Extrinsic origin of ferromagnetism in doped ZnO (original) (raw)
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Magnetic properties of doped ZnO prepared by different synthetic routes
Journal of Magnetism and Magnetic Materials, 2007
We have studied the magnetic properties of Zn 0.96 M 0.04 O (M ¼ Mn, Fe, Co) compounds prepared using several routes. The low temperature ceramic synthesis gave multiphasic samples and show ferromagnetic behavior. Single phases can be obtained by heating at higher temperatures ($900-1100 1C). The use of very low oxygen pressure also favours the preparation of single-phases. We were also successful in preparing single-phase samples at very low temperature ($400 1C) by using a sol-gel method. All of the samples without noticeable secondary phases in the X-ray patterns behave as conventional paramagnets. This is true even for the samples with very low grain size. Samples exhibiting secondary phases reveal spontaneous magnetization even at room temperature in some cases. Our results strongly support that ferromagnetism at room temperature is always due to the presence of secondary phases and not to the doping of ZnO. r
Structural and magnetic properties of Mn-doped ZnO nanocrystals
Abstract: Mn-doped ZnO nanocrystals were successfully prepared using a novel sol–gel method followed by drying in autoclave under supercritical conditions. The estimated crystallite size is in the range of 30–50 nm, in agreement with TEM analysis. Rietveld refinements confirm the formation of pure Mn-doped ZnO for lower Mn concentration. i.e. less than 5%. The lattice parameters increase with increasing Mn content according to Vegard's law due to the larger ionic radius of Mn2+ compared to that of Zn2+. Magnetic analysis reveals that increasing the doping level of Mn above 2% is not helping the long range ferromagnetic order in the sample but only enhancing the paramagnetic component. The paramagnetic susceptibility is found to increase linearly with increasing Mn concentration which suggests the formation of uncoupled magnetic moment. The estimated values for the magnetic moment per Mn atom are found to be in the range of 2–3.5 µB/Mn. Ab-initio calculations also have been performed which showed that doping diamagnetic bulk ZnO with Mn induces ferromagnetic at room temperature, the total magnetic momentum increases with increasing Mn content whereas the magnetic moment of Mn is predicted to be in the range of 3–3.5 μB/Mn atom which is consistent with the values obtained from magnetic measurements.
Absence of room temperature ferromagnetism in bulk Mn-doped ZnO
Journal of Applied Physics, 2004
Structural and magnetic properties have been studied for polycrystalline Zn1−xMnxO (x =0.02, 0.03, 0.05). Low-temperature (∼ 500 • C) synthesis leaves unreacted starting ZnO and manganese oxides. Contrary to a recent report, no bulk ferromagnetism was observed for single-phase materials synthesized in air at temperatures above 900 • C. Single-phase samples show paramagnetic Curie-Weiss behavior.
Structural and magnetic properties of chemically synthesized Fe doped ZnO
Journal of Applied Physics, 2009
We report on the synthesis of Fe-doped ZnO with nominal composition of Zn 0.99 Fe 0.01 O by using a coprecipitation method. X-ray diffraction and selective area electron diffraction studies reveal a single phase wurtzite crystal structure without any secondary phase. Field emission transmission electron microscopy measurements infer that Zn 0.99 Fe 0.01 O have nanorod-type microstructures. Magnetic hysteresis measurement performed at different temperatures show that Zn 0.99 Fe 0.01 O exhibits a weak ferromagnetic behavior at room temperature. A detailed investigation of the electronic and local structure using O K-, Fe L 3,2 near edge x-ray absorption fine structure suggests that Fe is substituting Zn in ZnO matrix and is in Fe 3+ state.
Magnetism in Mn-doped ZnO nanoparticles prepared by a co-precipitation method
Nanotechnology, 2006
We report the synthesis of nominal 2 and 5 at.% Mn-doped ZnO nanocrystalline particles by a co-precipitation method. Rietveld refinement of x-ray diffraction data revealed that Mn-doped ZnO crystallizes in the monophasic wurtzite structure and the unit cell volume increases with increasing Mn concentration. DC magnetization measurements showed ferromagnetic ordering above room temperature with H c ∼ 150 Oe for nominal 2 at.% Mn-doped ZnO nanoparticles annealed at 675 K. A distinct ferromagnetic resonance (FMR) signal was observed in the EPR spectra of the 2 at.% Mn-doped ZnO nanoparticles annealed at 675 K. EPR measurements were used to estimate the number of spins participating in ferromagnetic ordering. Of the total Mn present in the 2 at.% Mn ZnO lattice, 25% of the Mn 2+ ions were responsible for ferromagnetic ordering, whereas nearly 5% of the Mn 2+ ions remained uncoupled (isolated spins). A well resolved EPR spectrum of 5% Mn-doped ZnO samples annealed at 875-1275 K (g = 2.007, A = 80 G, D = 210 G and E = 15 G) confirmed that Mn was substitutionally incorporated into the ZnO lattice as Mn 2+. On increasing the temperature of annealing beyond 1075 K an impurity phase emerges in both the 2 and 5 at.% Mn-doped ZnO samples, which has been identified as a variant of (Zn 1−X Mn(II) X)Mn(III) 2 O 4 with T c ∼ 15 K. Our results indicate that the observed room temperature ferromagnetism in Mn-doped ZnO can be attributed to the substitutional incorporation of Mn at Zn-sites rather than due to the formation of any metastable secondary phases.
Structure and magnetism of manganese-doped ZnO powder samples
Crystal Research and Technology, 2009
The magnetic and structural properties of manganese-doped ZnO powder samples prepared by a solid state method are reported. Magnetization measurements indicate ferromagnetic behavior, with hysteresis observed in the M vs. H behavior at 300 K. Coercive fields were <100 Oe at 300 K. Temperature-dependent magnetization measurements showed evidence for ordering temperatures of >300 K. However, the results show that ferromagnetism originates from the doped matrix rather than any type of magnetic cluster and the ferromagnetism is due to the defects and/or oxygen vacancies confined to the surface of the grains.
Magnetic properties of Co-doped ZnO prepared by sol–gel method
Materials Science and Engineering: B, 2009
A l 2 O 3 SUBSTITUTED nickel zinc nano-ferrite Al x Ni 0.5-x Zn 0.5 Fe 2 O 4 (x = 0.0, 0.1, 0.2, 0.3) was prepared by the citrate sol-gel methodusing nickel, zinc, iron and aluminum nitratesand sintered at 800 o C for 3 h in air. Prepared nano-sized ferrites were characterized by X-ray diffraction, High resolution transmission and scanning electron microscopy and Fourier transforms infrared spectroscopy. The estimated crystallite sizes were in the range of 24.7-32.9 nm. HR-TEM data appears that all nano-ferrite samples are composed of more or less agglomerated nanoparticles with the average particle size of nanocrystallites is ~31 nm. The impact of introducing Al ions by Ni on themagnetic properties of the prepared nano-ferrite was investigated utilizing magnetic measurements at room temperature. The changing of crystallite sizes, lattice parameter and the surface area with increasing the Al content were determined. The saturation magnetization (Ms) and coercivity (Hc) values vary from 47.894-32.314 and 70.37-60.117 G; respectively.
Synthesis, Structure, and Room Temperature Ferromagnetism of Mn and/or Co Doped ZnO Nanocrystalline
Nanostructures of pure and (Mn, Co) doped and codoped ZnO have been prepared by chemical coprecipi-tation method. The crystal structure, morphology, and fer-romagnetism of the prepared samples were investigated by using X-ray diffraction, Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy, and vibrating sample magnetometer. The X-ray diffraction results demonstrated that all samples having a single phase with hexagonal wurtzite structure of ZnO. FTIR spectra showed the characteristic absorption bands of ZnO in different samples. The TEM images revealed that pure and Co-doped ZnO samples having uniform nanoparticle shapes with an average particle size of 18–20 nm. whereas, Mn-doped ZnO sample has nanorod structure and (Mn, Co) codoped ZnO sample has a mixture of nanoparticle and nanorod structures. The magnetic hysteresis loop of pure ZnO nanoparticles at room temperature revealed a saturation magnetization of 0.22 emu/g and a coercivity of 91.6 Oe. Alternatively, the hysteresis loops of Zn 0.99 Mn 0.01 O, Zn 0.99 Co 0.01 O, and Zn 0.98 Mn 0.01 Co 0.01 O samples illustrated more pronounced magnetic properties with saturation magnetization of 0.788, 0514, and 0.918 emu/g and coerciv-ity of 141.9, 452.5, and 185.6 Oe, respectively. The effects of doping ions on the structure and ferromagnetism of the investigated samples are discussed.
Journal of Alloys and Compounds, 2013
This paper reports the influence of the temperature annealing (T a ) on the structural, morphological, Raman and EPR characteristics of Mn-doped ZnO nanoparticles. XRD studies reveal a wurzite-type structure, while the formation of ZnMnO 3 secondary phase was evidenced only for the nanoparticles annealed at T a = 700°C. This impurity phase was also identified by Raman scattering in the samples thermally treated at 600°C and higher temperatures. TEM investigations reveal that the average particle size of samples starts from 13 nm. With the increase of synthesis temperature, the average particle size reaches a value of 55 nm for 700°C. From the XPS spectrum of Mn 2p core-level doublet only the 2+ valence state for manganese ions was evidenced. EPR investigations show that depending on the annealing temperature, Mn ions are incorporated either in the interior of ZnO nanoparticles (T a = 425 and 500°C) or at their surfaces (T a > 500°C). For the sample annealed at T a = 425°C, a new broad resonance line arises, which was attributed to a ferromagnetic phase. We assume that this ferromagnetic phase could be due to the interaction between the Mn 2+ ions and uncompensated acceptor defects incorporated into the ZnO lattice during the thermal treatment of the samples. Our investigations show that the ferromagnetism in Mn-doped ZnO nanoparticles could appear in the low-temperature annealed samples and disappears in the samples thermally treated at high temperatures.
Nanocrystalline ZnO Doped with Fe2O3- Magnetic and Structural Properties
Acta Physica Polonica A, 2011
We have studied the magnetic properties of ZnO nanocrystals doped with Fe2O3 in the magnetic dopant range from 5 to 70 wt%. The nanocrystals were synthesized by wet chemical method. The detailed structural characterization was performed by means of X-ray diffraction and micro-Raman spectroscopy measurements. The results of systematic measurements of magnetic AC susceptibility as a function of temperature and frequency are presented. We observed different types of magnetic behavior. For ZnO samples doped with low content of Fe2O3, the results of low-field AC susceptibility are satisfactorily explained by superparamagnetic model including inter-particle interactions. With the increase of magnetic Fe2O3 content, the spin-glass-like behavior is observed.