Defect-induced magnetism in nitride and oxide nanowires: Surface effects and quantum confinement

Defect-induced local moment formation in GaN and ZnO nanowires is investigated using density functional theory-based first-principles electronic-structure methods. We find that quantum confinement and surface effects, coupled with strong structural relaxations in nanowires, significantly enhance the spin-polarization energy and reduce the defect formation energy. These results are consistent with the fact that unconventional magnetism is often found in nanostructures and thin films.

Figure 1

(Color online) Nanowire structure studied in this paper.

Figure 2

(Color online) (a) Total DOS plot for ${\text{Ga}}_{108}{\text{N}}_{108}$ nanowire and the bulk. The DOS for the NW shows surface states (${\text{S}}_{\text{N}}$ and ${\text{S}}_{\text{Ga}}$) that are derived from the under-coordinated gallium and nitrogen on the surface. (b)Projected DOS for the peripheral nitrogen and gallium atoms showing the origin of ${\text{S}}_{\text{N}}$ and ${\text{S}}_{\text{Ga}}$.

Figure 3

(Color online) Spin-resolved DOS of a ${\text{Ga}}_{107}{\text{V}}_{\text{Ga}}{\text{N}}_{108}$ nanowire with a Ga vacancy created inside the nanowire. The DOS shows a large spin-splitting between majority- and minority-spin defect states.

Figure 4

(Color online) Spin-density isosurface for the ${\text{Ga}}_{107}{\text{V}}_{\text{Ga}}{\text{N}}_{108}$ nanowire showing the localization of the defect states on the four nitrogen atoms surrounding the gallium vacancy. Here the smallest spheres (blue/light gray spheres) are nitrogen atoms and larger spheres (red/dark gray) are gallium atoms.

Figure 5

(Color online) (a) Spin-resolved DOS for a 216-atom GaN nanowire with a Ga vacancy on the surface. (b) Projection of the DOS onto the $p$ orbitals of the three nitrogen atoms near the Ga vacancy.

Figure 6

(Color online) Spin-density isosurface for the 216-atom GaN nanowire with a cation vacancy on the surface. The three nitrogen atoms near the Ga vacancy are labeled ${\text{N}}_1$, ${\text{N}}_2$, and ${\text{N}}_3$.

Figure 7

(Color online) Spin-resolved DOS for the ${\text{Zn}}_{107}{\text{V}}_{\text{Zn}}{\text{O}}_{108}$-nanowire with a Zn vacancy created on the surface of the nanowire.

Figure 8

(Color online) Spin-Density isosurface for the GaN-nanowire with a ${\text{V}}_{\text{Ga}}{\text{-O}}_{\text{N}}$ complex. Here, the black ball is the substitutional oxygen.