Impact of Surface on the Ferromagnetism of Lithium-Doped Zinc Oxide Nanowires

We present a first-principles study on the impacts of the surface on the ferromagnetism (FM) induced by non-magnetic defects (lithium substitutional dopants at zinc sites Li_Zn and zinc vacancies V_Zn) of zinc oxide (ZnO) nanowires. We show that unlike bulk ZnO structures, both Li_Zn and V_Zn are able to induce d⁰ FM in ZnO nanowires. While the structural confinement at the nanowire surface is found to play a key role in the FM induced by Li_Zn, the reconstruction of the surface tunes the defect-induced FM. By suppressing the surface reconstruction, we find that a Li_Zn and a V_Zn, regardless of their position, induce the same magnetic moments of 0.71 and 2.00 μ_B (μ_B is the magneton Bohr), respectively. The steady correlation between the defect-induced FM, the dangling bonds introduced at these defects, and the local structural reconstruction surrounding these defects is unveiled. Finally, we discuss the saturation of the magnetic moment as the defect density increases.