Synthesis of nanocrystalline Co-Ni alloys by precursor approach and studies on their magnetic properties

Nanocrystalline Co-Ni alloys with different compositions were prepared by polyol reduction of mixed cobalt nickel hydroxides. The precursors (mixed cobalt nickel hydroxides) were prepared by co-precipitation. Powder X-ray diffraction analysis indicated the formation of fcc phase in the alloys and their crystallite size in the range 17-25 nm. Scanning electron microscopy and transmission electron microscopy studies revealed the morphology of the particles as being close to spherical, and the energy dispersive X-ray analysis showed the stoichiometry of the alloys. The magnetization as a function of field and temperature of the alloys, measured using a superconducting quantum interference device, showed superparamagnetic behavior with negligible coercivity and remanence values.     

Influence of Sm-substitution on structure and electromagnetic properties of Ba3−xSmxCo2Fe24O41 powders

Sm-substituted barium hexaferrites, Ba_3−xSm_xCo₂Fe₂₄O₄₁ (x=0-0.25), were prepared by a conventional ceramic sintering method. The microstructure, complex permittivity, complex permeability and static magnetic properties of the samples were studied using powder X-ray diffraction, field emission scanning electron microscopy, vector network analyzer and vibrating sample magnetometry. The results reveal that by introducing a relatively small amount of Sm³⁺ instead of Ba²⁺ an important modification of both structure and high-frequency electromagnetic properties can be obtained. Doping of Sm³⁺ suppressed the grain growth and gave rise to a decrease of the grain size. As the Sm content increases, the static magnetic properties continuously increase. The real part and imaginary part of complex permittivity initially increase with Sm content, and then decreases when x>0.10. The imaginary part of complex permeability decreases after Sm³⁺ is doped. There is no obvious change in the real part of the complex permeability for different Sm contents. The reasons are discussed using electromagnetic theory.     

Monte Carlo simulation of magnetic phase transitions in Mn-doped ZnO

The magnetic properties of Mn-doped ZnO semiconductor have been investigated using the Monte Carlo method within the Ising model. The temperature dependences of the spontaneous magnetization, specific heat and magnetic susceptibility have been constructed for different concentrations of magnetic dopant Mn and different carrier concentrations. The exact values of Mn concentration and carrier concentration at which high temperature transition occurs are determined. An alternative for the explanation of some controversies concerning the existence and the nature of magnetism in Mn diluted in ZnO systems is given. Other features are also studied.     

Effect of Fe substitution on multiferroic hexagonal YMnO3

Polycrystalline YMn_1−xFe_xO₃ (x=0.02-0.20) powders were synthesized by means of modified citrate method. Powder X-ray diffraction gives evidence that all the samples are single phase and exhibit hexagonal structure with P6₃cm space group as observed for YMnO₃. The solubility limit of Fe was determined as about 6 wt.%. Cell parameter values were found to increase with Fe content, since Fe³⁺ and Mn³⁺ have the same ionic radii. This can be attributed to the increase of the tilting of MnO₅ bipyramid and the buckling of Y atoms. In addition, ⁵⁷Fe Mössbauer spectrometry provides evidence of two Fe³⁺ sites attributed to two different nearest atomic neighbours. Magnetic properties reveal a paramagnetic-to-antiferromagnetic transition, a possible increase of the magnetic anisotropy, and a competition between ferromagnetic and antiferromagnetic interactions.     

Electromagnetic properties and microwave absorption properties of BaTiO3-carbonyl iron composite in S and C bands

BaTiO₃ powders are prepared by sol-gel method. The carbonyl iron powder is prepared via thermal decomposition of iron pentacarbonyl. Then BaTiO₃-carbonyl iron composite with different mixture ratios was prepared using the as-prepared material. The structure, morphology, and properties of the composites are characterized using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction, scanning electron microscopy (SEM), and a network analyzer. The complex permittivity and reflection loss of the composites have been measured at different microwave frequencies in S- and C-bands employing vector network analyzer model PNA 3629D vector. The effect of the mass ratio of BaTiO₃/carbonyl iron on the microwave loss properties of the composites is investigated. A possible microwave absorbing mechanism of BaTiO₃-carbonyl iron composite has been proposed. The BaTiO₃-carbonyl iron composite can find applications in suppression of electromagnetic interference, and reduction of radar signature.     

Effect of pH value on the structural and magnetic properties of nanocrystalline strontium hexaferrite thin films

Strontium hexaferrite SrFe₁₂O₁₉ thin films have been synthesized at different pH, adjusted by NH₄OH, on the Si (1 0 0) substrate using a spin coating sol-gel process. Fourier transform infrared spectroscopy analysis and theoretical calculations were conducted for determination and controlling metal citrates in solution precursors. X-ray diffraction, scanning electron microscopy and vibrating sample magnetometer were applied to evaluate the composition, microstructure, crystallite size and magnetic properties of the SrFe₁₂O₁₉ thin films. Using the solution with pH 7, the approximately single phase strontium hexaferrite thin films with optimum physical properties can be obtained at calcination temperature of 800 °C. The SrFe₁₂O₁₉ thin films derived from the solution with pH 7 after calcination at 800 °C exhibited crystallite size of 42 nm and magnetic properties of M_s=267 emu/cm³ (at 10 kOe), M_r=134 emu/cm³ and H_c=4290 Oe.     

ASYMPTOTIC PROPERTIES OF THE MOMENT CONVERGENCE FOR NA SEQUENCES

It is well-known that the complete convergence theorem for i.i.d. random variables has been an active topic since the famous work done by Hsu and Robbins [6]. Chow [4]obtained a moment version of Hsu and Robbins series. However, the series tends to infinity whenever ∈ goes to zero, so it is of interest to investigate the asymptotic behavior of the series as ∈ goes to zero. This note gives some limit theorems of the series generated by moments for NA random variables.     

Dynamic fuzzy reliability models of degraded hold-down structures for folded solar array

The hold-down structures are of considerable importance to the launch of solar array. Due to the difficulties in obtaining sufficient load specimen, it is imprecise to construct the stress as random variables. Therefore, dynamic fuzzy reliability models are developed in this paper, which resolve the problems in dealing with the interaction between the fuzzy stress process and the stochastic strength process. Even for a deterministic fuzzy stress process, the influences of material statistical properties on reliability can be affected by the level α of fuzzy stress. Meanwhile, the level α relates to investment in the collection of information about the fuzzy stress on hold-down bar. Hence, the models can be used for the economic analysis and optimal design of hold-down bar. Finally, key fuzzy parameters of stress, which have significant influences on both the reliability behavior and the effects of material statistical properties on reliability, are identified and some suggestions for the reliability enhancement of hold-down bar are provided in this paper.     

The uniqueness property for networks with several origin–destination pairs

We consider congestion games on networks with nonatomic users and user-specific costs. We are interested in the uniqueness property defined by Milchtaich (2005) as the uniqueness of equilibrium flows for all assignments of strictly increasing cost functions. He settled the case with two-terminal networks. As a corollary of his result, it is possible to prove that some other networks have the uniqueness property as well by adding common fictitious origin and destination. In the present work, we find a necessary condition for networks with several origin–destination pairs to have the uniqueness property in terms of excluded minors or subgraphs. As a key result, we characterize completely bidirectional rings for which the uniqueness property holds: it holds precisely for nine networks and those obtained from them by elementary operations. For other bidirectional rings, we exhibit affine cost functions yielding to two distinct equilibrium flows. Related results are also proven. For instance, we characterize networks having the uniqueness property for any choice of origin–destination pairs.