Structural characterization and ferromagnetic behavior of Fe-doped TiO2 powder by high-energy ball milling

Fe-doped TiO₂ powder was prepared by high-energy ball milling, using TiO₂ Degussa P-25 and α-Fe powders as the starting materials. The structure and magnetic properties of the Fe-doped TiO₂ powder were studied by X-ray diffraction, ⁵⁷Fe Mossbauer spectroscopy and vibrating sample magnetometer. The Reitveld refinement of XRD revealed that ball milling not only triggered incorporation of Fe in TiO₂ lattice but also induced the phase transformation from anatase to rutile in TiO₂ and consequently the milled Fe-doped TiO₂ powder contained only rutile.⁵⁷Fe Mössbauer effect measure showed that Fe atoms existed in Fe²⁺ and Fe³⁺ state, which were assigned to the solid solution Fe_xTi_1−xO₂. The magnetization measurements indicated that the milled Fe-doped TiO₂ powder was ferromagnetic above room temperature. The ferromagnetism in our milled Fe-doped TiO₂ powder seemingly does not come from Fe and iron oxides particles/clusters but from the Fe-doped TiO₂ powder matrices.     

Epitaxial Properties of Co-Doped ZnO Thin Films Grown by Plasma Assisted Molecular Beam Epitaxy

High quality Co-doped ZnO thin films are grown on single crystalline Al2O3（0001） and ZnO（0001） substrates by oxygen plasma assisted molecular beam epitaxy at a relatively lower substrate temperature of 450℃. The epitaxial conditions are examined with in-situ reflection high energy electron diffraction （RHEED） and ex-situ high resolution x-ray diffraction （HRXRD）. The epitaxial thin films are single crystal at film thickness smaller than 500nm and nominal concentration of Co dopant up to 20%. It is indicated that the Co cation is incorporated into the ZnO matrix as Co^2＋ substituting Zn^2＋ ions. Atomic force microscopy shows smooth surfaces with rms roughness of 1.9 nm. Room-temperature magnetization measurements reveal that the Co-doped ZnO thin films are ferromagnetic with Curie temperatures Tc above room temperature.     

Room-Temperature Ferromagnetism in Zn1-xMnxO Thin Films Deposited by Pulsed Laser Deposition

Zn1-xMnxO （x = O.Olq3.1） thin films with a Curie temperature above 300K are deposited on Al2O3 （0001） substrates by pulsed laser deposition. X-ray diffraction （XRD）, ultraviolet （UV）-visible transmission and Raman spectroscopy are employed to characterize the microstructural properties of these films. Room temperature ferromagnetism is observed by superconducting quantum interference device （SQUID）. The results indicate that Mn doping introduces the incorporation of Mn^2＋ ions into the ZnO host matrix and the insertion of Mn^2＋ ions increases the lattice defects, which is correlated with the ferromagnetism of the obtained films. The doping concentration is also proven to be a crucial factor for obtaining highly ferromagnetic Zn1-xMnxO films.     

The role of Co impurities and oxygen vacancies in the ferromagnetism of Co-doped SnO2: GGA and GGA+U studies

The electronic structure and ferromagnetic stability of Co-doped SnO₂ are studied using the first-principle density functional method within the generalized gradient approximation (GGA) and GGA+U schemes. The addition of effective U_Co transforms the ground state of Co-doped SnO₂ to insulating from half-metallic and the coupling between the nearest neighbor Co spins to weak antimagnetic from strong ferromagnetic. GGA+U_Co calculations show that the pure substitutional Co defects in SnO₂ cannot induce the ferromagnetism. Oxygen vacancies tend to locate near Co atoms. Their presence increases the magnetic moment of Co and induces the ferromagnetic coupling between two Co spins with large Co-Co distance. The calculated density of state and spin density distribution calculated by GGA+U_Co show that the long-range ferromagnetic coupling between two Co spins is mediated by spin-split impurity band induced by oxygen vacancies. More charge transfer from impurity to Co-3d states and larger spin split of Co-3d and impurity states induced by the addition of U_Co enhance the ferromagnetic stability of the system with oxygen vacancies. By applying a Coulomb U_O on O 2 s orbital, the band gap is corrected for all calculations and the conclusions derived from GGA+U_Co calculations are not changed by the correction of band gap.     

Room-Temperature Ferromagnetic ZnMnO Thin Films Synthesized by Plasma Enhanced Chemical Vapour Deposition Method

Room-temperature ferromagnetic Mn-doped ZnO films are grown on Si （001） substrates by plasma enhanced chemical vapour deposition （PECVD）. X-ray diffraction measurements reveal that the Znl-xMn.O films have the single-phase wurtzite structure. X-ray photoelectron spectroscopy indicates the existence of Mn^2＋ ions in Mndoped ZnO films. Furthermore, the decreasing additional Raman peak with increasing Mn-doping is considered to relate to the substitution of Mn ions for the Zn ions in ZnO lattice. Superconducting quantum interference device （SQUID） measurements demonstrate that Mn-doped ZnO films have ferromagnetic behaviour at room temperature.     

Influence of Gamma-Ray Irradiation on Absorption and Fluorescent Spectra of Nd：YAG and Yb：YAG Laser Crystals

We investigate the influence of gamma-ray irradiation on the absorption and fluorescent spectra of Nd^3＋：Y3A15 O12 （Nd： YAG） and Yb^3＋ ： Y3A15 O12 （Yb： YAG） crystals grown by the Czochralski method. Two additional absorption （AA） bands induced by gamma-ray irradiation appear at 255nm and 340nm. The former is contributed due to Fe^3＋ impurity, the latter is due to Fe^2＋ ions and F-type colour centres. The intensity of the excitation and emission spectra as well as the fluorescent lifetime of Nd：YAG crystal decrease after the irradiation of 100 Mrad gamma-ray. In contrast, the same dose irradiation does not impair the fluorescent properties of Yb：YAG crystal. These results indicate that Yb： YA G crystal possesses the advantage over Nd： YA G crystal that has better reliability for applications in harsh radiant environment.     

The relationship between magnetic behaviour and local structure around Fe ions in Fe-doped TiO2 rutile

The magnetic and structural characterization of Ti_1−xFe_xO₂ (x=0.025, 0.05, 0.07, 0.125, and 0.15) samples prepared by mechano-synthesis using TiO₂ and Fe₂O₃ as starting materials are reported. XANES measurements performed at the Fe K-edge show that Fe ions are in 3+ oxidation state in the 7 at% Fe-doped sample and in a mixture of 2+ and 3+ oxidation states in the other samples. EXAFS results show the incorporation of Fe ions substituting Ti ones in the rutile TiO₂ structure. They also reveal a strong correlation between the number of oxygen nearest neighbours and the Fe²⁺ fraction, i.e the number of oxygen near neighbours decreases when the Fe²⁺ fraction increases. All samples present ferromagnetic-like behaviour at room temperature. We found a clear dependence between saturation magnetization and coercivity with the fraction of Fe²⁺ and/or the number of Fe near neighbour oxygen vacancies.     

Relationship between structural and magnetic properties in (Ti,Fe)O2 powders obtained by mechanical milling

Fe-doped TiO₂ samples with different Fe content were prepared by mechanical alloying starting from TiO₂ rutile and FeO. The samples were structurally and magnetically characterized by XRD, Mössbauer spectroscopy, X-ray absorption spectroscopy (XAS), AC-susceptibility and magnetization measurements. XAS results showed that Fe ions were incorporated into the rutile phase with oxygen coordination that was lower than that expected in this phase. The oxygen coordination number decreased with the increase of Fe²⁺ ions such as it was previously found in the milled samples of TiO₂ doped with hematite. The RT Mössbauer spectra were reproduced using two paramagnetic interactions, one corresponding to Fe²⁺ (δ∼0.87 mm/s) and the other to Fe³⁺ (δ∼0.31 mm/s). Magnetometry measurements showed the presence of paramagnetic and ferromagnetic-like interactions at room temperature. Although saturation and coercivity of the ferromagnetic phase increased with iron, the effective magnetic moment per iron atom decreased, probably due to the precipitation of Fe rich antiferromagnetic structures.     

Microstructure, magnetic and optical properties of sputtered polycrystalline ZnO films with Fe addition

Microstructure, magnetic and optical properties of polycrystalline Fe-doped ZnO films fabricated by cosputtering with different Fe atomic fractions (x_Fe) have been examined systematically. Fe addition could affect the growth of ZnO grains and surface morphology of the films. As x_Fe is larger than 7.0%, ZnFe₂O₄ grains appear in the films. All the films are ferromagnetic. The ferromagnetism comes from the ferromagnetic interaction activated by defects between the Fe ions that replace Zn ions. The average moment per Fe ion reaches a maximum value of 1.61 μ_B at x_Fe = 4.8%. With further increase in x_Fe, the average moment per Fe ion decreases because the antiferromagnetic energy is lower than the ferromagnetic one due to the reduced distance between the adjacent Fe ions. The optical band gap value decreases from 3.245 to 3.010 eV as x_Fe increases from 0% to 10%. Photoluminescence spectra analyses indicate that many defects that affect the optical and magnetic properties exist in the films.     

The influence of the substrate nature on the iron repartition in the titania matrix

Fe-doped titania films were deposited by RF sputtering onto different substrates (glass and ITO/glass) in the same deposition run. The rutile nanocrystalline structure of Fe-doped thin films deposited on glass substrates and anatase nanocrystalline structure of Fe-doped thin films deposited on ITO/glass substrates were evidenced by XRD. SEM investigations showed a smooth surface with a dense nanostructure. XPS study evidenced an almost stoichiometric composition with different iron contents. EPR and XPS studies evidenced that iron entered into TiO₂ lattice by substitution, as isolated and dimer species. In Fe-doped thin films deposited on ITO/glass substrates the iron content is ten times higher than in Fe-doped thin films deposited on glass substrates and that a part of them entered as Fe²⁺.     

Magneto-optical spectroscopy of diluted magnetic oxides TiO2−δ: Co

We report an experimental study on transversal Kerr effect (TKE) in magnetic oxide semiconductors TiO_2−δ:Co. The TiO_2−δ: Co thin films were deposited on LaAlO₃ (0 0 1) substrates by magnetron sputtering in the argon-oxygen atmosphere at oxygen partial pressure of 2×10⁻⁶-2×10⁻⁴ Torr. It was obtained that TKE spectra in ferromagnetic samples are extremely sensitive to the Co-volume fraction, the crystalline structure, and technology parameters. The observed well-pronounced peaks in TKE spectra for anatase Co-doped TiO_2−δ films at low Co (<1%) volume fraction are not representative for bulk Co or Co clusters in TiO_2−δ matrix that indicates on intrinsic ferromagnetism in these samples. With increase of Co-volume fraction up to 5-8% the fine structure of TKE spectra disappears and magneto-optical response in reflection mode becomes larger than that for thick Co films     

Magnetism due to oxygen vacancies and/or defects in undoped semiconducting and insulating oxide thin films

Room temperature ferromagnetism was observed in HfO₂, TiO₂, and In₂O₃ films grown on yttrium-stabilized zirconia, LaAlO₃, and MgO substrates, respectively. While the magnetic moment is rather modest in the case of In₂O₃ films, it is very large in the other two cases. Thin film form, which might create necessary defects and/or oxygen vacancies, must be the main reason for undoped semiconducting and insulating oxides to become ferromagnetic. From the results, a serious question arises if a transition-metal doping indeed plays any essential role in producing ferromagnetism (FM) in non-magnetic oxides.     

Evolution of Structural Defects in SiOx Films Fabricated by Electron Cyclotron Resonance Plasma Chemical Vapour Deposition upon Annealing Treatment

We study the structural defects in the SiO, film prepared by electron cyclotron resonance plasma chemical vapour deposition and annealing recovery evolution. The photoluminescence property is observed in the as-deposited and annealed samples. [-SiO3]^2- defects are the luminescence centres of the ultraviolet photoluminescence （PL） from the Fourier transform infrared spectroscopy and PL measurements. [-SiO3]^2- is observed by positron annihilation spectroscopy, and this defect can make the S parameters increase. After 1000℃ annealing, [-SiO3]^2- defects still exist in the films.     

An Artificially Garnet Crystal Materials Using In Terahertz Waveguide

A hypothesis is brought forward that the materials with low propagation loss in both optical and microwave band may exhibit good performance in terahertz （THz） band because THz wave band interspaces those two wave bands. For the purpose-of exploring a kind of low-loss material for THz waveguide, Lu2.1Bi0.9Fe5O12（LuBiIG） garnet films are prepared by liquid phase epitaxy （LPE） method on a gadolinium gallium garnet （GGG） substrate from lead-free flux because of the good properties in both optical and microwave bands. In microwave band, the ferromagnetic resonance （FMR） linewidth of the film 2△H = 2.8-5.1Oe; in optical band, the optical absorption coefficient is 600cm^-1 at visible range and about 100-170cm^-1 when the wavelength is longer than 800nm. In THz range, our hypothesis is well confirmed by a THz-TDS measurement which shows that the absorbance of the film for THz wave is 0.05-0.3 cm 1 and the minimum value appears at 2.3 THz. This artificial ferromagnetic material holds a great promise for magnetic field tunable THz devices such as waveguide, modulator or switch.     

Study on Irradiation Damage of Bi-Doped PbWO4 Crystal

The luminescence and point defects of pure lead tungstate crystals (PbWO₄) and Bismuth (Bi) doping crystal (PbWO₄:Bi)grown by modified Bridgman method are studied. It is found that irradiation results in the great change of the transmission and X-ray excited emission after γ-ray irradiation about 4 Mrad dose. The defects in PbWO₄ crystal have been studied by means of positron annihilation lifetime and X-ray photoelectron spectra. The results show that Bi dopant suppresses the concentrations of positron capture centers and low-valent oxygen ions.After γ-ray irradiation,in the pure crystal the concentration of lead vacancy (V_Pb) is decreased and that of low-valent oxygen increased; on the contrary,in Bi dopant crystal the concentrations of positron capture centers increased and that of low-valent oxygen ions suppressed. It is tentatively proposed that Bi³⁺ dopants would mainly occupy the sites of lead vacancies resulted from Pb volatilization. And irradiation changes the chemical valence of Bi element,which is Bi³⁺→Bi⁵⁺.The Bi⁵⁺ will replace the lattice W⁶⁺ ions and it will cause some (WO₄)^2－ replaced by (BiO₃+V_O)^－.     

Magnetic Properties and Antiferromagnetic Coupling in Inhomogeneous Zn1-xFexO Magnetic Semiconductor

Zn1-xFexO inhomogeneous oxide magnetic semiconductor films with high Fe concentration are prepared by sputtering, and fast annealing is carried out at different temperatures. It is found that magnetic properties are greatly modulated by controlling the composition inhomogeneity and subsequently fast annealing. Both ferromagnetic and paramagnetie components are found to coexist in the as-deposited Zn1-xFexO magnetic semiconductor. In particular, the antiferromagnetic coupling between the neighbouring local ferromagnetic regions is found in the as-deposited Zn0.23Fe0.77O film, and the antiferromagnetic coupling strength increases with increasing temperature from 110K to 300 K. We believe that this unusual antiferromagnetic coupling is mediated by thermally activated hopping carriers.     

Magnetism in spin-coated pristine TiO2 thin films

Spin coated pristine TiO₂ thin films show magnetic behaviors that are similar to those of pulsed laser ablated TiO₂ thin films that were reported previously. It seems that in this kind of material, ferromagnetism (FM) is indeed intrinsic, and it can be achieved by various deposition techniques. The fact that oxygen annealing degrades the magnetic moment implies that the observed magnetism is likely due to defects or/and oxygen vacancies. Moreover, thick films that were deposited under the same growth conditions have the magnetic ordering degraded enormously. It is found that as for FM in undoped TiO₂ films made by the chemical solution deposition, not only do defects/oxygen vacancies play a role, but also the confinement effects seem to be important.     

Enhanced ferromagnetic properties of Fe+N codoped TiO2 anatase

The undoped, Fe-doped, N-doped and Fe+N codoped titanium dioxide (TiO₂) samples were synthesized. Detailed analysis shows that all the samples are pure anatase with the shape of a nanorod, and N and Fe ions are incorporated into the TiO₂ lattice. For all the samples, the saturation magnetization at room temperature is in the order of the Fe+N codoped TiO₂>N-doped TiO₂>Fe-doped TiO₂>undoped TiO₂. Upon N doping, enhanced ferromagnetic properties were observed. The N content in Fe+N codoped TiO₂ is about two times as large as that in the N-doped TiO₂, which may account for the largest saturation magnetization observed in Fe+N codoped TiO₂. It is suggested that metal ion and N codoping may provide a new approach for increasing the saturation magnetization in TiO₂-based dilute magnetic semiconductors.     

Effects of Annealing Temperature on Structural and Optical Properties of ZnO Thin Films

The effects of annealing temperature on the structural and optical properties of ZnO films grown on Si （100） substrates by sol-gel spin-coating are investigated. The structural and optical properties are characterized by x-ray diffraction, scanning electron microscopy and photoluminescence spectra. X-ray diffraction analysis shows the crystal quality of ZnO films becomes better after annealing at high temperature. The grain size increases with the temperature increasing. It is found that the tensile stress in the plane of ZnO films first increases and then decreases with the annealing temperature increasing, reaching the maximum value of 1.8 GPa at 700℃. PL spectra of ZnO films annealed at various temperatures consists of a near band edge emission around 380 nm and visible emissions due to the electronic defects, which are related to deep level emissions, such as oxide antisite （OZn）, interstitial oxygen （Oi）, interstitial zinc （Zni） and zinc vacancy （VZn^-）, which are generated during annealing process. The evolution of defects is analyzed by PL spectra based on the energy of the electronic transitions.     

Effect of W co-doping on the optical,magnetic and electrical properties of Fe-doped BaSnO3

The effect of W co-doping on the optical, magnetic and electrical properties of Fe-doped BaSnO₃ has been studied. Polycrystalline BaSnO₃, BaSn_0.96Fe_0.04O₃ and BaSn_0.95Fe_0.04W_0.01O₃ samples were prepared using solid state reaction. In the analysis of powder X-ray diffraction patterns, the samples were found to be free of secondary phases. Diffuse reflectance spectra evidenced the substitution of Fe and W for Sn in the host BaSnO₃. Micro-Raman spectra confirmed the existence of oxygen vacancies in the samples. Upon W-1% co-doping, the ferromagnetic character of Fe-4% doped BaSnO₃ is suppressed drastically and its Curie temperature is reduced to 310 K from 462 K. The existence of F-centers and ferromagnetic interactions at room temperature is evidenced by the electron paramagnetic resonance and ferromagnetic resonance signals observed in the electron spin resonance spectra of the undoped and Fe-4% doped, (Fe-4% and W-1%) co-doped BaSnO₃ samples respectively. Suppression of ferromagnetism upon W co-doping is due to the fact that each W⁶⁺ ion donates two electrons to the host lattice and it reduces the number of oxygen vacancies that are essential for ferromagnetism to exist in the Fe-doped BaSnO₃ samples.