多铁性BiFeO3纳米颗粒的尺寸依赖磁性能研究

采用乙二胺四乙酸杂化溶胶法制备了不同晶粒尺寸的纯相BiFeO₃纳米颗粒,并利用X射线衍射仪、扫描电镜、超导量子干涉仪和Mossbauer 谱系统研究了其结构、形貌以及磁性能.结果表明: BiFeO₃纳米颗粒具有明显的弱铁磁性,并呈现强烈的尺寸依赖特性; 这种弱铁磁性主要源于纳米材料的尺寸限制效应,而非杂质相或Fe²⁺ 的存在所致.     

Effects of doping site and pre-sintering time on microstructure and magnetic properties of Fe-doped BaTiO3 ceramics

The A-site substituted BaTiO₃ ceramics were prepared by solid-state reaction via partial substitution of Fe for Ba²⁺. By comparison with the B-site substituted sample made under similar conditions, the effect of Fe doping site on microstructure and magnetism was investigated using X-ray diffraction, Mössbauer spectroscopy and vibrating sample magnetometer. It is found that A-site substitution can be realized to a certain extent at 7 at% Fe addition, whereas impurities are observed at higher Fe concentrations. In the nominal (Ba_0.93Fe_0.07)TiO₃ sample, the Fe ions are present as Fe²⁺ and Fe³⁺, respectively, replacing A-site Ba²⁺ and octahedral B-site Ti⁴⁺ in hexagonal perovskite lattice. The double-exchange Fe²⁺-O²⁻-Fe³⁺ interactions produce ferromagnetism well above room temperature, but the saturation magnetization and the Curie temperature are both obviously lower than those for B-site substitution due to different magnetic exchange mechanisms. In the B-site substituted sample Ba(Ti_0.93Fe_0.07)O₃, the super-exchange interactions between Fe³⁺ on pentahedral and octahedral Ti⁴⁺ sites are responsible for ferromagnetism. These results mean that B-site substitution is a better way for Fe-doped BaTiO₃ system to obtain high-Curie-temperature ferromagnetism. Moreover, increasing pre-sintering time can further improve the magnetism of B-site substituted samples, through which the saturation magnetization for Ba(Ti_0.93Fe_0.07)O₃ is enhanced ∼6 times.     

Preparation and characterization of the ZnO:Al/Fe65Co35/ZnO:Al multifunctional films

The ferromagnetic transparent conducting film is a multifunctional film which has high visible transmittance, low resistivity and room-temperature ferromagnetism, simultaneously. In this article, ferromagnetic transparent conducting ZnO:Al/Fe₆₅Co₃₅/ZnO:Al multilayer films were fabricated by inserting a middle magnetic Fe₆₅Co₃₅ layer into aluminum-doped zinc oxide (ZnO:Al) matrix using a magnetron sputtering apparatus at substrate temperature ranging from room temperature (RT) to 400^∘C. The total film thickness was about 400 nm and the middle Fe₆₅Co₃₅ alloy layer was 4 nm. The influences of substrate temperature (T _s ) on the structural, electrical, optical and magnetic properties of the multilayer films were systemically investigated. The results showed that the microstructure and performance of the composite multilayer films strongly depended on the substrate temperature. The present results also showed that the inserted middle Fe₆₅Co₃₅ alloy thin layer played an important role in providing the RT ferromagnetism and decreasing the resistivity of the multilayer films. Therefore, it is possible to obtain a multifunctional film material with the combination of good optical transparency, high electrical conductivity and RT ferromagnetism.     

57Fe Mössbauer study of sol–gel synthesized Sn1 − x − y Fe x Sb y O2 − δ powders

Samples of SnO₂ doped with different amount of Fe (10–20%) and Sb (5–25%) were prepared by sol–gel method. Room temperature ferromagnetism was found to increase as a result of co-doping with Sb, as compared to SnO₂ doped only with Fe. ⁵⁷Fe Mössbauer spectra of almost all samples exhibited two paramagnetic doublets and a small subspectrum referring to magnetic relaxation at room temperature. Only the samples Sn_0.65Fe_0.2Sb_0.15O_2???δ and Sn_0.85Fe_0.1Sb_0.05O_2???δ with 4 h long annealing time showed well developed sextets and larger magnetic coercivity compared to that of the other samples. The sextet observed was considered to be due to precipitates like Sn doped α-Fe₂O₃. The results suggest that the origin of the magnetic interactions is enhanced by the presence of magnetic defects, which can interact with the iron ions by free carrier electrons. For the sample with precipitates, the grain boundary defects may play an important role of enhanced ferromagnetism.     

X-ray absorption and magnetic circular dichroism characterization of Mo1–xFexO2 (x = 0–0.05) thin films grown by pulsed laser ablation

The electronic and magnetic properties of well characterized Mo_1???xFe_xO₂ (x = 0–0.5) thin films that show ferromagnetism at room temperature (RT) have been investigated by the means of near edge x-ray absorption fine structure (NEXAFS) and x-ray magnetic circular dichroism (XMCD) experiments at the O K-, Fe L-, and Mo M-edges. The NEXAFS spectra at O K- and Mo M_3,2 -edges show a strong hybridization of O 2p-4d Mo orbitals, and Mo ions change their symmetry with the substitution of Fe ions into MoO₂ matrix. The Fe 2p NEXAFS/XMCD spectra exhibit multiple absorption peaks and an appreciable XMCD signal that persists even at RT. These results demonstrate that Fe is in a mixed valence state of Fe^2?+?–Fe^3?+?, substituting at the Mo site and that the Fe^2?+?/3?+? ions are ferromagnetically polarized.     

Simultaneous regulation of photoabsorption and ferromagnetism of NaTaO3 by Fe doping

NaTa_1-xFe_xO₃ (0 ≤ x ≤ 0.40) nanocubes were synthesized by a relatively low temperature hydrothermal method, using Ta₂O₅, FeCl₃ and NaOH as the precursors. The UV–vis diffuse reflectance spectra showed that NaTa_1-xFe_xO₃ had significant visible-light-absorbing capability, and the absorption edge of NaTaO₃ shifted to longer wavelength with the increase of Fe dopants. Moreover, NaTa_1-xFe_xO₃ exhibited room-temperature ferromagnetism when Fe³⁺ occupied Ta⁵⁺ sites in NaTaO₃ crystal lattice. The ferromagnetism is mainly attributed to the superexchange interactions between doped Fe³⁺, rather than the contribution of oxygen vacancies caused by Fe doping. Therefore, Fe doping can simultaneously regulate the optical and magnetic properties of NaTaO₃ semiconductor, which will enable its potential applications in multifunctional optical-electronics and optical-spintronics devices.     

Magnetic property and Mössbauer analysis of SrSn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript><Emphasis Type="Bold">x</Emphasis></Subscript>O<Subscript>3</Subscript> prepared by a sol-gel method

The dilute magnetic properties of SrSn_1?xFe_xO₃ (x = 0.01 ? 0.15) prepared by sol-gel and thermal decomposition methods were investigated by ⁵⁷Fe Mössbauer spectrometry, magnetometry, and X-ray diffractometry. It was found that SrSnO₃ doped with 2–8 % Fe show weak ferromagnetism although only paramagnetic doublets are observed in ⁵⁷Fe Mössbauer spectra at room temperature (RT), whereas SrSnO₃ doped with 10–15 % Fe show relatively strong ferromagnetism, and the sextets are additionally observed in the ⁵⁷Fe Mössbauer spectra at RT. The weak ferromagnetism by doping 2–8 % Fe is considered to be caused by the induced magnetic defects, and the ferromagnetism by doping 10–15 % Fe are considered mainly due to the magnetic coupling between dilute Fe ³⁺ partially substituted at Sn ⁴⁺ sites in the orthorhombic structure of SrSnO_3?δ accompanying the oxygen deficiencies. It is further remarkable that poor crystalline 8 % Fe doped SrSnO_3?δ obtained by annealing at 600 ^°C shows relatively high saturation magnetization and low coercivity.     

Sol-gel synthesis and dilute magnetism of nano MgO powder doped with Fe

Mg oxides doped with 1 % ⁵⁷Fe were prepared by a sol-gel method, and annealed at various temperatures. Nano-size Mg oxides were characterized by Mössbauer spectrometry, magnetization and XRD measurements. The crystalline size of MgO increases with increase of annealing temperature. Samples annealed at 600 °C and 800 °C gave only doublet peaks of paramagnetic Fe³⁺ in Mössbauer spectra although Fe³⁺ doping into MgO induced a distorted structure and showed weak ferromagnetism. It is considered that the magnetic property is due to defect induced magnetism by doping Fe³⁺ into MgO. For a sample heated at 1000 °C, it is found from low temperature Mössbauer spectra that Fe³⁺ species are located at the core and shell of fine MgFe₂O₄ grains and diluted in MgO matrix.     

High temperature ferromagnetism of the vacuum-annealed (In1−xFex)2O3 powders

(In_1−xFe_x)₂O₃ (x = 0.02, 0.05, 0.2) powders were prepared by a solid state reaction method and a vacuum annealing process. A systematic study was done on the structural and magnetic properties of (In_1−xFe_x)₂O₃ powders as a function of Fe concentration and annealing temperature. The X-ray diffraction and high-resolution transmission electron microscopy results confirmed that there were not any Fe or Fe oxide secondary phases in vacuum-annealed (In_1−xFe_x)₂O₃ samples and the Fe element was incorporated into the indium oxide lattice by substituting the position of indium atoms. The X-ray photoelectron spectroscopy revealed that both Fe²⁺ and Fe³⁺ ions existed in the samples. Magnetic measurements indicated that all samples were ferromagnetic with the magnetic moment of 0.49-1.73 μ_B/Fe and the Curie temperature around 783 K. The appearance of ferromagnetism was attributed to the ferromagnetic coupling of Fe²⁺ and Fe³⁺ ions via an electron trapped in a bridging oxygen vacancy.     

Coexistence of paramagnetism and ferromagnetism in Fe-TiO2 nanoparticle

Magnetic properties of pure and Fe doped rutile TiO₂ and TiO_2-ε are investigated using the first principle density functional theory. The results show that the considered systems are ferromagnetic. Furthermore, the origin of ferromagnetism is discussed and it is found that the double exchange and super-exchange are the main interactions in these compounds. Based on the calculations, the magnitude of the magnetic moment depends on the concentration of impurities and oxygen vacancies and the largest magnetic moment corresponds to the Fe_xTi_1-xO_2-ε. Moreover, using a model based on the bound magnetic polarons, the coexistence of ferromagnetic and paramagnetic phases can occur in Fe_xTi_1-xO₂ containing different impurity ions such as Fe⁺² and Fe⁺³ with different Curie temperatures. The finding may presents the potential application of the considered system as diluted magnetic semiconductor.     

Strangeness production in AA collisions at SIS18

Aluminium oxides doped with 1% ⁵⁷Fe were prepared by sol-gel method, and annealed for 3 hours at various temperatures between 550°C and 1100°C. Amorphous phases were obtained below 1000°C, and crystalline α–Al₂O₃ was formed at 1100°C. Although Al₂O₃ itself shows diamagnetism, the light doping of Fe ions into aluminium oxide induced a very weak ferromagnetism, but the ferromagnetism disappeared by longer annealing. M?ssbauer spectra were composed of paramagnetic Fe^2?+? and Fe^3?+? species for samples heated below 750°C, and of paramagnetic Fe^3?+? above 850°C, in addition to a magnetic sextet and relaxation peaks of Fe^3?+?. The magnetic and quadrupole interactions of the sextet and the relaxation peaks and the density functional calculations suggest that the lightly doped Fe^3?+? ions are substituted at Al sites in the Al₂O₃ lattice.     

Effect of annealing atmosphere on magnetism for Fe-doped BaTiO3 ceramic

Ba(Ti_0.3Fe_0.7)O₃ ceramic was prepared by solid-state reaction and post-annealed in vacuum and oxygen, respectively. The as-prepared and annealed samples are all single-phase, crystallizing in a 6H-BaTiO₃-type hexagonal perovskite structure. Room-temperature ferromagnetism is exhibited in all ceramics. For the as-prepared sample, the super-exchange interactions of Fe³⁺ in different occupational sites (pentahedral and octahedral sites) are expected to produce the ferromagnetism observed. After annealing in vacuum, the magnetization is reduced while the exchange mechanism remains unchanged. On the contrary, O₂ annealing can effectively enhance the magnetization due to the presence of Fe⁴⁺, an unusual valence for iron. The simultaneous presence of Fe³⁺ and Fe⁴⁺ allows new exchange mechanism responsible for the ferromagnetic interaction. The exchange couplings of Fe ions with mixed valences (Fe³⁺ and Fe⁴⁺) determine the magnetic behavior.     

脉冲激光沉积法制备的Ni1-xFexO稀磁半导体的结构和磁性研究

利用脉冲激光沉积方法制备出了具有室温铁磁性的Ni_1-xFe_xO(x=0.02,0.05)稀磁半导体.X射线衍射(XRD)结果表明Ni_1-xFe_xO的晶体结构为NaCl结构,并且在Fe含量较高的Ni_0.95Fe_0.05O中出现了少量的α-Fe₂O₃物相.X射线吸收近 关键词： 1-xFe_xO')" href="#">Ni_1-xFe_xO XANES 脉冲激光沉积方法     

Origin of ferromagnetism in iron implanted rutile single crystals

⁵⁷Fe doped titanium oxide monocrystals, prepared by ion implantation at different temperatures and subsequent thermal treatment, were characterized by conversion electron Mössbauer spectrometry, synchrotron radiation x-ray diffraction and superconducting quantum interference device magnetometry. After implantation at room temperature Fe is present in divalent state. Upon annealing in high vacuum Fe^2?+? is reduced to metallic Fe for the most part. After implantation at 623 K most iron is in metallic state. During annealing on air Fe is gradually oxidized from Fe^2?+? to Fe^3?+?. Depending on preparation conditions and thermal treatment the role of different nanosized secondary phases is discussed in terms of their influence on the magnetic properties of Fe:TiO₂. α-Fe nanoparticles are found to be responsible for ferromagnetism observed in TiO₂.     

Coexistence of paramagnetism and ferromagnetism in Fe-TiO2 nanoparticle

Magnetic properties of pure and Fe doped rutile TiO₂ and TiO_2-ε are investigated using the first principle density functional theory. The results show that the considered systems are ferromagnetic. Furthermore, the origin of ferromagnetism is discussed and it is found that the double exchange and super-exchange are the main interactions in these compounds. Based on the calculations, the magnitude of the magnetic moment depends on the concentration of impurities and oxygen vacancies and the largest magnetic moment corresponds to the Fe_xTi_1?xO_2?ε. Moreover, using a model based on the bound magnetic polarons, the coexistence of ferromagnetic and paramagnetic phases can occur in Fe_xTi_1?xO₂ containing different impurity ions such as Fe⁺² and Fe⁺³ with different Curie temperatures. The finding may presents the potential application of the considered system as diluted magnetic semiconductor.     

Laser reactivation of Room-Tc ferromagnetism in Mn-doped insulating TiO2 thin films

Room-T _c ferromagnetic insulating Mn-doped TiO₂ thin films were prepared by plasma assisted molecular beam epitaxy (PAMBE). The thin films show obvious decay of ferromagnetism when aged in air at room temperature for one year without any special treatment. A distinct reactivation of ferromagnetism, together with a decrease in coercive force is achieved in these samples under laser irradiation treatment. The possible mechanism for the phenomena is also discussed. It is suggested that the bound magnetic polaron (BMP) forming via photo-generated defect-bound carriers accounts for the laser reactivation of room-T _c ferromagnetism.     

Magnetism and thermal induced characteristics of Fe2O3 content bioceramics

Magnetic properties of Li₂O–MnO₂–CaO–P₂O₅–SiO₂ (LMCPS) glasses doped with various amounts of Fe₂O₃ were investigated. There is a dramatic change in the magnetic property of pristine LMCPS after the addition of Fe₂O₃ and crystallized at 850 °C for 4 h. Both the electron paramagnetic resonance and magnetic susceptibility measurements showed that the glass ceramic with 4 at% Fe₂O₃ exhibited the coexistence of superparamagnetism and ferromagnetism at room temperature. When the Fe₂O₃ content was higher than 8 at%, the LMCPS glasses showed ferromagnetism behavior. The complex magnetic behavior is due to the distribution of (Li, Mn)ferrite particle sizes driven by the Fe₂O₃ content. The thermal induced hysteresis loss of the crystallized LMCPS glass ceramics was characterized under an alternating magnetic field. The energy dissipations of the crystallized LMCPS glass ceramics were determined by the concentration and Mn/Fe ratios of Li(Mn, Fe)ferrite phase formed in the glass ceramics.     

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.     

Internal field distribution around spin-glas freezing temperature in AuFe-alloys

We demonstrate an application of PAC to the study of magnetic ordering in random alloys. Using¹¹¹Cd as a probe the spin-glass freezing inAuFe-alloys of different concentrations could be observed. In the Au₈₂Fe₁₈ alloy no indication of ferromagnetism above the freezing temperature was found, due to the fact that the¹¹¹In probe nuclei are located in a Au rich environment.     

Relaxation of magnetic clusters in Sr and Fe doped cobaltate perovskites

Mössbauer study of La_1???x Sr _x ⁵⁷Fe_0.025Co_0.975O₃ (0.15 ≤?x?≤ 0.25) perovskites below their magnetic ordering temperature shows cooperative electronic and magnetic phase separation for all compositions investigated. It is shown that the increase of the Sr concentration results in a transition from superparamagnetism to long range ferromagnetism of Co clusters between x = 0.18 and 0.2. In-field Mössbauer measurements revealed the existence of frustrated magnetic interactions at x = 0.22.