Magnetism Of Co-doped ZnO Research Articles

Optimizing physical properties of Co-doped ZnO nanoparticles: Controlling oxygen vacancy and carrier concentration

The origination of magnetism in Co-doped ZnO-based dilute magnetic semiconductor (DMS) with room temperature ferromagnetism (RTFM) has some controversy. In this paper, Zn1-xCoxO nanoparticles (x = 0.00, 0.01, 0.03 and 0.05) were prepared using sol-gel method. The relationship between oxygen vacancy or carrier concentration and magnetic properties were investigated. The hexagonal wurtzite structure of all the samples was confirmed by X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR). The morphological features were investigated by scanning electron microscopy (SEM) and transmission electron microscope (TEM), and the optical properties, including energy gap, were analyzed by ultraviolet–visible spectroscopy (UV–vis). The number of oxygen vacancies (VO) was measured by X-ray photoelectron spectroscopy (XPS), which is positively correlated with the Co content. The RTFM properties were analyzed by physical properties measurement system (PPMS). The results reveal that a higher carrier concentration is critical to promoting ferromagnetism. And, the magnetism of Co-doped ZnO may originate from VO defects or the carrier.

Effects on the magnetic and optical properties of Co-doped ZnO at different electronic states

Both blue and red shifts in the absorption spectrum of Co-doped ZnO have been reported at a similar concentration range of doped Co. Moreover, the sources of magnetism of Co-doped ZnO are controversial. To solve these problems, the geometry optimization and energy of different Co-doped ZnO systems were calculated at the states of electron spin polarization and nonspin polarization by adopting plane-wave ultra-soft pseudopotential technology based on density function theory. At the state of electron nonspin polarization, the total energies increased as the concentration of Co-doped increased. The doped systems also became unstable. The formation energies increased and doping became difficult. Furthermore, the band gaps widened and the absorption spectrum exhibited a blue shift. The band gaps were corrected by local-density approximation + [Formula: see text] at the state of electron spin polarization. The magnetic moments of the doped systems weakened as the concentration of doped Co increased. The magnetic moments were derived from the coupling effects of [Formula: see text]–[Formula: see text]. The band gaps narrowed and the absorption spectrum exhibited a red shift. The inconsistencies of the band gaps and absorption spectrum at the states of electron spin polarization and nonspin polarization were first discovered in this research, and the sources of Co-doped ZnO magnetism were also reinterpreted.

Structure, valence, and magnetism of Co-doped ZnO at the coalescence limit (invited)

Zn0.8Co0.2O epitaxial films have been grown by reactive magnetron sputtering and investigated with regard to the local structural properties, valence, and magnetism using x-ray absorption spectroscopy and both linear as well as magnetic circular dichroism. Samples were found to be phase pure and paramagnetic even at the coalescence of the Co doping and a small magnetic polarization of the Zn can be inferred. The changes of structure, valence, and magnetism upon increasing the oxygen content in the sputter gas are researched as well.

Influence of defects on magnetism of Co-doped ZnO

Two kinds of Zn0.97Co0.03O powders were prepared by precursor thermal decomposition under different conditions. One grown at low temperature has a positive Curie–Weiss temperature Θ, while the other grown at high temperature has a negative Θ. Both of them contain oxygen vacancies. There are more shallow donors in the former than those in the latter. It is proposed that coexistence of oxygen vacancies and shallow donors is necessary to induce ferromagnetic coupling between Co ions.

Effect of oxygen vacancy defect on the magnetic properties of Co-doped ZnO

The influence of oxygen vacancy on the magnetism of Co-doped ZnO has been investigated by the first-principles calculations. It is suggested that oxygen vacancy and its location play crucial roles on the magnetic properties of Co-doped ZnO. The exchange coupling mechanism should account for the magnetism in Co-doped ZnO with oxygen vacancy and the oxygen vacancy is likely to be close to the Co atom. The oxygen vacancy (doping electrons) might be available for carrier mediation but is localized with a certain length and can strengthen the ferromagnetic exchange interaction between Co atoms.

Enhanced ferromagnetism in single crystalline Co-doped ZnO thin films by Al codoping

High-quality (Co, Al)-codoped ZnO single crystalline films have been grown on a-plane sapphire substrates using molecular-beam epitaxy. Al codoping yields the Co-doped ZnO thin films to exhibit metallic conducting behavior with high fee carrier concentration. X-ray absorption studies confirm that nearly all Co ions are in divalent state and actually substituted into the ZnO lattice without formation of any detectable secondary phase. Compared with weak ferromagnetism (0.32 μ B/Co 2+) in the Co-doped ZnO single crystalline film, the film with additional Al codoping was found to have much stronger ferromagnetism (0.83 μ B/Co 2+) at room temperature. The magnetic anisotropy was also observed in the (Co, Al)-codoped ZnO thin film, indicating the ferromagnetism is intrinsic. Our experimental observations suggest that additional electron doping is effective to strengthen the ferromagnetism in Co-doped ZnO films, which is well consistent with the recent theoretical description of carrier-mediated magnetism in Co-doped ZnO.

The origins of ferromagnetism in Co-doped ZnO single crystalline films: From bound magnetic polaron to free carrier-mediated exchange interaction

High-quality Co-doped ZnO single crystalline films with a wide range of carrier concentration and good reproducibility have been grown by molecular beam epitaxy. After the systematic studies of the magnetic and transport properties of the films, we suggest that there are two distinct ferromagnetic mechanisms in different conductivity regimes. In the insulating regime, carriers tend to be localized, favoring the formation of bound magnetic polarons, which leads to ferromagnetism. In the metallic regime, however, most carriers are weakly localized and the free carrier-mediated exchange is dominant. Our experimental observations are well consistent with the recent theoretical description of magnetism in Co-doped ZnO and helpful for understanding the ferromagnetic mechanism in oxide-based diluted magnetic semiconductors.

Effect of Co and O defects on the magnetism in Co-doped ZnO: Experiment and theory

The electronic structure of ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{O}$ ($x=0.02$, 0.06, and 0.10) diluted magnetic semiconductors is investigated using soft x-ray emission spectroscopy and first-principles calculations. X-ray absorption and emission measurements reveal that most Co dopants are incorporated at the Zn sites and that free charge carriers are absent over a wide range of Co concentrations. The excess Co interstitials appear in the samples with high Co concentration ($10\phantom{\rule{0.3em}{0ex}}\mathrm{at.}\phantom{\rule{0.2em}{0ex}}%$) but are isolated without any direct exchange interaction with substitutional Co atoms. The lack of free charge carriers and the direct Co-Co interactions is responsible for the absence of ferromagnetism in the samples. First-principles calculations suggest that the exchange interaction between substitutional Co atoms induces only an antiferromagnetic coupling, and strong ferromagnetism in Co-doped ZnO requires not only free charge carriers but also the Co interstitials directly interacting with substitutional Co atoms.