Local investigation of hyperfine interactions in pure and Co-doped ZnO (original) (raw)
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Physical Review B, 2010
We report on investigations of the electronic structure and magnetic properties of ZnO doped with 15 at. % Co and postgrowth annealed at temperatures ranging between 250 and 800°C. In particular, we demonstrate how the presence of Co 3+ , indicative of secondary phases, is manifested in spectroscopy. Through resonant photoemmision spectroscopy we have found that x-ray diffraction in some cases underestimates or does not reveal the presence of secondary phases, possibly due to unrelaxed structures or structural arrangements with sizes below the detection limit. The magnetic properties are in most cases understood by assuming small antiferromagnetic clusters but can also show a behavior indicative of ferromagnetic interactions.
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.
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.
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.
Electronic Structure of Co-doped ZnO Thin Films by X-ray Absorption and Emission Spectroscopy
Journal of The Korean Physical Society, 2009
The electronic structure of Co-doped ZnO thin films, synthesized with a nominal composition of Zn1−xCoxO (x = 0.03, 0.05, 0.7, 0.10, and 0.15) by using spray pyrolysis method, has been investigated using near edge X-ray absorption fine structure (NEXAFS) measurements at the O K-and the Co L3,2-edges and using resonant inelastic X-ray scattering (RIXS) measurements at Co L3,2-edge. All the prepared Zn1−xCoxO thin films showed ferromagnetic behavior at room temperature, as measured by using an alternating gradient force magnetometer (AGFM). The intensity of the pre-edge spectral feature at the O K -edge increases with the Co concentration, which clearly reveals that there is strong hybridization of O 2p − Co 3d atoms in the ZnO matrix. Spectral features of the Co L3,2-edge NEXAFS exhibit multiple absorption peaks and are similar to those of Co 2+ ions coordinated in tetrahedral symmetry by four oxygen atoms. These results clearly demonstrate that Co is in a 2+ state, substituting at the Zn site. Co L3,2-edge RIXS measurements show that the substitution by Co atoms can explain the magnetic interaction in Co-doped ZnO.
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.
Physical Review B, 2008
We present results of electronic structure and magnetization measurements of Co:ZnO and Co:ZnO codoped with Al thin-film samples fabricated by solution-based methods together with ab initio electronic structure calculations. Electronic structure measurements indicate that the Co states lie close to the valence-band edge with pinning of the Fermi level primarily due to native defects yielding a heavily n-doped material. The findings in the electronic structure measurements are corroborated by results from theoretical calculations. We find that it is necessary to go beyond the local-density approximation to achieve agreement with experiments. Moreover, the theoretical calculations indicate a tendency for the formation of Co clusters, giving rise to an antiferromagnetic exchange interaction between the Co atoms. The magnetization measurements are well in line with the theoretical predictions, showing a dominating superparamagnetic behavior arising from small antiferromagnetic clusters containing uncompensated spins.
Nearest neighbor exchange in Co and Mn-doped ZnO
2005
We calculate the magnetic interactions between two nearest neighbor substitutional magnetic ions (Co or Mn) in ZnO by means of density functional theory and compare it with the available experimental data. Using the local spin density approximation we find a coexistence of ferro- and antiferromagnetic couplings for ZnO:Co, in contrast to experiment. For ZnO:Mn both couplings are antiferromagnetic but deviate quantitatively from measurement. That points to the necessity to account better for the strong electron correlation at the transition ion site which we have done by applying the LSDA+U method. We show that we have to distinguish two different nearest neighbor exchange integrals for the two systems in question which are all antiferromagnetic with values between -1.0 and -2.0 meV in reasonable agreement with experiment.