Mechanisms of enhancing magnetic properties of Zn 1− x Co x O films prepared by the sol–gel method (original) (raw)
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Structural, optical and magnetic properties of diluted magnetic semiconducting Zn 1− x Mn x O films
Solid State Communications, 2002
With a view to understand the influence of nano size on various properties of cobalt-doped ZnO-based diluted magnetic semiconductors, a series of materials were prepared by the citrate gel route. The phase and morphology studies have been carried out by X-ray diffraction and transmission electron microscopy, respectively. All the samples of the present investigation are found to have hexagonal wurtzite structure and crystallite sizes are found to vary from 25 nm to 65 nm. From the optical absorption measurements it has been observed that upon doping with cobalt, the energy band gap is found to shift towards lower energy side (red shift) while it shifts towards higher energy side (blue shift) when the crystallite size is increased continuously. It has been observed from the XPS results that oxidation state of Cobalt is +2 and that the difference in binding energies of Co 2p 3/2 and Co 2p 1/2 is found to increase continuously with increasing crystallite size. Finally, all the samples are found to exhibit room temperature ferromagnetism and the specific magnetization decreases with increasing crystallite size.
Ferromagnetism study of Co 0.2 Mg x Zn 0.8− x O films prepared by the sol–gel method
Journal of Sol-gel Science and Technology, 2009
Co0.2Mgx Zn0.8−x O films prepared with different molar ratio of magnesium acetate to zinc acetate were deposited on substrates by the sol–gel technique. X-ray diffraction, photoluminescence (PL) and ferromagnetism measurements were used to characterize the Co0.2Mgx Zn0.8−x O diluted magnetic semiconductors. The acceptor-like defects were determined in the PL band and the intensity of the acceptor-related PL increased with increasing Mg concentration. Therefore, an increase in the number of the acceptor-like defects (zinc vacancies especially) in the Co0.2Mgx Zn0.8−x O film may lead to the enhancement of the magnetic properties. It is worth noting that changes in Mg concentration and the number of the acceptor-like defects are important issues for producing strong ferromagnetism Co0.2Mgx Zn0.8−x O films prepared by the sol–gel method.
Structural, optical and magnetic properties of Zn 1− x Co x O thin films prepared by spray pyrolysis
A series of Co-doped ZnO films were fabricated by a spray pyrolysis method. The X-ray diffraction patterns of the films showed that the undoped and Co-doped ZnO films exhibit wurtzite crystal structure. Surface morphology of the films obtained by scanning electron microscopy reveals that pure ZnO film has the hexagonal shaped microrods. The introduction of Co content in the structure did not affect the surface morphology of the films significantly. Magnetic hysteresis loops were observed at room temperature, indicating that ferromagnetism can be realized with Co doping into ZnO. The values of remnant magnetization and coercive field slightly decreased when the concentration of Co was increased beyond 1 at.%. The calculated band gap value of undoped and 1 at.% Co doped samples was found to be around 3.08 eV. The band gap energy increases upon 3 at.% Co concentration followed by a decrease for further increase in Co concentration toward 5 at.%. Published by Elsevier B.V.
Structural, optical, magnetic and electrical properties of Zn1−x Co x O thin films
Journal of Materials Science: Materials in Electronics, 2009
Despite a considerable effort aiming at elucidating the nature of ferromagnetism in ZnO-based magnetic semiconductor, its origin still remains debatable. Although the observation of above room temperature ferromagnetism has been reported frequently in literature by magnetometry measurement, so far there has been no report on correlated ferromagnetism in magnetic, optical and electrical measurements. In this paper, we investigate systematically the structural, optical, magnetic and electrical properties of Zn 1-x Co x O:Al thin films prepared by sputtering with x ranging from 0 to 0.33. We show that correlated ferromagnetism is present only in samples with x > 0.25. In contrast, samples with x < 0.2 exhibit weak ferromagnetism only in magnetometry measurement which is absent in optical and electrical measurements. We demonstrate, by systematic electrical transport studies that carrier localization indeed occurs below 20-50 K for samples with x < 0.2; however, this does not lead to the formation of ferromagnetic phase in these samples with an electron concentration in the range of 6 × 10 19 cm -3 ~ 1 × 10 20 cm -3 . Detailed structural and optical transmission spectroscopy analyses revealed that the anomalous Hall effect observed in samples with x > 0.25 is due to the formation of secondary phases and Co clusters.
Journal of Alloys and Compounds, 2013
In this research, Zn 1Àx Mn x O (0 6 x 6 0.2) thin films were synthesized by sol-gel technique for ferromagnetic and paramagnetic responses at high and low temperatures. In this respect, the produced films were characterized through X-ray diffraction (XRD), scanning electron microscope (SEM), UV-vis spectrometer and superconducting quantum interference device (SQUID) magnetometer. Based on the characterizations and measurements, the influence of pH on the structural, optical and magnetic properties of a Zn 1Àx Mn x O thin films was scrutinized in detail. The analyzed results indicate that the films possess a good crystal quality and homogeneous, smooth surfaces with a pure hexagonal zincsite syn structure of ZnO without any Mn based phases. By increasing the Mn content, the films exhibited increase or decrease in both the c-axis lattice constant and fundamental band gap energy depending on their respective pH. It is clear that the Zn 1Àx Mn x O (x = 0.02 and 0.05) thin films prepared at pH of 4.6 exhibited both a ferromagnetic at high temperatures (100, 200, and 300 K) and a paramagnetic response at low temperature (5 K), whilst Zn 1Àx Mn x O (0.01 6 x 6 0.2) films prepared at pH value of 7.56 have paramagnetic behavior and antiferromagnetic contributions. The observed room temperature ferromagnetic behavior of the Zn 1Àx Mn x O (x = 0.02 and 0.05) thin films at pH of 4.6 may be related to the interaction between Mn doping and the oxygen vacancies, an oxygen-vacancy-stabilized metastable phase Mn 2Àx Zn x O 3Àd or clustering of Mn ions.
Atomic structure and magnetic properties of cobalt doped ZnO thin films prepared by sol-gel method
The ability of a sol-gel route for the synthesis of cobalt doped ZnO (Zn 1-x Co x O, x = 0.04-0.11) films grown on Si (100) and glass substrates is investigated. A homogeneous and stable Zn 1-x Co x O sol was prepared by dissolving zinc nitrate hexahydrate and cobalt acetate tetrahydrate in a PVP (polyvinylpyrrolidone) solution, followed by annealing at 800 ºC. Local structure studies of the Zn 1-x Co x O thin films by XANES (X-ray absorption near-edge structure) and EXAFS (extended X-ray absorption fine structure) proved the coexistence of a few amount of small metal cobalt aggregates with Co in nonstoichiometric Co n O m aggregates, with n > m, and m ~ 4. Low temperature magnetooptical Kerr effect (Faraday rotation) measurements of the magnetization versus field give a direct proof of a superparamagnetic behavior of the magnetic aggregates and allowed an estimate of the number of magnetically active atoms in Co aggregates, which is close to the number of Co excess atoms inferred from XANES and EXAFS considerations: n-m ~ 3 atoms per aggregate. To this main superparamagnetic phase, a weak ferromagnetic phase with coercitive field of ~ 50 Oe is superimposed, most probably due to metal cobalt nanoclusters.
Structural and Magnetic Properties of Codoped Zn O based Diluted Magnetic Semiconductors
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.
Study of Zn1−xCoxO (0.02≤x≤0.08) dilute magnetic semiconductor prepared by mechanosynthesis route
Journal of Magnetism and Magnetic Materials, 2010
Single-phase Zn 1 À x Co x O (0.02 r xr 0.08) dilute magnetic semiconductor is prepared by mechanical milling process. The shift of XRD peaks towards the higher angle and a redshift in the band gap compared to the undoped ZnO ensure the incorporation of Co 2 + ions in the semiconductor host lattice. Pure Zn x Co 1 À x O phases show the paramagnetic behavior in the temperature range 80 K r Tr 300 K. The room temperature volume magnetic susceptibility (w v) estimated in case of Zn 0.96 Co 0.04 O is $ 10 À 5 emu/Oe cm 3. The temperature dependence of susceptibility w v can be fitted well with Curie law confirming the paramagnetic interaction. The observed crystal-field splitting of 3d levels of Co 2 + ions inside Zn 1 À x Co x O has been successfully interpreted using Curie law.
Physica B: Condensed Matter, 2009
In this paper we report on ZnO-CoO thin films grown by sol-gel technology with using different solvent. As precursors zinc acetate dehydrate (Zn(CH 3 COO) 2 ?2H 2 O), cobalt acetate tetrahydrate (Co(CH 3-COO) 2 ?4H 2 O) as well as manganese chloride (MnCl 2) were used. The films grown from solvent with methanol have a highly preferred orientation along the (1 0 0) plane. The films deposited from C = 0.5 and 0.7 mol/l solvent with isopropanol and monoethanolamin and in smaller degree the C = 0.3 mol/l film consist of disordered crystallites whereas the C =0.1 mol/l film has the intensive (0 0 2) peak, revealing a more obvious [0 0 2] preferred orientation. The all obtained films are highly transparent. The PL of ZnO-CoO films are more intensive in ultraviolet region in comparison the PL of ZnO film. The ESR spectra of ZnO-CoO multilayer films were investigated.
Paramagnetism of the Co sublattice in ferromagnetic Zn1−xCoxO films
Physical Review B, 2007
Using the spectroscopies based upon x-ray absorption, we have studied the structural and magnetic properties of Zn1−xCoxO films (x = 0.1 and 0.25) produced by reactive magnetron sputtering. These films show ferromagnetism with a Curie temperature TC above room temperature in bulk magnetization measurements. Our results show that the Co atoms are in a divalent state and in tetrahedral coordination, thus substituting Zn in the wurtzite-type structure of ZnO. However, x-ray magnetic circular dichroism at the Co L2,3 edges reveals that the Co 3d sublattice is paramagnetic at all temperatures down to 2 K, both at the surface and in the bulk of the films. The Co 3d magnetic moment at room temperature is considerably smaller than that inferred from bulk magnetisation measurements, suggesting that the Co 3d electrons are not directly at the origin of the observed ferromagnetism. PACS numbers: 75.50.Dd, 75.30.Hx, 61.10.Ht