Magnetic Properties Of Co-doped ZnO Research Articles

Optical and magnetic properties of Co-doped ZnO synthesized by magnetic assisted hydrothermal method

Cobalt(Co) doped ZnO nanoparticles were synthesized by a hydrothermal method with a magnetic field pretreatment(in the hydrolysis reaction). The pretreatment time(t) was set as 0h(no pretreatment), 1h, 2h and 4h. The crystallite structure, the lattice constant have been estimated by X-ray diffraction (XRD) with Rietveld refinement, which shows slight lattice expansion due to Co doping. Scanning electron microscopy (SEM) reveals flower-like microstructure and transmission electron microscopy (TEM) indicates the single crystalline in nature. The defect, the optical and magnetic properties of the four obtained samples were systematically investigated by Raman spectroscopy, photoluminescence (PL) spectra and vibrating sample magnetometer (VSM) respectively. The results show that the pretreatment time of magnetic field has a significant influence on optical and magnetic properties of Co-doped ZnO, and the effective time of magnetic field in the hydrolysis reaction on the magnetic property is between 1h and 2h. When 1h ≤ t ≤ 2h, more Co atoms are incorporated into ZnO and the saturation magnetization (MS) increases by nearly three times, comparing with the sample without pretreatment. Samples of different pretreatment time show changes in the emission band of PL spectra and defect related bands of Raman spectra. The characterization of Raman scattering combined with the analysis of PL spectra suggests that the original of room temperature ferromagnetism(RTFM) may arise from the Co2+–VO+–Co2+ bound magnetic polaron(BMP).

Twin grain boundary mediated ferromagnetic coupling in Co-doped ZnO: First-principles calculations

Abstract First principle calculation, based on density functional theory, is applied to study the electronic and magnetic properties of Co-doped ZnO ∑7 (12 3 0) twin grain boundary. Co atoms substituting Zn at the threefold-coordination sites have the lowest formation energy, compared with other sites. More importantly, the configuration can result in the stable formation of ferromagnetic state (FM). Meanwhile, the strong Co-Co interaction is found to be responsible for the ferromagnetic state. Due to the structural character of the twin grain boundary, periodical defects can be offered, which favors the macroscopic FM ordering. The result also gives us a new thinking to understand the origin of FM in transition metal doped ZnO.

The electronic, magnetic and optical properties of ZnO doped with doubles impurities (Cr, Fe): An LDA-SIC and Monte Carlo study

Electronic structure, magnetic and optical properties of ZnO doped with single and double impurities Zn1−xCrxO, Zn1−xFexO, and Zn1−2xCrxFexO (x=0.03 and 0.06) are investigated using first-principles calculations. Based on the Korringa–Kohn–Rostoker method combined with the coherent potential approximation, we investigated the half-metallic ferromagnetic behavior of doubles impurities (Cr, Fe) doped ZnO. To support our results, we apply the self-interaction-corrected local density approximation (SIC-LDA) to study the electronic structure, optical and magnetic properties of Co-doped ZnO with doubles impurities (Cr, Fe) showing that the half-metallic ferromagnetic state still persists. The stability of the ferromagnetic state compared with the spin-glass state is investigated by comparing their total energies. The exchange interactions obtained from first principle calculations and used in a classical Ising model by a Monte Carlo approach resulted in ferromagnetic states with high Neel temperature.

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 Zn1−xCoxO (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 Co2+ 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 Co2+ 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.

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.

Electronic and magnetic properties of Co-doped ZnO: First principles study

In order to investigate the electronic and magnetic properties of Co–ZnO alloys, we used a full potential linearized augmented plane wave (FPLAPW) method within the density functional theory (DFT), as implement in the WIEN2K package. This work is carried out within the LSDA approximation as the exchange-correlation potential. We have modeled ZnO doped with 6.25, 12.5, and 18.75% of Co. It pointed out that the band gap decrease and the magnetic moments increase with the atomic fraction of Cobalt. The ZnxCo1–xO is found to be a semiconductor, where the filled-states are located in the valence bands and the empty ones above the conduction band edge. The filled and empty d-states are also shown to shift downwards and upwards in the valence and the conduction bands, respectively, with increase in the U potential. The analysis of the partial density of states reveals that the reduction of the ZnO band gap is due principally to the strong p–d interaction of O and Co.

Electronic and magnetic properties of Co-doped ZnO diluted magnetic semiconductor

The effect of low level Co doping (5%) on polycrystalline ZnO samples has been investigated to correlate the observed changes in their magnetic state vis à vis changes in their electronic properties. Rietveld refinement of the XRD patterns confirms single phase crystallization of the samples in the wurtzite type lattice , with no evidence of any secondary phases. The as-synthesized Co-doped sample shows a paramagnetic (PM) state, however, when hydrogenated for ∼6 h, it shows a strong ferromagnetic (FM) ordering. The magnetic moment suppressed significantly when hydrogen ions were evaporated by heating and the sample turned completely paramagnetic upon long heating in air. The Co 2p X-ray photoelectron spectroscopy (XPS) results show that the substituted Co ions are in 2+ oxidation state that incorporate at Zn 2+ sites and no evidence of metallic Co is observed upon hydrogenation. The XRD refinement results and the O 1s XPS results show clear evidence of oxygen depletion upon hydrogenation, followed by a complete regain upon their long heating in air. A plausible explanation for the observed room temperature ferromagnetism (RTFM) is presented in terms of oxygen vacancies, in the framework of bound magnetic polarons model. Our results evidence that the FM ordering can be switched between “on” and “off” by introducing (upon hydrogenation) or removing (by re-heating), respectively, the oxygen vacancies in the Co-doped ZnO matrix.

O-vacancy-mediated spin-spin interaction in Co-doped ZnO: First-principles total-energy calculations

The effects of O vacancy (VO) on the electronic and magnetic properties of Co-doped ZnO are examined through first-principles total-energy calculations. Our results suggest that due to the presence of VO, O 2p states are coupled with the Co 3d states. Co 3d empty minority states are broadened and move up towards the Fermi level. VO at metastable sites can induce the ferromagnetic (FM) coupling between nearest-neighbor (NN) Co ions, and the atoms around the FM-coupled NN Co ions are spin-polarized which leads to the long-ranged FM coupling between Co ions. Our results based on first-principles total-energy calculations give a possible explanation for the controversial magnetic properties of Co-doped ZnO reported in the literature.

H-impurity induced high-temperature ferromagnetism in Co-doped ZnO

Through first-principles total-energy calculations, the effect of H-impurity on the magnetic properties of Co-doped ZnO is studied. Instead of an antibonding location, a bond-centered location of Co–O is the most stable location for isolated H in Co-doped ZnO with a strong bond with oxygen which results in the Co neighbor displaced from the host site to form a Co dimer with the other Co. At the most stable position, due to the strong hybridization between the H-impurity states and the Co 3 d - t 2 g minority spin states at the Fermi level in the gap, H-impurity can mediate a strong short-ranged and long-ranged ferromagnetic spin–spin interaction between neighboring Co atoms. Results based on first-principles total-energy calculations show that H-impurity is a very effective agent that can make Co-doped ZnO process high-temperature ferromagnetism.

Magnetic properties of Co-doped ZnO prepared by sol–gel method

Zn 1− x Co x O (0 < x ≤ 0.09) were prepared by sol–gel method. The structure and magnetism analysis indicate that Co 2+ ions substitute for Zn 2+ ions to form a solid solution with wurtzite structure of ZnO. Zn 1− x Co x O (0.03 ≤ x ≤ 0.09) are paramagnetic at 300 K, but ferromagnetic component seems to exists in the Zn 1− x Co x O ( x = 0.01 and 0.02). Low-temperature (about 150 K) ferromagnetism is found in Zn 1− x Co x O (0 < x ≤ 0.09). Both maximum moment/Co at 80 K and the phase-transition temperature (Curie temperature) decrease with the increase of Co content. This magnetic phenomenon can be understood in the framework of the mean-field Zener model.

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

Zn095Co005O films were prepared under different oxygen partial pressure P by magnetron sputtering. The effect of P on the magnetic and electrical properties was investigated. The effect of oxygen vacancy on the magnetic properties was also calculated by first-principles calculation. The experimental results indicated that Zn095Co005O films showed room-temperature ferromagnetism and high electron concentration when they were deposited under high vacuum. The ferromagnetism disappeared and the electron concentration decreased sharply when P was increased. The calculated results indicated that the energy of ferromagnetic states could be decreased by introducing oxygen vacancy in Co-doped ZnO system. The stability of ferromagnetism was determined by the distance between oxygen vacancy and Co atoms.

Ferromagnetism induced by defect complex in Co-doped ZnO

The effect of Al donor and O vacancy (VO) on the magnetic properties of Co-doped ZnO has been studied by first-principles calculations. It is found that only Al donor cannot induce ferromagnetism (FM) in Co-doped ZnO but can provide additional electrons, which results in the enhancement of Fermi level. The presence of VO makes the Co empty 3d-t2g minority state broadened, and a t2g-VO hybrid level at the conduction band minimum forms. The combination of Al donor and VO results in a charge transfer to the Co empty 3d-t2g minority states, which induces a strong carrier-mediated FM interaction. Our results give an explanation for the controversial magnetic properties of (Co, Al)-codoped ZnO reported in the literature.

Process-dependent magnetic properties of Co-doped ZnO in bulk and thin film form

Structural, optical and magnetic studies of Co-doped ZnO have been carried out for bulk as well as thin films. The magnetic studies revealed the superparamagnetic nature for low-temperature synthesized samples, indicating the presence of cobalt metallic clusters, and this is supported by the optical studies. For the high-temperature sintered samples one obtains paramagnetism. The optical studies reveal the presence of Co 2+ ions in the tetrahedral sites indicating proper doping. Interestingly, the films deposited by laser ablation from the paramagnetic target showed room temperature ferromagnetism. It appears that the magnetic nature of this system is process dependent.

Effect of oxygen partial pressure on the local structure and magnetic properties ofCo-doped ZnO films

Zn0.95Co0.05O films were prepared under different oxygen partial pressures(PO2) by inductively coupled plasma enhanced physical vapor deposition. The effect ofPO2 on the local structure and magnetic properties was investigated. The x-ray absorptionspectroscopy at the Co K-edge, Co L-edge, and O K-edge revealed that the maindefects were oxygen vacancies when the films were deposited under very lowPO2.The change from room-temperature ferromagnetism to paramagnetism was observed with increasingPO2. It was experimentally demonstrated that the oxygen vacancy defect is absolutelynecessary to induce ferromagnetic couplings in Co-doped ZnO films.

Structural and Magnetic Properties of Codoped ZnO based Diluted Magnetic Semiconductors

Zn1-xCoxO (x = 0.01, 0.02, 0.05, 0.10 and 0.20) diluted magnetic semiconductors are prepared by the sol-gel method. The structural and magnetic properties of the samples are studied using x-ray diffraction (XRD), extended x-ray absorption fine structure (EXAFS) and superconducting quantum interference device (SQUID). The XRD patterns does not show any signal of precipitates that are different from wurtzite type ZnO when Co content is lower than x = 0.10. An EXAFS technique for the Co K-edge has been employed to probe the local structures around Co atoms doped in ZnO powders by fluorescence mode. The simulation results for the first shell EXAFS signals indicate that Zn sites can be substituted by Co atoms when Co content is lower than x = 0.05. The SQUID results show that the samples (x < 0.05) exhibit clear hysteresis loops at 300 K, and magnetization versus temperature from 5 K to 350 K at H = 100 Oe for the sample x = 0.02 shows that the samples have ferromagnetism above room temperature. A double-exchange mechanism is proposed to explain the ferromagnetic properties of the samples.

Electron-induced ferromagnetic ordering of Co-doped ZnO

The electronic and magnetic properties of Co-doped ZnO are investigated based on the B3LYP hybrid spin-density functional method. The calculated electronic structures obtained from B3LYP agree well with the experimental results. B3LYP predicts that antiferromagnetic (AFM) ordering between the Co ions is favored over ferromagnetic (FM) ordering in intrinsic Co-doped ZnO, and reveals that the FM ordering can be induced by electron doping when the doping level reaches 1 electron per Co ion. These results agree well with the FM ordering observed in highly conductive n-type Zn1−xCoxO films. Charge transfer to the minority-spin d states of Co atoms and the consequent double-exchange interaction are the primary origins of FM ordering. Since Ni has one more electron than Co, we also investigate the electronic and magnetic properties of intrinsic Ni-doped ZnO. Qualitatively different from the local-density-approximation results, B3LYP predicts that Ni-doped ZnO is an insulator and favors AFM ordering.

The magnetic properties of Co-doped ZnO diluted magnetic insulator films prepared by direct current reactive magnetron co-sputtering

Co-doped ZnO ferromagnetic films were prepared by direct current reactive magnetron co-sputtering at significantly low growth temperature ( ∼ 200 ∘ C ) . Employing complementary characterization we show that a solid solution of Co throughout the ZnO films, where Co is in the 2+ state substituting for Zn. Room temperature ferromagnetism with magnetic moment of 1.1 μ B / Co and a high Curie temperature T C of 750 K are observed in (4 at%) Co : ZnO films, which is not carrier mediated, but co-exists with the dielectric state. The combination of film growth by certain concentration of cobalt doping ( ∼ 4 at % ) and low growth temperature deposition is proved to be key in enhancing the ferromagnetism. The mechanisms responsible for the ferromagnetism in insulating Co : ZnO films are discussed, which is of help for a better understanding of ferromagnetism in transition-metal-doped oxides.

Magnetic properties of Co-doped ZnO diluted magnetic semiconductors prepared by low-temperature mechanosynthesis

We report on the magnetic and structural properties of 2% and 4% Co-doped ZnO diluted magnetic semiconductors prepared by mechanosynthesis. We show that this low-temperature preparation technique does not allow obtaining ferromagnetism in such materials, as already observed in samples prepared by high-temperature techniques. Although X-ray diffraction and UV–Vis spectroscopy indicate that Co ions are well inserted in the ZnO würtzite structure and that mechanosynthesis is susceptible to generate thermodynamically unstable defects, ferromagnetism observed at 5 and 295 K is systematically attributed to parasitic phases. The pure ZnO:Co phase presents no ferromagnetism in agreement with the high temperature prepared samples.