Co50Fe50-xSix合金的结构相变和磁性

利用实验测量和理论计算相结合的方法,研究了介于B2结构CoFe低有序合金和L21结构Co2FeSi高有序合金之间的Co50Fe50-xSix合金的结构相变、磁相变、分子磁矩和居里温度.采用考虑Coulomb相互作用的广义梯度近似（GGA＋U）方法计算了合金的能带结构.研究发现,合金出现较强的原子有序倾向,表现出较强的共价成相作用.合金的晶格常数、磁矩、居里温度随Si含量的增加而线性地降低,极限成分Co2FeSi合金的分子磁矩和居里温度分别达到5.92μB和777℃.原子尺寸效应导致合金晶格发生变化,但并未成为居里温度和分子磁矩变化的主导因素.分子磁矩的变化符合Slater-Pauling原理,但发现原子磁矩的变化并非线性,据此提出了共价成相对磁性影响的观点.采用Stearns理论解释了居里温度的变化趋势,排除了原子间距对居里温度的主导影响作用.能带计算的结果还表明,Co2FeSi作为半金属材料并非十分完美,可能在实际应用中会出现自旋极化率降低的问题.发现该系列合金的结构相变和磁相变随着成分的变化聚集在窄小的成分和温度范围内.     

Nanostructural,magnetic and Mössbauer studies of nanosized Co1−xZnxFe2O4 synthesized by co-precipitation

This work presents a systematic investigation on the structural and magnetic properties of Co_1−xZn_xFe₂O₄ (0.5<x<0.75) nanoparticles synthesized by the chemical co-precipitation method. The X-ray diffraction analysis, the Fourier Transform Infrared (FTIR) and the Vibrating Sample Magnetometer were carried out at room temperature to study the micro-structural and magnetic properties. The X-ray measurements revealed the production of a broad single cubic phase with the crystallite size within the range of 6–10 nm. The FTIR measurements between 400 and 4000 cm⁻¹ confirmed the intrinsic cation vibrations of the spinel structure. The magnetic measurements show that the saturation magnetization and coercivity decrease by increasing the zinc content. Furthermore, the results reveal that the sample with a chemical composition of Co_0.3Zn_0.7Fe₂O₄ exhibits the super-paramagnetic behavior and the Curie point of 97 °C.     

Hydrothermal synthesis and magnetic properties of gadolinium-doped CoFe2O4 nanoparticles

CoFe_2−xGd_xO₄ (x=0-0.25) nanoparticles were synthesized via a simple hydrothermal process at 200 °C for 16 h without the assistance of surfactant. The as-synthesized powders were characterized by X-ray diffraction, transmission electron microscopy, and a vibrating sample magnetometer. The X-ray diffraction results showed that the as-synthesized powders were in the pure phase with a doping amount of ≤0.25, and the peaks could be readily indexed to the cubic spinel cobalt ferrite. Transmission electron microscopy and high resolution transmission electron microscopy observations revealed that the gadolinium-doped cobalt ferrite nanoparticles were single crystal, roughly spherical, uniformly distributed, and not highly agglomerated. The room temperature magnetic field versus magnetization measurements confirmed a strong influence of gadolinium doping on the saturation magnetization and coercivity due to large lattice distortion and grain growth of small particles.     

Synthesis and magnetic properties of antiferromagnetic Co3O4 nanoparticles

Antiferromagnetic Co₃O₄ nanoparticles with diameter around 30 nm have been synthesized by a solution-based method. The phase identification by the wide-angle X-ray powder diffraction indicates that the Co₃O₄ nanoparticle has a cubic spinel structure with a lattice constant of 0.80843(2) nm. The image of field emission scanning electron microscope shows that the nanoparticles are assembled together to form nanorods. The magnetic properties of Co₃O₄ fine particles have been measured by a superconducting quantum interference device magnetometer. A deviation of the Néel temperature from the bulk is observed, which can be well described by the theory of finite-size scaling. An enhanced coercivity as well as a loop shift are observed in the field-cooled hysteresis loop. The exchange bias field decreases with increasing temperature and diminishes at the Néel temperature. The training effect and the opening of the loop reveal the existence of the spin-glass-like surface spins.     

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.     

CoFe2O4纳米颗粒的结构、磁性以及离子迁移

聚乙烯醇(PVA)溶胶凝胶法制备出CoFe₂O₄纳米微粉，用X射线衍射研究了铁氧体纳米颗粒的结构.测量了CoFe₂O₄纳米颗粒80—873 K的变温穆斯堡尔谱，发现纳米颗粒的磁转变温度范围为793—813 K，比块体材料的磁性转变温度要低.CoFe₂O₄纳米颗粒的德拜温度θ_A=674 K，θ_B=243 K，比块体材料要小.CoFe₂O₄纳米颗粒超精细场H_f随温度的变化符合T^3/2+T^5/2定理.当温度较高时，平均同质异能移IS随温度的升高而减小，并呈线性关系. 关键词： 纳米颗粒 磁性 穆斯堡尔谱     

Effect of grain size on the magnetic properties of superparamagnetic Ni0.5Zn0.5Fe2O4 nanoparticles by co-precipitation process

Ni_0.5Zn_0.5Fe₂O₄ (NZFO) spinel-type nanoparticles were directly fabricated by the chemical co-precipitation process using metal nitrate and acetate as precursors since nitrogen and carbon would be taken away in the forms of oxynitride and oxycarbide, respectively, after the precursors were annealed and then investigated in detail by employing X-ray diffraction (XRD), magnetic measurement and Raman spectroscopy. XRD analysis indicates that the as-prepared nanocrystals are all of a pure cubic spinel structure with their sizes ranging from 20.8 to 53.3 nm, as well as peaks of some samples shifting to lower angles due to lattice expansion. Calculations from the derived XRD data indicate that the activation energy is 30.83 kJ/mol. The magnetic measurements show that these samples are superparamagnetic. The saturation magnetization increases with annealing temperature, which may be explained by super-exchange interactions of Fe ions occurring at A- and B-sites. The variation of coercivity with particle size is interpreted on the basis of domain structure and crystal anisotropy. Furthermore, these nanoparticles exhibit a redshift phenomenon at lower temperatures seen in the Raman spectra, which could be related to ionic substitution.     

Structural and magnetic characteristics of Co1−xNix/2Srx/2Fe2O4 nanoparticles

Co_1−xNi_x/2Sr_x/2Fe₂O₄ (x=0–0.5 in steps of 0.1) ferrite nanoparticles have been synthesized at room temperature, without calcination, using a reverse micelle process. The site preference was determined by Mössbauer spectroscopy at 300 K. The hyperfine parameters were obtained, for the whole series of solid solutions. For the X≤0.20 samples, the spectra were fitted with two discrete sextets and for the X>0.20 samples, a magnetic hyperfine field distribution and a doublet were also imposed in the fit procedure. Hysteresis loops were measured using a superconducting quantum interference device magnetometer at 2 K and 300 K. The results indicate that the relative decrease in saturation magnetization of nanoparticles compared to the submicron particles could be attributed to a surface spin termination and disorder. Magnetic dynamics of the nanoparticles was studied by the measurement of ac magnetic susceptibility versus temperature at different frequencies and it is found that the results are well described by the Vogel–Fulcher model.     

Structural and magnetic properties of MnxCo1−xFe2O4 ferrite nanoparticles

We report on the structural and magnetic properties of nanoparticles of Mn_xCo_1−xFe₂O₄ (x=0.1, 0.5) ferrites produced by the glycothermal reaction. From the analysis of XRD spectra and TEM micrographs, particle sizes of the samples have been found to be about 8 nm (for x=0.1) and 13 nm (for x=0.5). The samples were characterized by DC magnetization in the temperature range 5-380 K and in magnetic fields of up to 40 kOe using a SQUID magnetometer. Mössbauer spectroscopy results show that the sample with higher Mn content has enhanced hyperfine fields after thermal annealing at 700 °C. There is a corresponding small reduction in hyperfine fields for the sample with lower Mn content. The variations of saturation magnetization, remnant magnetization and coercive fields as functions of temperature are also presented. Our results show evidence of superparamagnetic behaviour associated with the nanosized particles. Particle sizes appear to be critical in explaining the observed properties.     

Fabrication and magnetic properties of Ni0.50.5Zn0.5Fe2O4 nanofibres by electrospinning

NiZn ferrite/polyvinylpyrrolidone composite fibres were prepared by sol–gel assisted electrospinning.Ni0.5Zn0.5Fe2O4 nanofibres with a pure cubic spinel structure were obtained subsequently by calcination of the composite fibres at high temperatures.This paper investigates the thermal decomposition process,structures and morphologies of the electrospun composite fibres and the calcined Ni0.5Zn0.5Fe2O4 nanofibres at different temperatures by thermogravimetric and differential thermal analysis,x-ray diffraction,Fourier transform infrared spectroscopy and field emission scanning electron microscopy.The magnetic behaviour of the resultant nanofibres was studied by a vibrating sample magnetometer.It is found that the grain sizes of the nanofibres increase significantly and the nanofibre morphology gradually transforms from a porous structure to a necklace-like nanostructure with the increase of calcination temperature.The Ni0.5Zn0.5Fe2O4 nanofibres obtained at 1000 C for 2 h are characterized by a necklace-like morphology and diameters of 100–200 nm.The saturation magnetization of the random Ni0.5Zn0.5Fe2O4 nanofibres increases from 46.5 to 90.2 emu/g when the calcination temperature increases from 450 to 1000 C.The coercivity reaches a maximum value of 11.0 kA/m at a calcination temperature of 600 C.Due to the shape anisotropy,the aligned Ni0.5Zn0.5Fe2O4 nanofibres exhibit an obvious magnetic anisotropy and the ease magnetizing direction is parallel to the nanofibre axis.     

Structural and magnetic properties of free-standing Ni0.23Cu0.11Zn0.66Fe2O4 thick films prepared using a modified tape-casting method

Free-standing Ni_0.23Cu_0.11Zn_0.66Fe₂O₄ thick films have been prepared using a modified tape-casting method from a viscous paint. The surface morphology, saturation magnetization (M_s), coercivity (H_c) and complex permeability of the thick films were studied. It is demonstrated that the thick films have relatively larger complex permeability and higher resonance frequency as compared to their bulk material counterparts.     

The effect of urea-to-nitrates ratio on the morphology and magnetic properties of Mn0.8Mg0.2Fe2O4

Mn_0.8Mg_0.2Fe₂O₄ ferrite was synthesized using flash auto-combustion technique using urea as fuel. The effect of the urea-to-nitrates ratio was examined and found to affect the samples characteristics. The as-burnt powder was crystallized in single-phase spinel structure of cubic symmetry. The lattice parameter was decreased with increase in the urea-to-nitrates ratio (n) while the crystal size increased from 21 to 42 nm with n changing from 6.67 to 10. The coercivity increases while the saturation and remanence magnetization decreases with increase in n. This was attributed to the disturbance of the spin order as a result of the surface effects.     

Heat dissipation and magnetic properties of surface-coated Fe3O4 nanoparticles for biomedical applications

In this study, the influence of surface coating on the magnetic and heat dissipation properties of Fe₃O₄ nanoparticles was investigated. Fe₃O₄ nanoparticles that ranged in size between (particle sizes of 20 and 30 nm) were coated with polyethylenimine (PEI), oleic acid, and Pluronic F-127. Surface coatings that were composed of thick layers of oleic acid and Pluronic F-127 reduced dipole interactions between the particles, and resulted in reduced coercivity and decreased Néel relaxation times. The ac magnetization measurements revealed that the heat dissipation of the PEI-coated Fe₃O₄ nanoparticles was induced by hysteresis loss and Brownian relaxation loss and that of the oleic-acid-coated Fe₃O₄ nanoparticles was mainly induced by hysteresis loss and Néel relaxation loss.     

Dielectric behavior of hexaferrites BaCo2−xZnxFe16O27

The dielectric constant (′) and dielectric loss (tan δ) for hexaferrites BaCo_2−xZn_xFe₁₆O₂₇ have been studied as a function of frequency (f), temperature (T) and composition (x). The experimental results indicate that ′ and tan δ above the relaxation frequency only decrease as the frequency increases and as the temperature decreases. Tan δ shows the dielectric relaxation at certain critical frequencies which rise as temperature increases. The activation energy for the dielectric relaxation (E_D), ′, and tan δ are found to be minimum for x = 0.8.     

Magnetic and structural studies on CoFe2O4 nanoparticles synthesized by co-precipitation,normal micelles and reverse micelles methods

Cobalt ferrite nanoparticles were synthesized by the chemical co-precipitation, normal micelles and reverse micelles methods of iron and cobalt chlorides. X-ray diffraction analysis, Fourier Transform Infrared (FTIR) and Vibrating Sample Magnetometer were carried out at room temperature to study the structural and magnetic properties. X-ray patterns revealed the production of a broad single cubic phase with the average particle sizes of ∼12 nm, 5 nm and 8 nm for co-precipitation, normal micelles and reverse micelles methods, respectively. The FTIR measurements between 400 and 4000 cm⁻¹ confirmed the intrinsic cation vibrations of spinel structure for each one of the three methods. Moreover, the average particle sizes were lower than the single domain size (128 nm) and higher than the super-paramagnetic size (2–3 nm) at room temperature. The results revealed that the magnetic properties depend on the particle size and cation distribution, whereas the role of particle size is more significant.     

Structural and magnetic properties of Co1−xZnxFe2O4 nanoparticles by co-precipitation method

Co_1−xZn_xFe₂O₄ nanoparticles were prepared by co-precipitation method with x varying from 0 to 1.0. The powder samples were characterized by X-ray diffraction (XRD), vibrating sample magnetometer (VSM) and Fourier transform infrared spectroscopy (FTIR). The average crystallite sizes of the particles were determined from XRD. X-ray analysis showed that the samples were cubic spinel. The average crystallite size (D_aveXR) of the particles precipitated was found to vary from 6.92 to 12.02 nm decreasing with the increase in zinc substitution. The lattice constant (a_o) increased with the increase in zinc substitution. The specific saturation magnetization (M_S) of the particles was measured at room temperature. The magnetic parameters such as M_S, H_c, and M_r were found to decrease with the increase in zinc substitution. FTIR spectra of the Co_1−xZn_xFe₂O₄ with x varying from 0 to 1.0 in the range 400–4000 cm⁻¹ were reported. The spinel structure and the crystalline water adsorption of Co_1−xZn_xFe₂O₄ nanoparticles were studied by using FTIR.     

Phase formation and change of magnetic properties in mechanical alloyed Ni0.5Co0.5Fe2O4 by annealing

The effects of milling time and annealing temperature on phase formation, microstructure and magnetic properties of nickel-cobalt ferrite synthesized from oxide precursors by mechanical alloying were studied. The study of milling time effects on phase formation of milled materials showed that if milling continues up to 55 h, single phase nano-sized nickel-cobalt ferrite is obtained. Also, magnetic properties of powders versus milling time and annealing at different temperatures extensively changed, so that annealing at 1200 °C increased the magnetization saturation of the as-milled powder from 15.1 to 53.6 emu/g. X-ray powder diffraction technique (XRD) with Cu-Ka radiation was employed for phase identification. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were also used to determine the morphology and size of the particles. The magnetic properties were measured by a vibration sample magnetometer (VSM).     

Sintering temperature dependence of room temperature magnetic and dielectric properties of Co0.5Zn0.5Fe2O4 prepared using mechanically alloyed nanoparticles

Co_0.5Zn_0.5Fe₂O₄ nanoparticles were prepared using mechanical alloying (MA) and sintering. The crystallite size, coercivity, retentivity and saturation magnetization were also measured. The frequency dependence of dielectric and the magnetic parameters, namely, real permittivity ε′, loss tanget tan δ, real permeability μ′ and loss factor μ″ were measured at room temperature for samples sintered from 600 to 1000 °C, in the frequency range 10 MHz to 1.0 GHz. The results show that the crystallite size of the resulting products ranges between 16 and 67 nm for as-milled sample and the sample sintered at 1000 °C, respectively. The sample sintered at 1000 °C, measured at room temperature exhibited a saturation magnetization of 37 emu g⁻¹. The values of permittivity remain constant within the measured frequency, but vary with sintering temperature. The permeability values, on the other hand however vary with both the sintering temperature and the frequency, thus, the absolute value of the permeability decreased after the natural resonance frequency.     

Heat generation of aqueously dispersed CoFe2O4 nanoparticles as heating agents for magnetically activated drug delivery and hyperthermia

Self-heating from magnetic nanoparticles under AC magnetic field can be used either for hyperthermia or to trigger the release of an anti-cancer drug, using thermo-responsive polymers. The heat generated by applying an AC magnetic field depends on the properties of magnetic nanoparticles (composition, size, crystal structure) as well as the frequency and amplitude of the magnetic field. Before these systems can be efficiently applied for in vitro or in vivo studies, a thorough analysis of the magnetically induced heating is required. In this study, CoFe₂O₄ nanoparticles were synthesized, dispersed in water, and investigated as heating agents for magnetic thermo-drug delivery and hyperthermia. The temperature profiles and infrared (IR) camera images of heat generation of CoFe₂O₄ nanoparticles under various AC magnetic fields of 127–700 Oe at 195, 231, and 266 kHz were measured using an IR thermacam, excluding the external AC magnetic field interruption. The CoFe₂O₄ nanoparticles were successfully dispersed in water using an 11-mercaptoundecanoic acid ligand exchange method to exchange the solvent used for synthesis of hexane for water. During the heating experiments, each of CoFe₂O₄ nanoparticle solutions reached a steady state where the temperature rose between 0.1 and 42.9 °C above ambient conditions when a magnetic field of 127–634 Oe was applied at 231 or 266 kHz. The heat generation was found to be dependent on the intensity of AC magnetic field and applied frequency. Therefore, the desired heating for magnetically triggered drug delivery or hyperthermia could be achieved in water-dispersed CoFe₂O₄ nanoparticles by adjusting the AC magnetic field and frequency.     

Magnetoresistance and magnetic properties of Fe3O4 nanoparticle compacts

In this paper,we report on the magnetic properties of Fe3O4 nanoparticles with different grain sizes under different pressures.In all the samples,the saturated magnetization Ms shows a linear decrease with increasing pressure.The thickness of the magnetic dead layer on the nanoparticle surface nuder different pressures was roughly estimated,which also increases with increasing pressure.The transport measurements of the nanoparticle Fe3O4 compacts show that the low-field magnetoresistance (MR) value is insensitive to the grain size in a wide temperature range;however,the high-field MR value is dependent on grain size,especially at low temperatures.These experimental results can be attributed to the different surface states of the nanoparticles.