Dependence of structural and magnetic properties of CoFe2O4/SiO2 nanocomposites on annealing temperature and component ratio

Cobalt ferrite (CoFe₂O₄) nanoparticles embedded in amorphous silica can be synthesized by using tetraethylorthosilicate (TEOS) and metallic nitrates as precursors. A well-established silica matrix network provides nucleation locations for CoFe₂O₄ nanoparticles, thus confining their growth and aggregation. The structural and magnetic properties show strong dependence on the variation of particle size caused by annealing temperature and CoFe₂O₄ ratio, resulting in higher crystallization, saturation magnetization M_s and remanent magnetization M_r as the annealing temperature and CoFe₂O₄ ratio increase. But the variation of coercivity H_c is not in accordance with that of M_s and M_r, indicating that H_c is not determined by the size of CoFe₂O₄ nanoparticles only. The realization of the adjustable particle sizes and the controllable magnetic properties makes the applicability of CoFe₂O₄ even more versatile.     

Exchange coupling behavior in bimagnetic CoFe2O4/CoFe2 nanocomposite

In this work we report a study of the magnetic behavior of ferrimagnetic oxide CoFe₂O₄ and ferrimagnetic oxide/ferromagnetic metal CoFe₂O₄/CoFe₂ nanocomposite. The latter compound is a good system to study hard ferrimagnet/soft ferromagnet exchange coupled. Two steps were followed to synthesize the bimagnetic CoFe₂O₄/CoFe₂ nanocomposite: (i) first, preparation of CoFe₂O₄ nanoparticles using a simple hydrothermal method, and (ii) second, reduction reaction of cobalt ferrite nanoparticles using activated charcoal in inert atmosphere and high temperature. The phase structures, particle sizes, morphology, and magnetic properties of CoFe₂O₄ nanoparticles were investigated by X-Ray diffraction (XRD), Mossbauer spectroscopy (MS), transmission electron microscopy (TEM), and vibrating sample magnetometer (VSM) with applied field up to 3.0 kOe at room temperature and 50 K. The mean diameter of CoFe₂O₄ particles is about 16 nm. Mossbauer spectra revealed two sites for Fe³⁺. One site is related to Fe in an octahedral coordination and the other one to the Fe³⁺ in a tetrahedral coordination, as expected for a spinel crystal structure of CoFe₂O₄. TEM measurements of nanocomposite showed the formation of a thin shell of CoFe₂ on the cobalt ferrite and indicate that the nanoparticles increase to about 100 nm. The magnetization of the nanocomposite showed a hysteresis loop that is characteristic of exchange coupled systems. A maximum energy product (BH)_max of 1.22 MGOe was achieved at room temperature for CoFe₂O₄/CoFe₂ nanocomposites, which is about 115% higher than the value obtained for CoFe₂O₄ precursor. The exchange coupling interaction and the enhancement of product (BH)_max in nanocomposite CoFe₂O₄/CoFe₂ are discussed.     

Preparation and characterization of multi-functional CoFe2O4-ZnO nanocomposites

Multi-functional magnetic, photoluminescent and photocatalytic CoFe₂O₄-ZnO nanocomposites were successfully synthesized by a collosol method. The average diameter of the prepared CoFe₂O₄-ZnO nanocomposites was 30±5 nm, and a diffusion layer was formed to link CoFe₂O₄ and ZnO. The saturation magnetization of the CoFe₂O₄-ZnO nanocomposites was 8.99 emu/g. Generation of ZnO from Zn(OH)₂ collosol was nearly complete after thermal decomposition at about 380 °C. A photoluminescence emission peak was observed at 443 nm when excitated at 350 nm. Degradation of methyl orange is performed by CoFe₂O₄-ZnO nanocomposites under ultraviolet radiation, with a degradation rate of up to 93.9%.     

复合薄膜NiFe2 O4-BiFeO3 中的磁电耦合

采用化学溶液沉积法(CSD)在Au/Ti/SiO₂/Si衬底上通过自组装生长制备了BiFeO₃-NiFe₂O₄ (BFO-NFO)多铁性复合薄膜.X射线衍射图谱(XRD)显示形成了分离的钙钛矿结构的铁电相BFO和尖晶石结构的铁磁相NFO. NFO的引入导致复合薄膜的泄漏电流减小,剩余极化强度增加.相比于纯BFO薄膜,0.25NFO-0.75BFO样品泄漏电流下降了约两个数量级,剩余极化强度( M _{关键词： 多铁性复合薄膜 磁电耦合 铁电性 铁磁性}     

Structural, microstructural and magnetic properties of NiFe2O4, CoFe2O4 and MnFe2O4 nanoferrite thin films

The structural, microstructural and magnetic properties of nanoferrite NiFe₂O₄ (NF), CoFe₂O₄ (CF) and MnFe₂O₄ (MF) thin films have been studied. The coating solution of these ferrite films was prepared by a chemical synthesis route called sol-gel combined metallo-organic decomposition method. The solution was coated on Si substrate by spin coating and annealed at 700 °C for 3 h. X-ray diffraction pattern has been used to analyze the phase structure and lattice parameters. The scanning electron microscopy (SEM) and atomic force microscopy (AFM) have been used to show the nanostructural behavior of these ferrites. The values of average grain's size from SEM are 44, 60 and 74 nm, and from AFM are 46, 61 and 75 nm, respectively, measured for NF, CF and MF ferrites. At room temperature, the values of saturation magnetization, M_s∼50.60, 33.52 and 5.40 emu/cc, and remanent magnetization, M_r∼14.33, 15.50 and 1.10 emu/cc, respectively, are observed for NF, CF and MF. At low temperature measurements of 10 K, the anisotropy of ferromagnetism is observed in these ferrite films. The superparamagnetic/paramagnetic behavior is also confirmed by χ′(T) curves of AC susceptibility by applying DC magnetizing field of 3 Oe. The temperature dependent magnetization measurements show the magnetic phase transition temperature.     

Co3O4-ZnO hierarchical nanostructures by electrospinning and hydrothermal methods

A new hierarchical nanostructure that consists of cobalt oxide (Co₃O₄) and zinc oxide (ZnO) was produced by the electrospinning process followed by a hydrothermal technique. First, electrospinning of a colloidal solution that consisted of zinc nanoparticles, cobalt acetate tetrahydrate and poly(vinyl alcohol) was performed to produce polymeric nanofibers embedding solid nanoparticles. Calcination of the obtained electrospun nanofiber mats in air at 600 °C for 1 h, produced Co₃O₄ nanofibers with rough surfaces containing ZnO nanoparticles (i.e., ZnO-doped Co₃O₄ nanofibers). The rough surfaced nanofibers, containing ZnO nanoparticles (ZnNPs), were then exploited as seeds to produce ZnO nanobranches using a specific hydrothermal technique. Scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were employed to characterize the as-spun nanofibers and the calcined product. X-ray powder diffractometery (XRD) analysis was used to study the chemical composition and the crystallographic structure.     

The modification effect in magnetization behaviors for CoFe2O4–p-NiFe2O4 binary ferrofluids

The magnetization curves of CoFe₂O₄ ferrofluids, p-NiFe₂O₄ paramagnetic fluids and CoFe₂O₄–p-NiFe₂O₄ binary ferrofluids, in which the volume fraction of CoFe₂O₄ particles φ _Co is 0.6% and one of p-NiFe₂O₄ particles φ _Ni is 0.2%, 0.4%, 0.6% and 0.8% respectively, prepared by the Massart method, have been measured at room temperature. Comparison of the experimental data from the CoFe₂O₄ ferrofluids with the Langevin theory curves demonstrates a considerable difference between them, but a curve fitted using a model of a gas-like compression (MGC) agrees with the experimental data very well. The experimental results show that the magnetization of the CoFe₂O₄–p-NiFe₂O₄ binary ferrofluid is not a simple summation of the ferrimagnetic CoFe₂O₄ part and the paramagnetic p-NiFe₂O₄ part. From the fitted results, it was found that the saturation magnetization of the CoFe₂O₄ part of the binary ferrofluid depends non-monotonically on the p-NiFe₂O₄ particle volume fraction, and the CoFe₂O₄ part is a stronger “hard” magnet than CoFe₂O₄ in simple ferrofluids. The magnetization behavior of the binary ferrofluids is explained by the modification of the microstructure of CoFe₂O₄ nanoparticle system by the p-NiFe₂O₄ nanoparticle system.     

Interfacial strain-mediated magnetoelectric coupling as reflected in extended X-ray absorption spectra of CoFe2O4-dispersed Pb(Zr,Ti)O3 matrix composites

An atomic-scale picture of the strain-mediated magnetoelectric (ME) coupling is delineated by carefully examining the effect of an applied electric field on the extended X-ray absorption fine structure (EXAFS) spectra of a CoFe₂O₄-dispersed Pb(Zr,Ti)O₃ matrix (CFO-PZT) composite. These studies demonstrated a tensile-compressive strain relation between the PZT matrix and the dispersed CFO phase, thereby providing an X-ray spectroscopic evidence of the interfacial strain-mediated ME coupling. Both the dielectric anomaly observed at ∼480 °C and the decrease in the remanent magnetization under an applied electric field support the strain-mediated ME coupling in the CFO-PZT composite.     

Effects of Mn substitution of Co and Fe in spinel CoFe2O4 thin films

Effects of Mn substitution for Co and Fe on the structural and magnetic properties of inverse-spinel CoFe₂O₄ have been investigated. Mn_xCo_1−xFe₂O₄ and Mn_yCoFe_2−yO₄ thin films were prepared by a sol–gel method. The observed increase of the lattice constant of Mn_xCo_1−xFe₂O₄ indicates that Mn²⁺ ions substitute the octahedral Co²⁺ sites. Conversion electron Mössbauer spectroscopy data indicate that a fraction of octahedral Co²⁺ ions exchange sites with tetrahedral Fe³⁺ ions through Mn doping. Vibrating-sample magnetometry data exhibit a large increase of saturation magnetization for both Mn_xCo_1−xFe₂O₄ and Mn_yCoFe_2−yO₄ films compared to that of the CoFe₂O₄ film. Such enhancement of magnetization can be explained in terms of a breaking of ferrimagnetic order induced by the Co²⁺ migration.     

Magnetization reversal in Fe48Pt34B34 magnetic foils

Exchange-spring magnet L1-FePt/(Fe₂B+α-Fe) is fabricated by flash annealing a melt-spun Fe₄₈Pt₃₄B₃₄ foil. A coercivity of 8500 Oe (1 Oe = 79.5775 A/m), squareness (M_r/M_s) of 0.70, saturation magnetization of 10.2 kGs (1 Gs = 10^-4 T) and an effective anisotropy K_eff =2.0 × 10⁷ ergs/cm³ are obtained. A two-step magnetization reversal feature is characterized in this paper. An exchange bias phenomenon is also observed in a low saturation field.     

一维Ni0.5Zn0.5Fe2O4/SiO2复合纳米结构的制备及其磁性能

采用溶胶-凝胶法结合静电纺丝技术制备了Ni_0.5Zn_0.5Fe₂O₄/SiO₂复合纳米纤维.利用热重-差热分析、X射线衍射、场发射扫描电镜、高分辨透射电镜和振动样品磁强计研究了前驱体纤维的热分解及相转化过程以及焙烧温度和SiO₂含量对目标纳米纤维的相组成、微观结构、形貌及磁性能的影响.结果表明,在450 ℃焙烧时,立方尖晶石结构已基本形成.随着焙烧温度由450 ℃升高到100     

Structural and magnetic properties of sol-gel Co2xNi0.5−x Zn0.5−xFe2O4 thin films

Nanocrystalline Co_2xNi_0.5−xZn_0.5−xFe₂O₄ (x=0−0.5) thin films have been synthesized with various grain sizes by a sol-gel method on polycrystalline silicon substrates. The morphology as well as magnetic and microwave absorption properties of the films calcined at 1073 K were studied using X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and vibrating sample magnetometry. All films were uniform without microcracks. The Co content in the Co-Ni-Zn films resulted in a grain size ranging from 15 to 32 nm while it ranged from 33 to 49 nm in the corresponding powders. Saturation and remnant magnetization increased with increase in grain size, while coercivity demonstrated a drop due to multidomain behavior of crystallites for a given value of x. Saturation magnetization increased and remnant magnetization had a maximum as a function of grain size independent of x. In turn, coercivity increased with x independent of grain size. Complex permittivity of the Co-Ni-Zn ferrite films was measured in the frequency range 2-15 GHz. The highest hysteretic heating rate in the temperature range 315-355 K was observed in CoFe₂O₄. The maximum absorption band shifted from 13 to 11 GHz as cobalt content increased from x=0.1 to 0.2.     

压力对纳米CoFe2O4/SiO2复合材料结构的影响

 采用溶胶-凝胶工艺和高温高压实验技术,制备了纳米CoFe₂O₄/SiO₂复合材料。利用X射线衍射仪、扫描电子显微镜和振动样品磁强计,对样品的结构、微观形貌和磁性进行了研究,并对CoFe₂O₄中阳离子的占位情况进行了讨论。结果表明,随着处理压力的升高,样品的晶粒尺寸增大,晶格常数减小,比饱和磁化强度增大。通过计算结果可以推断,压力的升高导致CoFe₂O₄中的部分Fe³⁺从A位移向了B位,而部分Co²⁺则从B位移向了A位。     

Synthesis and magnetic properties of hard magnetic (CoFe2O4)-soft magnetic (Fe3O4) nano-composite ceramics by SPS technology

CoFe₂O₄/Fe₃O₄ nano-composite ceramics were synthesized by Spark Plasma Sintering. The X-ray diffraction patterns show that all samples are composed of CoFe₂O₄ and Fe₃O₄ phases when the sintering temperature is below 900 °C. It is found that the magnetic properties strongly depend on the sintering temperature. The two-step hysteresis loops for samples sintered below 500 °C are observed, but when sintering temperature reaches 500 °C, the step disappears, which indicates that the CoFe₂O₄ and Fe₃O₄ are well exchange coupled. As the sintering temperature increases from 500 to 800 °C, the results of X-ray diffractometer indicate the constriction of crystalline regions due to the ion diffusion at the interfaces of CoFe₂O₄/Fe₃O₄ phases, which have great impact on the magnetic properties.     

Microstructure evolution and electromagnetic properties improvement of Al18B4O33w/Co composite powders through heat-treatment

Al₁₈B₄O₃₃w/Co composite powders were prepared through electroless depositing Co on Al₁₈B₄O₃₃ whiskers and the microstructure of the prepared composite powders was adjusted through heat-treatment. The included Co oxide is reduced and the density as well as the crystal perfection of the coatings is improved when annealed at 400 °C in H₂ atmosphere. An increase of 10⁵ S m⁻¹ in conductivity together with an increase of 28% in M_s is obtained, resulting in a prominent increase of the permittivity and the permeability. The increase of permittivity, specifically the dielectric relaxation is attributed to the increase of conductivity. The increase of permeability is attributed to the increase of M_s and the microstructure evolution. The increase of electromagnetic parameters in 2-18 GHz band is believed to enhance the electromagnetic wave absorbing performance of the Al₁₈B₄O₃₃w/Co composite powders.     

Synthesis and magnetic properties of single-crystalline BaFe12O19 nanoparticles

Rod-like and platelet-like nanoparticles of simple-crystalline barium hexaferrite (BaFe₁₂O₁₉) have been synthesized by the molten salt method. Both particle size and morphology change with the reaction temperature and time. The easy magnetization direction (0 0 l) of the BaFe₁₂O₁₉ nanoparticles has been observed directly by performing X-ray diffraction on powders aligned at 0.5 T magnetic field. The magnetic properties of the BaFe₁₂O₁₉ magnet were investigated with various sintering temperatures. The maximum values of saturation magnetization (σ_s=65.8 emu/g), remanent magnetization (σ_r=56 emu/g) and coercivity field (H_ic=5251 Oe) of the aligned samples occurred at the sintering temperatures of 1100 °C. These results indicate that BaFe₁₂O₁₉ nanoparticles synthesized by the molten salt method should enable detailed investigation of the size-dependent evolution of magnetism, microwave absorption, and realization of a nanodevice of magnetic media.     

Electrophoretic deposition of BaTiO3/CoFe2O4 multiferroic composite films

Electrophoretic deposition was utilized for preparation of BaTiO₃/CoFe₂O₄ multiferroic composite thick films on indium-tin oxide substrates. The suspensions for electrophoretic experiments were prepared by dispersing BaTiO₃ and CoFe₂O₄ nanoparticles with different molar ratios into solvents composed of ethanol and acetylacetone. Polyvinyl butyral was added to the suspensions in order to enhance the adhesion and strength of deposit and prevent cracking. The zeta potential values of BaTiO₃/CoFe₂O₄ suspensions were measured to be 26.4-36.9 mV. The experiment results showed that deposited films were obtained only when the applied electric field was larger than a certain critical value. XRD and SEM analysis depicted the presence of constituent phases in composite films. The percolation threshold of composite films was improved through dispersing ferromagnetic phase into ferroelectric phase. Therefore, the ferroelectric properties of composite thick films were maintained when the ferromagnetic properties were enhanced significantly with increasing CFO content.     

Fabrication of NiCo2O4 nanofibers by electrospinning

The spinel NiCo₂O₄ nanofibers with diameters of 50-100 nm were prepared by high temperature calcinations of a simple inorganic-polymer composite fibers, which were obtained by electrospinning of the PVA/cobalt acetate/nickel acetate composite precursor. The crystallinity, purity, and surface morphology of the as-prepared NiCo₂O₄ nanofibers were investigated by XRD, FT-IR, SEM, respectively.     

Preparation of nanofibers consisting of MnO/Mn<Subscript>3</Subscript>O<Subscript>4</Subscript> by using the electrospinning technique: the nanofibers have two band-gap energies

In the present study, nanofibers consisting of manganese monoxide (MnO), which is hard to prepare because of the chemical activity of the manganese metal, and the popular Mn₃O₄ have been synthesized via the electrospinning technique. The nanofibers were obtained by electrospinning of an aqueous sol–gel consisting of manganese acetate tetra-hydrate and poly(vinyl alcohol). The obtained nanofiber mats were dried in vacuum at 80°C for 24 h and then calcined in argon atmosphere at 900°C for 5 h. According to X-ray diffraction results, the obtained nanofibers contain 65% MnO. Transmission electron microscope analysis reveals good crystallinity of the produced nanofibers. UV–visible spectroscopic analysis has indicated that the produced nanofibers have two band-gap energies, 2 and 3.7 eV, which enhances utilizing of the nanofibers in different applications.     

Room temperature magnetization measurements of some substituted rare earth iron garnets

The saturation magnetization (M _s) and the initial magnetic susceptibility (χ _in) of substituted yttrium iron garnet (YIG) Ho_3−x Y _x Fe₅O₁₂ where (x=0,0.3,1.5), pure terbium iron garnet Tb₃Fe₅O₁₂, and substituted aluminum iron garnet (AlIG) Ho₃Fe_5−x Al _x O₁₂ where (x=0.05,0.7) at room temperature are reported. It has been observed that M _s and χ _in decrease in a linear manner as Ho³⁺ replaces Y³⁺ in yttrium iron garnet. For substituted (AlIG), a drastic decrease in both M _s and χ _in as Al³⁺ ions replace Fe³⁺ ions. The rate of decrease of M _s and χ _in with the increase of Al³⁺ for substituted (AlIG) is greater than that for substituted (YIG). That is attributed to the decrease in the superexchange interaction (self and mutual iron interactions) with Al substitution.