Co doped ZnO semiconductor materials: structural, morphological and magnetic properties (original) (raw)
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IEEE Transactions on Magnetics, 2000
Both ZnO and Zn 0 99 Co 0 01 O semiconductors were synthesized through solid state reaction via mechanical milling and thermal treatment. Initially the wurtzite ZnO structures of the synthesized particles were characterized by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR). Since these techniques were unable to identify both contamination atoms and Co distribution, energy dispersive X-ray spectrometry (EDS) was used. EDS showed a successful doping of Co atoms with the atomic ratio of 0.9 0.1%, and also showed a contamination of tungsten (W) atoms, in the atomic ratio of 1.6 0.2% for Zn 0 99 Co 0 01 O, and 1.3 0.2% for ZnO. Substitutions of Co +2 ions with Zn +2 host atoms in the ZnO lattice were exposed through X-ray photo spectroscopy (XPS) data of Co 2p electronic energy levels. UV-vis absorption spectroscopy (UV-vis) was also used to prove Co substitutions in the ZnO lattice. This was revealed by a decrease in band gap from 3.25 0.01 eV to 3.03 0.01 eV, and the existence of newly permitted transitions between intra ionic levels. The ferromagnetic effect of Co doping in ZnO lattice was revealed by the coercivity of 154 50 Oe and positive Curie-Weiss temperature, 79 1 K. Beside ferromagnetic interactions, the calculated effective Bohr Magnetron ( e ), 0 32 0 01 B , suggested anti-ferromagnetic interactions due to be less than the theoretical spin based magnetic moment of Co 2+ ions, 3.0 B . Index Terms-Doping, magnetic semiconductors, zinc alloys.
Structural and magnetic properties of Zn1−xCoxO (0
Journal of Magnetism and Magnetic Materials, 2008
Pure and Zn 1-x Co x O nanoparticles have been synthesized by a simple sol-gel method at low temperature where neither a chelating agent nor subsequent annealing was required. The effect of Cobalt atomic fraction, 'x' ≤ 0.0625, on the structural and magnetic properties of the doped ZnO powders was evaluated. X-ray diffraction and Fourier-transform infrared spectroscopy analyses evidenced the exclusive formation of the ZnO-wurtzite structure; no isolated Co-phases were detected. The linear dependence of cell parameters a and c with 'x', suggested the actual replacement of Zn by Co ions in the oxide lattice. Micro Raman spectroscopy measurements showed a band centered at 534cm-1 , which can be assigned to a local vibrational mode related to Co species, in addition to the normal modes associated with wurtzite. The intensity and broadening of this band at 534 cm-1 were enhanced by increasing 'x'. In turn, the other bands corresponding to A 1 (E 2 , E 1) and E 2 High modes were red shifted at higher Co contents. Room-temperature magnetization measurements revealed the paramagnetic behavior of the Co-doped ZnO nanoparticles.
Soft-x-ray spectroscopic investigation of ferromagnetic Co-doped ZnO
2006
The electronic properties of cobalt-doped ZnO were investigated through site-selective and element-sensitive x-ray-absorption spectroscopy in the vicinity of the Co L 2,3 edge, the oxygen K edge, and at the Zn L 3 edge. The spectroscopic measurements of the ferromagnetic cobalt-doped ZnO films appear to have additional components in the O K edge x-ray-absorption spectrum not observed in the undoped films. The observed features may derive from both hybridization with unoccupied Co 3d states and also from lattice defects such as oxygen vacancies. Only minor changes in the Zn L 3 edge spectra were observed. These observations are consistent with a polaron percolation model in which the ferromagnetic coupling is mediated by shallow donor electrons trapped in oxygen vacancies and couples the Co atoms substituted on Zn sites in the hexagonal wurtzite ZnO structure.
Effect of Co and O defects on the magnetism in Co-doped ZnO: Experiment and theory
Physical Review B, 2007
The electronic structure of Zn 1−x Co x O ͑x = 0.02, 0.06, and 0.10͒ 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 at. %͒ 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.
Magnetic and Structural Properties of Co doped ZnO Nanoparticles
Semiconductor materials with dilute doping of transition metals are of great interest in spintronic applications. In order to investigate the magnetic properties of these diluted magnetic semiconductors, Co-doped Zinc Oxide nanoparticles and thin films are prepared by simple, economic and application oriented sol-gel method. Zinc acetate dihydrate and Cobalt nitrate are used as precursor materials. It is worth mentioning here that all the synthesis is carried out at room temperature. Thin films have been deposited by spin coating technique on glass substrates. Cobalt composition is varied in a range from 0-9wt%. The deposited thin films with the thickness of 80nm are then magnetically annealed at 300°C for 1 hour. XRD results show that Cobalt is successfully doped in zinc oxide crystal lattices while preserving the ZnO wurtzite structure. With increasing cobalt concentration peak broadening is observed in (101) peak which is high intensity peak. Surface morphology is studied by SEM and it indicates that the particle size less than 20 nm in case of nanoparticles and wrinkle network like structure with uniform distribution of size is clearly seen in case of thin films. With increasing cobalt concentration size of the nanostructure decreases well. The effect of cobalt doping on magnetic properties of the synthesized nanoparticles and thin films has been investigated by using VSM. Room temperature ferromagnetism is observed in almost all the samples. It is observed that the doping of cobalt plays important role in the presence of ferromagnetism as the difference in the radii of high spin cobalt (0.58 Å) and divalent Zinc (0.60 Å) is small. It is also observed that strong magnetic properties and crystal quality is obtained by doping with 1% to 10% Cobalt in ZnO to obtain diluted magnetic semiconductors while lower doping percentage results in the lower magnetization and higher doping percentage results in the presence of the secondary phase. Interaction of the transition metal (Cobalt in this case) and free carriers results in the ferromagnetism.
Structural, electronic, and magnetic properties of Co-doped ZnO
Density functional theory based calculations have been carried out to study structural, electronic, and magnetic properties of Zn 1−x CoxO (x = 0, 0.25, 0.50, 0.75) in the zinc-blende phase, and the generalized gradient approximation proposed by Wu and Cohen has been used. Our calculated lattice constants decrease while the bulk moduli increase with the increase of Co 2+ concentration. The calculated spin polarized band structures show the metallic behavior of Co-doped ZnO for both the up and the down spin cases with various doping concentrations. Moreover, the electron population is found to shift from the Zn-O bond to the Co-O bond with the increase of Co 2+ concentration. The total magnetic moment, the interstitial magnetic moment, the valence and the conduction band edge spin splitting energies, and the exchange constants decrease, while the local magnetic moments of Zn, Co, O, the exchange spin splitting energies, and crystal field splitting energies increase with the increase of dopant concentration.
Physica B: Condensed Matter, 2018
Understanding of origin of ferromagnetism in dilute magnetic oxides (DMO's) has become one of the most challenging research problems in condensed matter physics. Here we are reporting a detailed study of magnetic properties and electronic structure of two 5% Co-doped ZnO samples (the as-prepared sample Zn 0.95 Co 0.05 O and the hydrogenated sample Zn 0.95 Co 0.05 O:H). The as-prepared sample is found to be paramagnetic while through hydrogenation, we observed inducement of remarkable ferromagnetism in it. The H-mediated magnetic transition is accompanied by electronic structure modifications with no structural deviations. To get in-depth information into electronic structure correlations of the observed ferromagnetism, we have investigated their electronic properties in detail. For this purpose, we have employed the site-selective and element-sensitive X-ray-absorption spectroscopy (XAS) in the vicinity of the Cobalt L 2,3 edge, the oxygen K edge, and the Zinc L 3 edge using synchrotron radiation. The Co L 2,3 edge spectra clearly show that Co dopants reside at the Zn sites for both these samples and that they are tetrahedrally coordinated with the ligand O atoms. Very minor changes are observed in the Zn L 3 edge spectra. However, the O 1s edge spectra display dominant additional components in the ferromagnetic hydrogenated sample Zn 0.95 Co 0.05 O:H, not observed in the asprepared non-magnetic sample Zn 0.95 Co 0.05 O. We conclude that the observed spectral features can be attributed to the presence of O vacancies and the hybridization of Co 3d states with O 2p vacancy states. These two factors together are likely to play important role in inducement of ferromagnetic ordering in this Co-doped ZnO system. However, which of these two weighs more in this mechanism, cannot be pinpointed and more studies are required in this regard.
Magnetic properties of Fe doped, Co doped, and Fe+Co co-doped ZnO
Journal of Applied Physics, 2013
The structural, electronic and magnetic properties of Zn 0.95 Co 0.05 O, Zn 0.95 Fe 0.05 O and Zn 0.90 Fe 0.05 Co 0.05 O nanoparticles prepared by a sol-gel method are presented and discussed. Xray diffraction and optical analysis indicated that high spin Co 2+ ions substitute for the Zn 2+ ions in tetrahedral sites. 57 Fe Mössbauer spectroscopy showed the presence of isolated paramagnetic Fe 3+ ions in both Fe doped and Fe+Co co-doped ZnO, however, no evidence of ferromagnetically ordered Fe 3+ ions is observed. In the Zn 0.95 Fe 0.05 O sample, weak presence of ZnFe 2 O 4 was detected as an impurity phase, whereas Zn 0.90 Fe 0.05 Co 0.05 O was impurity-free. Results of these studies suggest that Fe and Co ions in the Fe+Co co-doped sample has a strong synergistic effect because they eliminated the presence of impurities and gave the strongest ferromagnetic signal. Possible role of charge transfer ferromagnetism involving mixed valence ions is considered as a potential mechanism in these nanoparticles. Presence of both Co 2+ and Fe 3+ might promote more efficient charge transfer in the co-doped Zn 0.90 Fe 0.05 Co 0.05 O, leading to the enhanced ferromagnetism observed in this sample. However, more evidence is necessary to confirm the role of charge transfer ferromagnetism.
Structural, chemical and magnetic properties of secondary phases in Co-doped ZnO
2011
Abstract. We have utilized a comprehensive set of experimental techniques such as transmission electron microscopy (TEM) and synchrotron-based x-ray absorption spectroscopy (XAS) and the respective x-ray linear dichroism and x-ray magnetic circular dichroism to characterize the correlation of structural, chemical and magnetic properties of Co-doped ZnO samples.
Chemical Communications, 2012
Surface-modified Zn 0.98 Co 0.02 O nanocomposites have been synthesized by a simple solvothermal technique using zinc nitrate hexahydrate (AR, SCRC), cobalt nitrate hexahydrate (AR, SCRC) and sodium hydroxide (AR, SCRC) as starting materials. Absolute ethanol was used as solvent and oleic acid (OA) was introduced as a capping molecule for nanoparticles. Detailed experimental synthesis processes are described as follows. Firstly, 0.1 M *98% Zn(NO 3)•6H 2 O/ 2% Co(NO 3)•6H 2 O and 0.3 M NaOH dissolved in 150 mL absolute ethanol at the room temperature, with a vigorous magnetic string to insure the solution homogenous. Secondly we divided the mixture into three equal parts. For each one we added a different concentration of oleic acid under vigorous stirring to obtain three solutions with the following Zn(Ⅱ)/OA molar ratios: 5:3, 5:6 and 5:9. After all of solutions were transferred into Teflon-lined stainless steel autoclave of capacity 100 mL. The autoclave were sealed and heated automatically to ca. 120°C, persisted for 12 hours and then cooled to room