First-principle study of magnetism in Co-doped SnO2

The effects of Co dopants and oxygen vacancies on the electronic structure and magnetic properties of the Co-doped SnO₂ are studied by the first-principle calculations in full-potential linearized augmented plane wave formalism within generalized gradient approximations. The Co atoms favorably substitute on neighboring sites of the metal sublattice. Without oxygen vacancies, the Co atoms are at low spin state independent of concentration and distribution of Co atoms, and only the magnetic coupling between nearest-neighbor Co atoms is ferromagnetic through direct exchange and super-exchange interaction. Oxygen vacancies tend to locate near the Co atoms. Their presence strongly increases the local magnetic moments of Co atoms, which depend sensitively on the concentration and distribution of Co atoms. Moreover, oxygen vacancies can induce the long-range ferromagnetic coupling between well-separated Co atoms through the spin-split impurity band exchange mechanism. Thus the room temperature ferromagnetism observed experimentally in the Co-doped SnO₂ may originate from the combination of short-range direct exchange and super-exchange interaction and the long-range spin-split impurity band exchange model.     

Effects of hydroxyls on the structural and room temperature ferromagnetic properties of Co doped SnO2 nanoparticles

Co doped SnO₂ nanoparticles have been prepared via a wet chemical method with different precipitation processes. The structure and morphology of Co doped SnO₂ nanoparticles demonstrate that the nanoparticles are in a rutile single phase and uniform, respectively. X-ray photoelectron spectroscopy shows that the Co dopants are in 2+ oxidation valence state and doped ∼2 atm% in SnO₂ nanoparticles. Moreover, Raman spectroscopy further confirms that Co doped SnO₂ nanoparticles have single phase crystallinity without forming any extra modes related to secondary phases. The magnetic measurements reveal that all nanoparticles exhibit room temperature ferromagnetism (RTFM) due to the presence of disorders and defects introduced by hydroxyls in the crystal structure. In addition, it has been clearly observed that the saturated magnetic moments are strongly affected by the precipitation processes which control the incorporation of hydroxyls into the lattice.     

Influence of dopant level on structural, optical and magnetic properties of Co-doped anatase TiO2 nanoparticles

Cobalt-doped TiO₂ nanoparticles were synthesized by sol-gel method. The associated structural, optical, compositional and magnetic properties of the nanoparticles as a function of cobalt concentration have been systematically studied. The X-ray powder diffraction reveals that all samples have pure anatase phase tetragonal system and the lattice parameter analysis indicated that Co ions may substitute into the lattice of TiO₂. The average particle size is 15 nm, when found through transmission electron microscope. Optical spectroscopy measurement showed that the bandgap value decreases upon increasing Co concentration. The magnetic measurements revealed that the enhanced room temperature ferromagnetism (RTFM) strongly depends on the doping content.     

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 ferromagnetism in pure and Mn doped SnO2 powders

We report here observation of ferromagnetism in pure and Mn doped SnO₂ powder with different Mn contents. Magnetic measurements revealed that all samples exhibit room temperature ferromagnetism (RTFM), which is identified as an intrinsic characteristic. The RTFM has been observed in the pure SnO₂ powder, which is believed to be defect induced, with a saturation magnetization of ～0.017 emu/g. The RTFM was enhanced considerably in the Mn doped samples and the magnetic properties strongly depend on doping content. A sample with 1% of Mn is ferromagnetic at room temperature with a saturation magnetization of ～0.98 emu/g, a remanent magnetization of ～27%, and a coercivity of ～270 Oe. The average magnetic moment per Mn atom decreases with increasing Mn content. Our results reveal that the large RTFM observed in Mn doped SnO₂ powder originates from a combination effect of oxygen vacancies and transition metal doping.     

Nitrogen doping-mediated room-temperature ferromagnetism in insulating Co-doped SnO2 films

Co-doped SnO₂ films codoped with nitrogen (N) have been prepared by magnetron sputtering to investigate the effect of p-type defects on magnetic properties. The incorporation of N modifies the preferential growth orientation of the films. Multiple characterization techniques reveal that the incorporated Co²⁺ and N³⁻ ions substitute for Sn⁴⁺ and O²⁻ sites in SnO₂ lattice, respectively. As N concentration increases, the band gap of the films decreases because of the formation of Sn-N bond. Room-temperature ferromagnetism is observed in (Co, N)-codoped SnO₂ films, and the saturated magnetic moment is sensitive to the incorporated N concentration. The variations in the magnetic properties as a function of N concentration are discussed on the basis of bound magnetic polaron model.     

Raman scattering and room temperature ferromagnetism in Co-doped SrTiO3 particles

Raman scattering has been used to study the influence of cobalt, an effective dopant to obtain SrTiO₃ magnetic oxide, on the lattice dynamics of SrTiO₃. It is found that Co doping increases the lattice defects and induces a Raman vibration mode of 690 cm⁻¹. On the other hand, the ferromagnetism dependence on the x and annealing temperature was clearly and coherently observed in SrTi_1−xCo_xO₃ (x = 0, 0.01, 0.03 and 0.05) nanoparticles. It is found that the ferromagnetism of SrTi_1−xCo_xO₃ nanoparticles is weakly related to crystal deformation and oxygen vacancies in SrTiO₃. So, F-center model can explain the origin of the ferromagnetism in the prepared Co-doped SrTiO₃ samples. At the same time, the finding of large room-temperature ferromagnetism (1.6 emu/g) in this system would stimulate further interest in the area of more complicated ternary oxides.     

Fe doping induced enhancement in room temperature ferromagnetism and selective cytotoxicity of CeO2 nanoparticles

In the present study, structural, optical, magnetic properties as well as cytotoxicity of undoped and Fe doped Ceria (CeO₂) nanoparticles synthesized by simple soft chemical method have been reported. SEM and XRD results have shown that the synthesized samples are comprised of ultrafine spherical nanoparticles having single phase cubic fluorite structure of CeO₂. Raman spectroscopy results have depicted a red shift in F_2g mode with Fe doping which reveals enhancement in the oxygen vacancies. The optical band gap calculated from UV–visible absorption spectra has been found to vary unsystematically with Fe doping which is associated with the creation of impurity level and abundance in oxygen vacancies with Fe doping. The oxygen vacancies have introduced the room temperature ferromagnetism (RTFM) in undoped and Fe doped CeO₂ nanoparticles. The saturation magnetization (M_s) value of pristine CeO₂ nanoparticles has been found to be 0.00083 emu/g which is increased up to 0.0126 emu/g for 7% Fe doped nanoparticles. For cytotoxicity tests, the synthesized nanoparticles induced effects on Neuroblastoma cancer cells & HEK-293 healthy cells have been analyzed via CCK-8 analysis. It has been observed that the prepared undoped and Fe doped CeO₂ nanoparticles have nontoxic nature towards healthy cells while they are extremely toxic towards cancerous cells. Furthermore, the anticancer activity is found to enhance with Fe doping. The selective toxicity and enhancement in anticancer activity with Fe doping has observed to be strongly correlated with reactive oxygen species (ROS) generation.     

Preparation of nanocrystalline Sn-TiO2−X via a rapid and simple stannous chemical reducing route

The Sn-TiO_2−X nanoparticles have been prepared via a rapid and simple stannous chemical reducing method. The as-prepared Sn-TiO_2−X nanoparticles were investigated by means of surface photovoltage spectroscopy (SPS), XPS, and DRS technology as well as photocatalytic degradation of RhB were studied under illumination. The experiment results revealed that the reduction of the TiO₂ particles raised their Fermi level, which can enhance the driven force of photoinduced electrons transferring from TiO₂ to adsorbed O₂ and SnO₂ on the surface of TiO₂. On the other hand, the amount of oxygen vacancies of the Sn-TiO_2−X increased after the stannous chemical reduction. The oxygen vacancies can also effectively inhibit the recombination of photoinduced electrons and holes pairs. These factors are favorable to the photocatalytic reaction.     

SnO$lt;sub$gt;2$lt;/sub$gt;纳米晶体的制备、结构与发光性质

使用软化学方法在碱性溶液中制备出了颗粒尺寸分布均匀的SnO₂纳米颗粒,使用透射电子显微镜（TEM）、X射线衍射（XRD）、光致发光谱（PL）和光吸收谱等方法分析与表征了SnO₂纳米颗粒的结构和光学性能.实验中通过表面活性剂的加入来控制纳米颗粒的结晶与凝聚.XRD,TEM的结果表明,原始制备出的SnO₂纳米颗粒的平均粒径小于4 nm,为完好的晶体状态.纳米颗粒经过400—1000 ℃退火后晶粒尺寸进一步增大.光吸收谱表明,相对于体材料,纳米颗粒的禁带宽度展宽并随颗粒尺寸增大而红移.光致发光谱测试表明,不同温度下退火的SnO₂纳米颗粒在350—750 nm有较强的发光,研究表明这是来源于颗粒表面的氧空位缺陷发光. 关键词： 氧化锡 表面活性剂 纳米颗粒 光致发光     

Effect of hydrogenation vs. re-heating on intrinsic magnetization of Co doped In2O3

Influence of Co doping for In in In₂O₃ matrix has been investigated to study the effect on magnetic vs. electronic properties. Rietveld refinement of X-ray diffraction patterns confirmed formation of single phase cubic bixbyite structure without any parasitic phase. Photoelectron spectroscopy and refinement results further revealed that dopant Co²⁺ ions are well incorporated at the In³⁺ sites in In₂O₃ lattice and also ruled out formation of cluster in the doped samples. Magnetization measurements infer that pure In₂O₃ is diamagnetic and turns to weak ferromagnetic upon Co doping. Hydrogenation further induces a huge ferromagnetism at 300 K that vanishes upon re-heating. Experimental findings confirm the induced ferromagnetism to be intrinsic, and the magnetic moments to be associated with the point defects (oxygen vacancies V_o) or bound magnetic polarons around the dopant ions.     

Synthesis of monodisperse In2O3 nanoparticles and their d0 ferromagnetism

Monodisperse indium oxide (In₂O₃) nanoparticles (NPs) with the average diameter of 11 nm were prepared by a solvothermal method. The In₂O₃ NPs were characterized by X-ray diffraction, Raman and transmission electron microscopy. The intrinsic nature of ferromagnetism in In₂O₃ NPs has been established with the experimental observation of magnetic hysteresis loop. Photoluminescence and UV–visible studies were employed to evidence the presence of oxygen vacancies and revealed that the oxygen vacancies contribute to the ferromagnetism. The origin of ferromagnetism in In₂O₃ NPs may be due to exchange interactions among localized electron spin moments resulting from oxygen vacancies.     

First-principles study of electronic structure and magnetic properties of doped SnO2 (rutile) with single and double impurities

Using the augmented spherical wave method, the electronic structure and magnetic properties of the rutile SnO₂ doped with single and double impurities: Sn_1−xMn_xO₂, Sn_1−xW_xO₂, and Sn_1−2xMn_xW_xO₂ with x=0.0625, have been studied. The scalar-relativistic implementation with a generalized gradient approximation functional has been used for treating the effects of exchange and correlation. The ground state of Mn-, and W-doped SnO₂ systems have a total magnetic moments of 3 and 2 μ_B, respectively. The half-metallic nature appears in Sn_1−2xMn_xW_xO₂, which makes them suitable as spintronic systems with total magnetic moment of 5 μ_B. The advantages of doping SnO₂ with double impurities are investigated in this work. The total moment of the system, the local magnetic moments of the impurities, and their oxidation states are also discussed. Since there are two possible couplings between the impurities, we studied both configurations (ferromagnetic and antiferromagnetic) for double-impurities-doped SnO₂. Magnetic properties and interatomic exchange have been computed for various distances between Mn and W. The indirect exchange between double impurities has similarities with the Zener mechanism in transition metal oxides. Based on the interaction between localized moments, via hybridization between impurities orbitals with the host oxygen, a double exchange mechanism is proposed to explain the ferromagnetism of our system.     

钴掺杂二氧化锡纳米粉的光致发光和磁学性质

研究了钴掺杂对二氧化锡纳米粉的光致发光性质和磁学性质的影响,发现钴掺杂对发光带的位置影响很小,但紫外发光带与蓝色发光带的强度之比随掺杂含量的增加而下降.当钴掺杂含量达到0.02时,样品中的铁磁性被完全破坏.讨论了样品中的磁相互作用的机理,认为掺杂离子的不均匀分布、自旋极化子与掺杂离子之间的耦合都可能导致反铁磁性的相互作用,这种反铁磁性的作用破坏了铁磁性. 关键词： 钴掺杂二氧化锡 光致发光 磁学性质     

On the longevity of H-mediated ferromagnetism in Co doped TiO2: A study of electronic and magnetic interplay

Recent studies on dilute magnetic semiconductors have largely been focused on hydrogen induced giant magnetization. On the contrary, there are hardly any attempts to establish whether the H-induced magnetization is a permanent feature or likely to degrade with time. This aspect warrants a firm confirmation prior to planning their possible applications in spintronics and microelectronics. Motivated by this, we have systematically investigated this behaviour in Co doped TiO₂. In one study, we gave successive re-heat treatment to hydrogenated pellets and studied their magnetic, transport and electronic properties versus re-heating time. In another, we stored the hydrogenated sample in dry atmosphere for several months and studied the ageing effect on the induced ferromagnetism. The H-induced magnetization quickly vanishes upon re-heating the specimen; in addition, it also tends to degrade gradually with passage of time. The electronic and transport properties show a close parallelism, without any departure from structural properties. On the other hand, the Co-induced ferromagnetism, albeit a weak effect, does not show such degradation effects. As regards the mechanism of ferromagnetism, the oxygen vacancies are suggested to mediate the coupling.     

Structure and magnetic properties of Sn1−xMnxO2

The microstructure and magnetic properties have been investigated systematically for Sn_1−xMn_xO₂ polycrystalline powder samples with x=0.02-0.08 synthesized by a solid-state reaction method. X-ray diffraction revealed that all samples are pure rutile-type tetragonal phase and the cell parameters a and c decrease monotonously with the increase in Mn content, which indicated that Mn ions substitute into the lattice of SnO₂. Magnetic measurements revealed that all samples exhibit room temperature ferromagnetism. Furthermore, magnetic investigations demonstrate that magnetic properties strongly depend on doping content, x. The average magnetic moment per Mn atom decreases with increase in the Mn content, because antiferromagnetic super-exchange interaction takes place within the neighbor Mn³⁺ ions through O²⁻ ions for the samples with higher Mn doping. Our results indicate that the ferromagnetic property is intrinsic to the SnO₂ system and is not a result of any secondary magnetic phase or cluster formation.     

Effects of Fe doping and Fe-N-codoping on magnetic properties of SnO2 prepared by chemical co-precipitation

The effects of Fe-doping and Fe-N-codoping on the magnetic properties of SnO₂, prepared by chemical co-precipitation technique, are investigated in details. We found that the paramagnetism is the dominant magnetic interaction in Fe doped SnO₂. A weak antiferromagnetic coupling between Fe²⁺ ions is also confirmed through Zero field-cooled (ZFC) and field-cooled (FC) magnetization studies. On the other hand, hystersis behavior is observed for Fe-N-codoped SnO₂ samples with coercivity H_c∼420 and 352 Oe for x=0.05 and 0.10, respectively. As no other secondary or impurity phase is detected by XRD study and the presence of N is confirmed by EDX analysis, this observed ferromagnetism is originated due to the substitution of N in Sn_1−xFe_xO₂. N doping at the oxygen site can be regarded as defect and introduces a hole in this system. As a result, a hole-induced ferromagnetism might be the origin of the observed ferromagnetism in Fe-N-codoped SnO₂ samples.     

Effects of temperature and atmosphere on the magnetic properties of Co-doped ZnO rods

Co-doped ZnO (Zn_0.95Co_0.05O) rods are fabricated by co-precipitation method at different temperatures and atmospheres. X-ray diffraction, Energy dispersive X-ray spectroscopy and Raman results indicate that the samples were crystalline with wurtzite structure and no metallic Co or other secondary phases were found. Raman results indicate that the Co-doped ZnO powders annealed at different temperatures have different oxygen vacancy concentrations. The oxygen vacancies play an important role in the magnetic origin for diluted magnetic semiconductors. At low oxygen vacancy concentration, room temperature ferromagnetism is presented in Co-doped ZnO rods, and the ferromagnetism increases with the increment of oxygen vacancy concentration. But at very high oxygen vacancy concentration, large paramagnetic or antiferromagnetic effects are observed in Co-doped ZnO rods due to the ferromagnetic-antiferromagnetic competition. In addition, the sample annealed in Ar gas has better magnetic properties than that annealed in air, which indicates that O₂ plays an important role. Therefore, the ferromagnetism is affected by the amounts of structural defects, which depend sensitively on atmosphere and annealing temperature.     

Controlling oxygen vacancies and properties of ZnO

Intrinsic defects in semiconductors play crucial roles on their electrical and optical properties. In this article, we report on a facile method to control concentration of oxygen vacancies inside ZnO nanostructures and related physical properties based on adjustment of thermal transformation conditions from ZnO₂ to ZnO, including annealing atmosphere and temperature. ZnO₂ spheres assembled with nanoparticles were formed through the reaction between zinc nitrate and hydrogen peroxide. Significantly, it was found that the adopted temperature and atmosphere have remarkable impact on the concentration of oxygen vacancies, which was revealed by the variations of featured Raman scattering peaks at 584 cm⁻¹. Furthermore, with the increase of oxygen vacancies inside ZnO, the optical band-gap was found to red-shift 350 meV and the room-temperature ferromagnetism became stronger up to 1.6 emu/mg. The defect formation and evolution were discussed according to the chemical equilibrium of decomposition reaction under special local heating environment. This work demonstrated that ZnO₂ decomposition is an effective process to control the defect states inside ZnO and related properties.     

Observation of room temperature ferromagnetism in pure La2O3 nanoparticles

In this paper, we report the observation of intrinsic room temperature ferromagnetism in pure La₂O₃ nanoparticles. Magnetism measurement indicates that all of the samples exhibit room temperature ferromagnetism and the saturation magnetization for the samples decreases with the increase in annealing temperature from 700 to 1,000 °C. X-ray photoelectron spectroscopy identifies the presence of oxygen vacancies in the La₂O₃ nanoparticles. The fitting results of the O 1s spectrum indicate that the variation of the oxygen vacancy concentration is in complete agreement with the change of the saturation magnetization. It is also found that the saturation magnetization of the La₂O₃ nanoparticles can be tuned by post-annealing in argon or oxygen atmosphere. These results suggest that the oxygen vacancies are largely responsible for the room temperature ferromagnetism in pure La₂O₃ nanoparticles.