| Spintronics, a new research field of science, allows one to explore the physics of previously unavailable combinations of electronic, optical, and magnetism in semiconductors, in which the spin (electrons or holes) is exploited to provide new functionality such as spin based information storage, data processing, spin-polarized laser, etc. It has been predicted, from theoretical simulations, that Mn-doped ZnO (ZnO:Mn) would have such a high Curie temperature. The integration of DMS materials into today's electronics requires low dimension circumstance in order to make use of the advantages offered by spin. single-wall ZnO NT can serve as a good template to study ZnO nanostructures theoretically.So far, the origin of ferromagnetism in the nanosized structures is still not very clear, The issue of whether the formation of magnetic nano-clusters is the origin of ferromagnetism in DMSs is still under debate. Understanding the formation and properties of magnetic clusters is important in providing a clear picture of the magnetic properties of such TM-doped DMSs.On the other hand, magnetic properties of nano-clusters itself is very interesting. For example, bulk TM oxides such as NiO and CoO are antiferromagnetic. However, it has been discovered that the nano-crystalline TM oxides can have very strong ferromagnetism which makes the nano-crystalline TM oxides potentially useful.First-principles calculations based on density functional theory with the generalized gradient approximation were performed to study the stable geometries, electronic structure and magnetic properties of the adsorption of a single Mn atom on a graphitic ZnO sheet and a (9, 0) single-wall ZnO nanotube,Mn-doped ZnO nanotube and NiO nano-clusters. The main work and results include:(1)we have calculated the stable geometries, electronic structure and magnetic properties of the adsorption of a single Mn atom on a graphitic ZnO sheet and a (9, 0) single-wall ZnO nanotube. For the graphitic ZnO sheet, the Mn atom prefers to reside above the center of a hexagon (H site), with a relatively large binding energy of 1.24 eV. The H site is also the most stable site for adsorption of an Mn atom inside the ZnO nanotube, with a large binding energy of 1.47 eV. In both of these cases, the total magnetic moment is 5.0?B per Mn atom, which is the same as that of a free Mn atom. When the Mn atom is adsorbed outside the tube, the most energetically favorable site is the atop oxygen site. The magnetic moment is 3.19?B for this configuration. The smaller magnetic moment is mainly due to the strong p–d mixing of O and Mn orbitals. The different adsorption behaviors are related to the curvatures of the nanostructures.(2)The electronic and magnetic properties of pure and Mn-doped armchair and zigzag ZnO nanotubes were studied. The calculated results show that all of the pure ZnO nanotubes are nonmagnetic and have relatively uniform band gap of 1.66 eV at?-point. Both the armchair and zigzag ZnO nanotubes are found to be direct gap and the band gaps are almost independent of tubular structures. For the Mn-doped ZnO nanotubes, it is noted that the band gaps are influenced by three factors, doping concentration, diameter, and chirality of the tube, while the bond lengths between Zn and O and between Mn and O for these structures are only decided by the diameter of the nanotube. The magnetic moments, however, are independent of the chirality from comparison between armchair and zigzag Mn-doped ZnO nanotube, and mainly governed by the doping concentration and diameter of the nanotube. Furthermore, it is found that large magnetic moments appears in Mn-doped ZnO nanotubes, which are principally due to the hybridization between the O 2p and Mn 3d states by our analysis.(3)The stable geometries and magnetic properties of NiO nano-cluster were studied using density functional theory with the generalized gradient approximation. It is found that AFM is generally preferred whenever spins of Ni atoms can be compensated in small clusters from the energetic point of view,medium sized clusters (~nm) favor ferromagnetic state due to high surface/volume ratio and uncompensated surface spins, however, Relatively large clusters have small magnetic moments which are mainly due to uncompensated surface spins. Our calculations for the Ni18O18 cluster showed that the optimized noncollinear structure was found to be more stable compared to the collinear one. when the size of the clusters increases, the clusters will become more stable because of the increasing binding energy, but the magnetic moment increase firstly, then decreases rapidly. These results are very meaningful for experiment. |
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