Structural, compositional and magnetic characterization of bulk V2O5 doped ZnO system

This paper investigates the structural, compositional and magnetic properties of vanadium doped ZnO bulk samples prepared by solid state reaction technique. The Rietveld refinement analysis for XRD results of samples showed small change in lattice parameters for 3 and 5% vanadium doped ZnO samples indicating the substitution of Zn²⁺ ions by vanadium ions in ZnO lattice. Raman spectroscopy reveals the change in ZnO modes positions due to vanadium doping. The appearance of E₁ and E₂ modes showed that the wurtzite structure of ZnO is still maintained after doping of vanadium oxide. XPS analysis confirms the presence of the different elements and oxidation states of vanadium ions. M-H curves obtained from VSM showed weak ferromagnetism in the samples. The observation of ferromagnetic behavior indicates the formation of ZnVO phase with V²⁺ ion substitution in the ZnO lattice. XPS scans of the etched bulk samples confirmed the 2+ oxidation state of vanadium ions in our samples explaining the origin of ferromagnetism.     

Electronic structure and ferromagnetism of polycrystalline Zn1−xCoxO (0≤x≤0.15)

The electronic structure of polycrystalline ferromagnetic Zn_1−xCo_xO (0.05≤x≤0.15) and the oxidation state of Co in it, have been investigated. The Co-doped polycrystalline samples are synthesized by a combustion method and are ferromagnetic at room temperature. XPS and optical absorption studies show evidence for Co²⁺ ions in the tetrahedral symmetry, indicating substitution of Co²⁺ in the ZnO lattice. However, powder XRD and electron diffraction data show the presence of Co metal in the samples. This give evidence to the fact that some Co²⁺ ion are incorporated in the ZnO lattice which gives changes in the electronic structure whereas ferromagnetism comes from the Co metal impurities present in the samples.     

Defect-mediated ferromagnetism in ZnO:Mn nanorods

In this work, the structural, chemical and magnetic properties of ZnO:Mn nanorods were investigated. Firstly, well-aligned ZnO nanorods with their long axis parallel to the crystalline c-axis were successfully grown by the vapor phase transport technique on Si substrates coated with a ZnO buffer layer. Mn metal was then diffused into these nanorods at different temperatures in vacuum. From SEM results, ZnO:Mn nanorods were observed to have diameters of ～100 nm and lengths of 4 μm. XPS analysis showed that the Mn dopant substituted into the ZnO matrix with a valence state of +2. Magnetic measurements performed at room temperature revealed that undoped ZnO nanorods exhibit ferromagnetic behavior which may be related to oxygen vacancy defect-mediated d ⁰ ferromagnetism. ZnO:Mn samples were seen to show an excess room temperature ferromagnetism that is attributed to the presence of oxygen vacancy defects forming bound magnetic polarons involving Mn.     

Ferromagnetism in carbon-doped ZnO films from first-principle study

First-principle calculations based on density functional theory have been performed on the nonmagnetic 2p light element C-doped ZnO thin films. The total energies and magnetism of the system are calculated with a ten-layer slab along () direction. The results show that the C-doped ZnO thin films are ferromagnetic. A single C is preferable to occupy the subsurface site. As the concentration of C atoms increases, the ferromagnetic coupling among the dopants is more favorable, and they tend to form a cluster around the Zn atom at the film surface. The ferromagnetism is predicted to be mainly from a p-d exchange-like p-p coupling interaction and a p-d exchange hybridization. The p-p coupling interaction is the dominative mechanism.     

Room-temperature ferromagnetism of Cu ion-implanted Ga-doped ZnO

1 MeV Cu²⁺ ions have been implanted into un-doped ZnO and Ga-doped ZnO films with a dose of 1 × 10¹⁷ ions/cm² at room-temperature. Cu ion-implanted Ga-doped ZnO had ferromagnetism at room-temperature and the saturation magnetization of this sample was estimated to be 0.12 μ_B per Cu, while the Cu ion-implanted un-doped ZnO did not show ferromagnetic behavior. Near-edge X-ray fine structure (NEXAFS) spectroscopy revealed that a partial amount of implanted Cu ions existed as Cu²⁺ (d⁹) state in Ga-doped ZnO film. On the other hand, almost Cu atoms existed as Cu¹⁺ (d¹⁰) state in un-doped ZnO film. However, the subsequent annealing at temperature above 800 °C on this ferromagnetic sample induced the annihilation of ferromagnetism due to the formation of non-ferromagnetic Cu₂O phase.     

Effect of thermal treatment on room-temperature ferromagnetism in Co-doped ZnO powders

The Co-doped ZnO powders were synthesized by sol-gel method, and treated at different temperatures (673-873 K) in the presence or absence of NH₃ atmosphere for 0.5 and 2 h, respectively. X-ray diffraction (XRD) and vibrating sample magnetometer (VSM) show that better crystal structure can cause larger ferromagnetism and the second phase (Co₃O₄) is the reason for saturation magnetization decrease of the sample sintered at higher temperature in air. XPS and nuclear magnetic resonance (NMR) prove the existence of Co²⁺ ions in the Zn_0.9Co_0.1O and the absence of Co clusters, indicating intrinsic ferromagnetism of the samples treated in air. However, strong ferromagnetism of the samples annealed in NH₃ is ascribed to cobalt nitride formed during annealing.     

First-principles study on the magnetic property of C-doped wurtzite ZnS

We have studied the structure, electronic and magnetic properties of wurtzite (WZ) ZnS semiconductor doped with one or two C atoms using first-principles calculations. The moderate formation energy implied that C-doped ZnS could be fabricated experimentally. The total magnetic moment of the 72 atom super cell was 2.02μB, mainly due to the 2p component of the C atom. Electronic structures showed ZnS doped with C atom was p-type half-metallic ferromagnetic (FM) semiconductor and hole mediation was responsible for the ferromagnetism. The large energy difference (154 meV) between the FM and antiferromagnetic (AFM) state implied room-temperature ferromagnetism for C-doped WZ ZnS, which has great potential in spintronic devices.     

Influence of Fe-doping on the structural,optical and magnetic properties of ZnO nanoparticles

Zn_1–xFe_xO (x=0–0.05) nanoparticles were synthesized without a catalyst by a two-step method. Fe was doped into ZnO by a source of metallic Fe sheets in a solid–liquid system at 80 °C, and the Zn_1−xFe_xO nanoparticles were obtained by annealing at 300 °C. X-ray diffraction, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy were used to characterize the structural properties of the as-grown Zn_1−xFe_xO. The optical properties were determined by Infrared and Ultraviolet–visible spectroscopy. The results confirm that the crystallinity of the ZnO is deteriorated due to Fe-doping. XPS results show that there is a mixture of Fe⁰⁺ and the Fe³⁺ in the representative Zn_0.95Fe_0.05O sample. The optical band gap of Zn_1−xFe_xO is enhanced with increasing of Fe-doping. Room temperature ferromagnetism was observed in all the Fe-doped ZnO samples.     

Effect of microstructural characteristics on the magnetic properties of sol-gel synthesized ZnMnO

In this paper we report the effect of microstructural characteristics on the magnetic properties of sol-gel synthesized Mn-doped ZnO. The microstructural characteristics of the samples (e.g., grain sizes and their distribution) have been varied by changing the sintering temperature (T_S) and sintering duration (T_H). Weak room temperature ferromagnetism (RTFM) has been observed in the samples sintered for ∼8 h at 500, 600, 700, 800 and 900 °C. The ferromagnetic fraction and the saturation magnetization, however, first increase as T_S increases from 500 to 600 °C and after that both start decreasing. On the other hand, the samples sintered for ∼12 h at the same temperatures show paramagnetic behavior at room temperature. Field emission scanning electron microscope (FESEM) results show enhancement in the grain sizes with the increase in both T_S and T_H. Energy dispersive X-ray (EDAX) results show increase in the oxygen content in the sample with increase in both T_S and T_H. X-ray diffractometer (XRD) measurements reveal that the basic crystal structure of all the samples corresponds to the wurtzite structure of pure ZnO together with some minor impurities. The correlation between the observed magnetic properties and the microstructural characteristics of the samples has been discussed in this paper.     

Structure and room-temperature ferromagnetism of Co-doped ZnO DMS films

Co-doped ZnO diluted magnetic semiconductor films were prepared on Si(100) substrates by magnetron sputtering system and the Co content varies from 0.01 to 0.15. The X-ray diffraction results showed ZnO of the wurtzite structure. The ferromagnetism was observed at room temperature. The X-ray near-edge absorption spectroscopy revealed that Co substitutes for Zn²⁺ ions in the valence of +2 state in the Co-doped ZnO films.     

A study on the magnetic properties of carbon-doped (1 1 2? 0) ZnO thin films

By using the first principle method based on density functional theory (DFT), a study on the electronic structure and the ferromagnetic stability in C-doped (1 1 2¯ 0) ZnO thin films was conducted. It was found that the thin films have a FM ground state for a majority of configurations. It was also found that C atoms in the thin films have a clear clustering tendency. The ferromagnetism (FM) can be attributed to the coupling between C energy levels. The results showed that oxygen vacancies cannot stabilize the FM coupling of C-doped ZnO thin films. However, zinc vacancies can stabilize the FM coupling of the thin films, which indicates that hole-carriers play a crucial role in the observed FM behavior. In addition, the strain effect on the FM of C-doped ZnO thin films was also analyzed.     

Characterization, sintering and dielectric properties of nanocrystalline zinc oxide prepared by a citric acid-based combustion route

Nanosized zinc oxide has been synthesized through a novel single step solution combustion route using citric acid as fuel. The X-ray diffraction (XRD) analysis revealed that the synthesized ZnO nanopowder has the pure wurtzite structure. The phase purity of the nanopowder has been confirmed using differential thermal analysis (DTA), thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FT-IR). The morphology and crystalline size of the as-prepared nanopowder characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that the powder consisted of a mixture of nanoparticles and nanorods. The nanocrystalline ZnO could be sintered to ∼97% of the theoretical density at 1200 °C in 4 h. The dielectric constant (ε_r) and dielectric loss (ε_i) of sintered ZnO pellets at 5 MHz were 1.38 and 9×10⁻², respectively, at room temperature.     

Surface of Zn-Mn-O and its role in room temperature ferromagnetism: An XPS analysis

The existence of ferromagnetism in Zn-Mn-O semiconductor samples and dependence on the preparation condition were investigated. We systematically examined the samples with manganese concentration ranging from 0 to 10 at.%, prepared by a solid state reaction route using (ZnC₂O₄·2H₂O)_1−x and (MnC₂O₄·2H₂O)_x as precursors. Thermal treatment was carried out in air at temperatures ranging from 400 to 900 °C. The samples were investigated by X-ray diffraction, transmission electron microscopy, magnetization measurements and XPS spectroscopy. XPS surface composition, chemical analysis and depth profiling were successfully employed on powder revealing the chemical composition at the surface of the grains and underneath. The present investigation suggests that physical properties and observed room temperature ferromagnetism might be due to grain surface effects. It seems that the ferromagnetic phase is correlated with oxygen build up at the surface.     

Effect of Dy substitution on structural,magnetic and optical properties of BiFeO3 ceramics

Bi_1−xDy_xFeO₃ (x=0.0, 0.03, 0.05, 0.07, 0.10 and 0.12) ceramics were synthesized by solid state reaction method. Effects of Dy substitution on structural distortion, magnetic and optical properties of BiFeO₃ were examined by X-ray diffraction, Raman and UV–Visible spectroscopy. The samples were found to crystallize in rhombohedral structure of BiFeO₃ with R3c space group. The reduction in lattice parameters and unit cell volume indicate the distortion in FeO₆ octahedra of the rhombohedral structure without any signature of phase transformation up to x=0.12. The predictable weak ferromagnetic hysteresis loops can be observed in the Dy doped samples with maximum remnant magnetization of 0.2103 emu/g for x=0.12. The weak ferromagnetism is ascribed to the suppressed spiral spin structure and magnetically active characteristic of Dy³⁺ ions together with ferromagnetic coupling between Dy³⁺ and Fe³⁺ ions. With optical band gap in visible region, Dy doped BiFeO₃ ceramics are potential material for optoelectronic device and solar cell applications.     

Electronic structure and magnetic properties of BaTi1-xMnxO3

The electronic structure and magnetic properties of polycrystalline BaTi_1-xMn_xO₃ (x = 0–0.1) compounds prepared by solid-state reactions were studied. The results revealed that the increase in Mn content (x) did not change the oxidation numbers of Ba (+2) and Ti (+4) in BaTi_1-xMn_xO₃. However, there is the change in Mn valence that Mn^3+,4+ ions coexist in the samples with x = 0.01–0.04 while Mn⁴⁺ ions are almost dominant in the samples with x = 0.06–0.1. We also point out that Mn³⁺ and Mn⁴⁺ ions substitute for Ti⁴⁺ and prefer locating in the tetragonal and hexagonal BaTiO₃ structures, respectively, in which the hexagonal phase constitutes soon as x = 0.01. Particularly, all the samples exhibit room-temperature ferromagnetism. Ferromagnetic order increases with increasing x from 0 to 0.02, but decreases as x ≥ 0.04. We think that ferromagnetism in BaTi_1-xMn_xO₃ is related to lattice defects and/or exchange interactions between Mn³⁺ and Mn⁴⁺ ions.     

Properties of Co/Ni codoped ZnO based nanocrystalline DMS

Nanoparticles of Co and Ni codoped zinc oxide, Zn_0.9Co_0.1−xNi_xO (x=0.0, 0.03, 0.06 and 0.09), diluted magnetic semiconductors (DMSs) are synthesized by the sol-gel method at annealing temperature of 500 °C. X-ray diffraction (XRD) patterns confirm the single phase character of the samples with x=0.0 and 0.03. However, minor NiO secondary phase is detected in the samples with x=0.06 and 0.09. All of them possess the hexagonal wurtzite structure. There is no significant change in the lattice parameters due to variation of doping concentration. The average particle size is found to be 19.31-25.71 nm. FTIR and UV-vis spectroscopic results confirm the incorporation of the dopants into the ZnO lattice structure. Magnetization data reveal the presence of room temperature ferromagnetism (RTFM). The XRD patterns rule out the formation of secondary phase of either metallic Co cluster or CoO in the samples. Nevertheless, the secondary phases are a concern in any DMS system as a source of spurious magnetic signals. Therefore, we carried out the XPS studies from which the oxidation states of Co and Ni are found to be Co²⁺ and Ni²⁺, respectively. Moreover, XPS O 1s spectra show evidence of the presence of the oxygen vacancy in the ZnO matrix.     

Synthesis and magnetic properties of Ni-doped zinc oxide powders

Polycrystalline Zn_1−xNi_xO diluted magnetic semiconductors have been successfully synthesized by an auto-combustion method. X-ray diffraction measurements indicated that the 5 at% Ni-doped ZnO had the pure wurtzite structure. Refinements of cell parameters from powder diffraction data revealed that the cell parameters of Zn_0.95Ni_0.05O were a little bit larger than ZnO. Transmission electron microscopy observation showed that the as-synthesized powders were of the size ∼60 nm. Magnetic investigations showed that the nanocystalline Zn_0.95Ni_0.05O possessed room temperature ferromagnetism with the saturation magnetic moment of 0.1 emu/g (0.29 μ_B/Ni²⁺).     

ZnO-Fe3O4 composite prepared by sol-gel method with H2 deoxidation

Fe doped ZnO powder samples (Fe/Zn=0.05 and 0.1) were prepared by sol-gel method with H₂ deoxidation at 450 ^°C for several hours or just heated in air at the same temperature. It was showed by vibrating sample magnetometer (VSM) that samples heat treated in H₂ could show strong ferromagnetism at room temperature while samples treated in air only show very weak magnetism. XRD using Co kα X-ray revealed that the samples heated in H₂ were not pure phase but like a granular system and the magnetism mainly results from Fe₃O₄ in samples while samples heated in air showed pure ZnO phase. Our work indicated that H₂ deoxidation treatment may be an effective technique to fabricate such magnetic semiconductor-like materials with Curie temperature higher than room temperature.     

Ferromagnetism induced by oxygen-vacancy complex in (Mn,in) codoped ZnO

Mn doped Zinc oxide (ZnO) thin films were prepared by metal organic chemical vapor deposition (MOCVD) technique. Structural characterizations by X-ray diffraction technique (XRD) and photoluminescence (PL) indicate the crystal quality of ZnO films. PL and Raman show a large fraction of oxygen vacancies (V_O²⁺) are generated by vacuum annealed the film. The enhancement of ferromagnetism in post-annealed (Mn, In) codoped ZnO could result from V_O²⁺ incorporation. The effect of V_O²⁺ on the magnetic properties of (Mn, In) codoped ZnO has been studied by first-principles calculations. It is found that only In donor cannot induce ferromagnetism (FM) in Mn-doped ZnO. Besides, the presence of V_O²⁺ makes the Mn empty 3d-t_2g minority state broadened, and a t_2g-V_O²⁺ hybrid level at the conduction band minimum forms. The presence of V_O²⁺ can lead to strong ferromagnetic coupling with the nearest neighboring Mn cation by BMP model based on defects reveal that the ferromagnetic exchange is mediated by the donor impurity state, which mainly consists of Mn 3d electrons trapped in oxygen vacancies.     

Optical properties of bulk Zn1−xCoxO magnetic semiconductors

Polycrystalline Zn_1−xCo_xO (x=0, 0.02, 0.05, 0.10 and 0.15) oxides have been synthesized by solid state reaction via sintering ZnO and Co powders in open air. X-ray diffraction analyses using Rietveld refinement indicate that a stoichiometric single phase with a wurtzite-like structure was found in Zn_1−xCo_xO samples with x up to 0.10. The elemental mapping using energy dispersive X-ray spectroscopic analyses presents a uniform distribution of Co. Optical transmittance measurements show that several extra absorption bands appear in the Co-doped ZnO, which is due to the transitions between the crystal-field-split 3d levels of tetrahedral Co²⁺ substituting Zn²⁺ ions. Raman measurements show that limited host lattice defects are induced by Co doping. Magnetization measurements reveal that the Co-doped ZnO samples are paramagnetic due to the absence of free carriers and in low temperature the dominant magnetic interaction is nearest-neighbor antiferromagnetic.