSe}}_{41}$ nanocrystals, respectively.

Figure 2

Spin-polarized electronic structure for passivated (a) ${\mathrm{Ge}}_{81}\mathrm{Mn}$, (b) ${\mathrm{Ga}}_{40}{\mathrm{MnAs}}_{41}$, and (c) ${\mathrm{Zn}}_{40}{\mathrm{MnSe}}_{41}$ nanocrystals. The $e$ and $t_2$ levels are doubly and triply degenerate, respectively. The Fermi level is located at the majority spin $t_2$ levels in all cases. Filled and empty circles denote electrons and holes, respectively. The majority levels are shown in red.

Figure 3

Energy separation of host HOMO and LUMO (squares), host HOMO$\uparrow$-$t_2^{\uparrow }$ (circles), and $t_2^{\uparrow }$-$e^{\downarrow }$ (diamonds) as a function of nanocrystalline diameter for (a) Ge:Mn, (b) GaAs:Mn, and (c) ZnSe:Mn.