Characterization and magnetic properties of Fe–Co ultrafine particles

Characterization and magnetic properties of Fe–Co ultrafine particles were investigated systematically by means of X-ray diffraction, Mössbauer spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, energy disperse spectroscopy analysis, chemical analysis, oxygen determination and magnetization measurement. In comparison with bulk iron–cobalt alloys, the corresponding Fe–Co ultrafine particles have significant difference in the phase structure and magnetic properties, depending on the condition of evaporating and subsequent quenching. The mechanism for the formation of the ultrafine particles as well as the origin of ferromagnetism and paramagnetism (or superparamagnetism) were discussed.     

Influence of dipolar interaction on magnetic properties of ultrafine ferromagnetic particles

We use Monte Carlo simulations to study the influence of dipolar interaction and polydispersity on the magnetic properties of single-domain ultrafine ferromagnetic particles. From the zero field cooling (ZFC)/field cooling (FC) simulations we observe that the blocking temperature T(B) clearly increases with increasing strength of interaction, but it is almost not effected by a broadening of the distribution of particle sizes. While the dependence of the ZFC/FC curves on interaction and cooling rate are reminiscent of a spin glass transition at T(B), the relaxational behavior of the magnetic moments below T(B) is not in accordance with the picture of cooperative freezing.     

Preparation and magnetic properties of ultrafine particles of Fe---Ni alloys

Ultrafine particles (UFP) of six kinds of Fe---Ni alloys were synthesized by the method of hydrogen plasma reaction. The prepared UFP samples were examined by X-ray diffraction, electron transmission microscopy and magnetic measurement. The spherical Fe---Ni UFP alloys with a mean particle size less than 35 nm can be prepared with a production rate much higher than by conventional methods. The phase constitution of UFP alloys is different from the equilibrium phase diagram owing to rapid condensation of evaporated metal gases. Although the magnetization for the UFP alloys has almost the same temperature dependence as that of the bulk alloys, the saturation magnetization remarkably decreases as the bulk alloys change into the UFP alloys.     

Structure and magnetic properties of the oxide layers on iron ultrafine particles

5 . The γ-Fe particles, because of their paramagnetic nature, are very convenient for investigation on the attributes of iron oxide layers formed on the particle surfaces. Structures, morphologies and magnetic properties of the oxide layers covering the iron ultrafine particles have been studied using transmission electron microscopy observation, magnetic property measurement, X-ray diffraction and annealing treatment. Convincing evidences established that the iron oxide layers are not continuous and consist of very fine crystallites, and that these layers are non-ferromagnetic and have no contribution to the saturation magnetization of the iron particles. The iron oxide layers formed at room temperature was determined to be Fe₃O₄. Additionally, a brief annealing of the iron particles in air were performed to examine magnetic properties of the formed iron oxide layers and ultrafine oxide particles. Received: 30 April 1996/Accepted: 5 November 1996     

磁场对碳纸上磁电沉积的AuPt颗粒性能的影响

AuPt纳米颗粒是一种催化锂空气电池空气电极上氧气还原反应和氧气析出反应的双功能催化剂．在电沉积制备AuPt颗粒的过程中施加了磁场,恒电流沉积条件下,随着磁场强度的增加,AuPt催化剂颗粒的粒径从1 μm减小到200 nm,同时催化活性增大．垂直于电场方向的磁场有助于提高AuPt/C空气电极的催化能力．使用脉冲电沉积,结晶粒径可减小至约100 nm．通过调节电场和磁场参数,可原位制备具有不同形貌、不同组成和催化性能的AuPt催化剂．     

Morphology and magnetic properties of ultrafine ZnFe2O4 particles

Well-crystallized ultrafine ZnFe₂O₄ particles of several nanometers in size have been prepared by the coprecipitation method, and their particle morphology and magnetic properties, especially at low temperatures, examined. Room-temperature X-ray diffraction, transmission electron microscopy, magnetization measurements at various temperatures from 300 K to 4.2 K, and Mössbauer spectroscopy at various temperatures from 300 K to 4.2 K, and at 4.2 K with a longitudinal magnetic field of 16.4 kOe applied have been employed. The formation of short-range and long-range magnetic order in small ZnFe₂O₄ particles above and below approximately 30 K is discussed. Below 30 K, the appearance of spontaneous magnetization and its hysteretic property is confirmed for small ZnFe₂O₄ particles.     

Preparation and magnetic properties of ultrafine particles of FeNi alloys

Ultrafine particles (UFP) of six kinds of FeNi alloys were synthesized by the method of hydrogen plasma reaction. The prepared UFP samples were examined by X-ray diffraction, electron transmission microscopy and magnetic measurement. The spherical FeNi UFP alloys with a mean particle size less than 35 nm can be prepared with a production rate much higher than by conventional methods. The phase constitution of UFP alloys is different from the equilibrium phase diagram owing to rapid condensation of evaporated metal gases. Although the magnetization for the UFP alloys has almost the same temperature dependence as that of the bulk alloys, the saturation magnetization remarkably decreases as the bulk alloys change into the UFP alloys.     

Effect of magnetic field on self-assembling of colloidal Co magnetic nanoparticles

In this paper the formation of 3-D structures composed of Co nanoparticles (NPs) is reported. Structures were obtained by drying a droplet of a colloidal solution of NPs in a magnetic field perpendicular to the substrate. The Co nanoparticles were prepared by thermolysis of Co₂(CO)₈. The 3-D NP structures were characterized by scanning electron microscopy (SEM) and atomic and magnetic force microscopy (AFM/MFM). It has been found that at the border of the droplet, NPs assemble into hexagonally ordered 3-D columns or they form a labyrinthine structure. The formation of the 3-D structures can be explained by the outflow of NPs to the border of the droplet during the drying process. Within this model the pattern formation is dependent on the concentration of the NPs and the degree of alignment of the magnetic moments of NPs in the 3-D columns.     

Shape-influenced magnetic properties of CoO nanoparticles

Despite uncertainty about the potential human health and environmental risks of nanotechnology, major stakeholders such as regulatory agencies and the nanotechnology industry are already negotiating the emerging regulatory framework for nanotechnology. Because of a relative lack of nano-specific regulations, the future of nanotechnology development will depend greatly on the views held by the nanotechnology industry. This study fills the research gap in understanding how the nanotechnology industry perceives the risks of nanotechnology. This is the first interview-based study of the nanotechnology industry in the United States. Semi-structured, open-ended phone interviews were conducted with 17 individuals involved in the commercialization of nanotechnology in the United States. Results indicate that while the industry acknowledges uncertainty about the potential risks of nanotechnology and takes significant precaution in ensuring the safety of their products, they do not see nanotechnology as novel or risky. They do not believe that uncertainty over risk ought to delay the further development of nanotechnology. The industry sees itself as the primary agent in ensuring consumer safety and believes that consumers are adequately protected. They are also largely benefit-centric and view product labeling as inefficacious.     

Effect of Co layer thickness on structural and magnetic properties of C/Co/C films

Granular C/Co/C films have been prepared by magnetron sputtering from C and Co onto glass substrates at room temperature and subsequent in situ annealing. It has been found that the structure and magnetic properties of the C/Co/C films depend strongly on the Co layer thickness. Vibrating sample magnetometer measurements indicate that the in-plane coercivities reach maximum in 20 nm Co thickness of both as-deposited and annealed films. The squareness ratio of annealed films was more than 0.8. X-ray diffraction shows that majority Co nanograins are formed as the hexagonal-close-packed (HCP) structure in 20 nm Co thickness with annealing at 400 °C. Scanning probe microscope was used to scan surface morphology and magnetic domain structures. The values of the surface roughness were lower than 0.6 nm in all annealed samples. The average magnetic cluster size was estimated to be about 10 nm in annealed 20 nm Co thickness films.     

Electromagnetic and magnetic properties of magnetophotonic crystal based on opal matrix with Co and CoO nanoparticles

Magnetic properties of magnetophotonic crystals based on opal matrices have been studied as well as their electromagnetic properties in millimeter waveband. The particles of cobalt oxide are embedded into the inter-sphere voids of the matrix. After annealing in hydrogen the cobalt oxide particles transform to metallic cobalt. It has been shown that if antiferromagnetic cobalt oxide remains besides ferromagnetic cobalt, the low-temperature magnetic hysteresis loop is shifted along the field axis. Magnetic field influences essentially on the microwave transmission and reflection coefficients only after annealing in hydrogen that is if the ferromagnetic phase presents in the sample. The spectra of magnetic resonance and antiresonance are studied.     

Thermal stability, oxidation behavior and magnetic properties of Fe–Co ultrafine particles prepared by hydrogen plasma–metal reaction

Six kinds of Fe–Co ultrafine particles (UFPs) with an average particle size of about 45 nm were prepared by a hydrogen plasma–metal reaction method (HPMR). Particle feature, thermal stability, oxidation and magnetic properties of these UFPs were investigated. Fe–Co UFPs are spherical in shape and have the same crystal structure as bulk samples, but the Co content in Fe–Co UFPs is slightly richer than that in the master samples. Characteristics such as particle shape, mean particle size and specific surface area do not vary remarkably with the Co content. Fe–Co UFPs are stable in Ar and air below 473 K, but oxidize quickly in air above 473 K and simultaneously vary from a spherical shape into acicular shape during oxidation. Although the saturation magnetizations of Fe–Co UFPs both before and after oxidation are smaller than those of bulk samples, they exhibit a composition dependence similar to that of the master alloys. High residual magnetization and coercive force are obtained for Fe–Co UFPs before and after oxidation.     

Tunneling mechanism through the nonlinear electrical transport in Co/CoO particles with core-shell nanostructure

We investigate the nonlinear electrical transport as a function of temperature in Co/CoO nanoparticles having core-shell nanostructure. Nanoparticle was synthesized by sol-gel citrate precursor technique where core-shell nanostructure is confirmed by the high resolution Transmission Electron Microscopy. Current-voltage (I-V) characteristics are measured over the temperature range 20-295 K. I-V curve exhibits ohmic behaviour at 295 K. Nonlinear electrical transport is observed at low temperature (T) for T?275 K. Electrical transport properties have been interpreted in terms of tunneling mechanism where tunneling between ferromagnetic Co nanoparticles takes place through the antiferromagnetic CoO layer. Analysis of dynamic conductance (G=dI/dV) indicates that the inelastic tunneling via localized states of antiferromagnetic CoO layers is dominant in the transport mechanism at low temperature.     

磁场对模板法制备的Co纳米线结构和磁性的影响

在用二次氧化法制备的高度有序的氧化铝模板上通过交流电化学方法制备了Co纳米线阵列．研究了外加磁场及电解液pH值对纳米线生长的影响．在pH值为6.0和6.5的电解液中分别在不加磁场和沿纳米线轴向施加0.3 T磁场情况下制备了hcp结构的Co纳米线阵列．实验数据表明,沉积时外加磁场和调节pH值能有效影响纳米线中hcp结构的Co晶粒的易磁化轴沿纳米线长轴方向生长．由于晶粒的磁晶各向异性和纳米线沿长度方向的宏观形状各向异性叠加,制备的Co纳米线阵列具有高垂直各向异性,高矫顽力和较高矩形比． 关键词： Co纳米线阵列 织构 磁性     

Effect of Co substitution on the magnetic properties of BiFeO3

Bismuth ferrite is doped with a dilute concentration of cobalt, _{BiFe1−XCoXO3}${\mathrm{BiFe}}_{1-X}{\mathrm{Co}}_X{\mathrm{O}}_3$; X=0, 0.01, 0.02 is prepared by sol-gel auto combustion technique. X-ray diffraction data refined via Reitveld method shows single phase and shrinkage in cell volume for Co doped BiFeO₃. Various magnetic ground states viz. superparamagnetic, glassy antiferromagnetic and glassy ferrimagnetic behavior is observed for X=0, 0.01, 0.02, respectively. A first-order magnetic transition is observed in the Arrott plot data of cobalt doped BiFeO₃. Possibility of thermally induced magnetic transition is also seen in the magnetization data of cobalt doped BiFeO₃. A model based on the existing spinoidal cyclic arrangement of spins is proposed to explain the observed data. Finally, a very dilute concentration of Co⁺³${\mathrm{Co}}^{+3}$ in BiFeO₃ is found sufficient to tailor the magnetic properties.     

Effect of bilayer repeats on magnetic properties of Au-buffered Co/Ni multilayers with perpendicular magnetic anisotropy

The effect of bilayer repeats (N) on the static and dynamic magnetic properties of Co/Ni multilayers was investigated. The effective perpendicular magnetic anisotropy constant of multilayers drops from $1.08\times {10}^6$ erg/cm³ to $0.78\times {10}^6$ erg/cm³ with N increasing from 5 to 11. For Co/Ni multilayers with $N\le 7$, sharp magnetization switching was observed. In contrast, Co/Ni multilayers with $N\ge 9$ have a long tail in the hysteresis loop. Ferromagnetic resonance measurements show that intrinsic Gilbert damping changes from 0.021 to 0.016 with increase in N and is inversely proportional to N. This study provides a deep understanding and effective control of magnetic properties of Co/Ni multilayers for spintronics devices.     

Effect of nickel doping concentration on structural and magnetic properties of ultrafine diluted magnetic semiconductor ZnO nanoparticles

The ZnO:Ni²⁺ nanoparticles of mean size 2-12 nm were synthesized at room temperature by the simple co-precipitation method. The crystallite structure, morphology and size were determined by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). The wurtzite structure of ZnO gradually degrades with the increasing Ni doping concentration and an additional NiO-associated diffraction peak was observed above 15% of Ni²⁺ doping. The change in magnetic behavior of the nanoparticles of ZnO with varying Ni²⁺ doping concentration was investigated using a vibrating sample magnetometer (VSM). Initially, these nanoparticles showed strong ferromagnetic behavior, however, at higher doping percentage of Ni²⁺, the ferromagnetic behavior was suppressed and paramagnetic nature was observed. The enhanced antiferromagnetic interaction between neighboring Ni-Ni ions suppressed the ferromagnetism at higher doping concentrations of Ni²⁺.     

Effect of magnetic field decay on the chemical heating of cooling neutron stars

The effect of magnetic field decay on the chemical heating and thermal evolution of neutron stars is discussed in this paper. Our main goal is to study how the chemical heating mechanism and thermal evolution are changed by the field decay and how the magnetic field decay is modified by the thermal evolution. We compare stars cooling with chemical heating with one without chemical heating and find that the decay of the magnetic field is delayed significantly by the chemical heating. We find that the effect of chemical heating has been suppressed through the decaying magnetic field by the spin-down of the stars at a later stage. Compared with typical chemical heating, we find the decay of the magnetic field can even cause the surface temperature to turn down at an older age. When we discuss the cooling of neutron stars, we should consider the coupling effect of the magnetic field and the rotational evolution of neutron stars on the heating mechanisms.     

Effect of nanocrystalline particles on the magnetic properties of Z-type hexaferrites

In order to improve the magnetic properties of Z-type hexaferrites such as high initial permeability and high-quality factor, the nanocrystalline hexaferrite particles (NHPs) with the same chemical composition were introduced. The influence of NHPs on the densification, microstructures and magnetic properties of the ceramics prepared by a combined method was investigated. The results show that these NHPs, which spread around the micron-sized hexaferrite particles (MHPs), enhance the densification by increasing the inter-diffusion of the particles due to the increase of contact area; simultaneously, the grain growth in the direction of c-plane is dominant. Due to the special microstructure, high sintering density, and no addition sintering aids with different chemical composition, relatively high initial permeability and Q-factor than those of the samples with 1.0 wt% Bi₂O₃ were obtained in the samples with proper nanocrystalline particles sintered at 900 °C.