Narrow Results

For continuous functions $A$ and $B$, which are allowed to change sign, we consider the nonlocal differential equation

Granular Co₃₀Pt₇₀/C and Co₄₅Pt₅₅/C films, consisting of nanoparticle CoPt phases embedded in a carbon matrix, have been made by co-sputtering from CoPt and C targets using a tandem deposition mode. X-ray diffraction shows the existence of hard CoPt phase embedded in an amorphous C matrix after annealing. The coercivities for CoPt/C are strongly dependent on C and Pt composition. Films with coercivity of up to 5.4 kOe and grain size of 7 nm can be obtained. The development of shoulder in hysteresis loop may be contributed to the co-existence of magnetic soft CoPt₃ and hard CoPt phases and the magnetic interactions between them. High remanence M_r/M_s (>0.8) found in our samples indicates the presence of the intergranular interactions in the samples. Observed positive contributions of δm also give evidence of the existence of exchange interaction. High coercivity and large M_r/M_s make granular CoPt/C film with magnetic nanoparticles very attractive for next-generation high-density recording.

Anatase Ti_1-xCo_xO₂ nanoparticles were prepared by a hydrothermal process at 180 °C. Ferromagnetic hysteresis loops of the as-prepared samples were measured at room temperature. The Ti_1-xCo_xO₂ (x = 0.0376) powder shows coercivity up to 700 Oe, which is the highest value reported for anatase Ti_1-xCo_xO₂. X-ray diffraction (XRD), transmission electron microscopy (TEM) and magnetic measurement results provided evidence that Co was incorporated into TiO₂ lattice. Combined with the fact that the preparation was carried out in an oxidized environment starting from cobalt (II) in the oxidized state, it is suggested that the homogeneous doping of Co into the lattice of anatase should be responsible for the improvement of coercivity in anatase Ti_1-xCo_xO₂ nanoparticles.

Nd₅₅Al₂₀Fe₂₅ bulk sample was prepared in the shape of rods 3 mm in diameter by suction casting. The sample exhibits typical amorphous characters in XRD pattern, distinct glass transition in DSC traces and hard magnetic properties. The distinct glass transition, which is invisible in DSC traces of previously reported Nd—Al—Fe ternary BMGs, allows us to investigate the glass forming ability (GFA) of Nd₅₅Al₂₀Fe₂₅ alloy using the reduced glass transition temperature T_rg and the recently defined parameter γ. However, it is found that the obtained diameter of the Nd₅₅Al₂₀Fe₂₅ glassy rod is much larger than the critical section thickness of the BMG predicted by either T_rg or γ. The microstructure of Nd₅₅Al₂₀ Fe₂₅ as-cast rod was studied and the apparent GFA of the alloy was supposed to be enhanced by the metastable nano-precipitates dispersed within the glassy matrix.

In this research, the effect of milling time and annealing temperature on a heat activated synthesis of barium hexaferrite from mechanically activated mixture of hematite and barium acetate was investigated. The properties of samples were examined by XRD, DTA/TGA, SEM and VSM methods. The initial mixture with Fe/Ba molar ratio of 11 was milled for 24–48 h and then annealed at 800–1100°C. The XRD results showed the reflections of hematite as a major phase and barium hydroxide as a minor phase during mechanical milling in all three samples. It was observed that the specimen milled for 48 h exhibits the smallest mean crystallite size of 25 nm for hematite, as major phase. XRD results showed that annealing at 1100°C produces more pure barium hexaferrite phase comparing to lower temperatures. So, it was concluded that single phase BaFe₁₂O₁₉ can be achieved by milling for 48 h and then annealing at 1100°C. The sample processed at these conditions exhibited nano-crystalline barium hexaferrite powders with a mean crystallite size of 46 nm, mean particle size of about 200 nm and high coercivity of 334.2 kA/m.

In this research, SrFe${}_{12-x}$Ni${}_x$O${}_{19}$ (x$=$ 0 − 1) hexagonal ferrites were prepared by sol-gel auto-combustion method. Effect of Ni substitution on structural, morphological and magnetic properties of nanoparticles was investigated by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), Transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM), respectively. The XRD results confirmed that all samples with x$\le$ 0.5 have single phase M-type strontium ferrite structure, whereas for the SrFe${}_{12-x}$Ni${}_x$O${}_{19}$ samples with x$>$ 0.5, the spinel NiFe₂O₄ phase has also appeared. The lattice parameters and crystallite sizes of the powders were concluded from the XRD data and Williamson–Hall method. Magnetic analyses showed that the coercivity of powders decreased from 5672 Oe to 639 Oe while the saturation magnetization increased from 74 emu/g to 81 emu/g with nickel substitution. The results of this study suggest that the strontium hexaferrites doped with Ni are suitable for applications in high density magnetic recording media as well as microwave devices because of their promising magnetic properties.

Co(5 nm)/Dy(t${}_{\mathrm{\text{Dy}}}$)/Co(20 nm)/S and Ni(5 nm)/Dy(t${}_{\mathrm{\text{Dy}}}$)/Ni(20 nm)/S trilayer films are prepared by electron-beam sputtering to investigate the influence of dysprosium layer thickness (t${}_{\mathrm{\text{Dy}}}$) and thermal annealing on the crystal structure, magnetoresistance (MR) and magnetic properties of thin films. The thickness of Dy layer changed in the range from 1 nm to 20 nm. The samples annealed for 20 min at 700 K. Electron diffraction patterns reveal that the as-deposited and annealed systems Co/Dy/Co and Ni/Dy/Ni had fcc-Co + hcp-Dy and fcc-Ni + hcp-Dy phase state, respectively. It is also shown that at the t${}_{\mathrm{\text{Dy}}}$ = 15 nm the transition from amorphous to crystalline structures of Dy layer is observed. An increase in the Dy layer thickness results in changes in the MR and magnetic properties of the trilayer systems. It is shown that MR is most thermally stable against annealing to 700 K at t${}_{\mathrm{\text{Dy}}}$ = 15 nm for Co/Dy/Co as well as for Ni/Dy/Ni. For t${}_{\mathrm{\text{Dy}}}$ = 15 nm the, value of MR for both system increases by two times compared to those of pure ferromagnetic (FM) samples. The coercivity (B${}_c$), remanent (M${}_r$) and saturation (M${}_s$) magnetization of the in-plain magnetization hysteresis loops are related to the Dy layer thickness too. The coercivity depends on the FM materials type and diffusion processes at the layer boundary. Accordingly, M${}_r$ and M${}_s$ are reduced with t${}_{\mathrm{\text{Dy}}}$ increasing before and after annealing for both trilayer systems.

Well-defined ZnO and Sm-doped nanorods have been successfully fabricated by a low temperature hyderthermal process. The XRD patterns of both compositions with single diffraction peak (002) show the same wurtzite hexagonal structure. The radius of Sm-ZnO nanorods observed by FE-SEM is smaller than that of pure ZnO indicating the reduction of growth rate by the doping of Sm. Ferromagnetism is observed from the results of magnetization measurement. The increase of the saturation magnetization and decrease of coercivity reveal an association with the increase of oxygen vacancies induced by the doping of the Sm in the nanorods.

Nd₂Fe${}_{13.6}$Zr${}_{0.4}$B hard magnetic material were prepared using arc-melting technique on a water-cooled copper hearth kept under argon gas atmosphere. The prepared samples, Nd₂Fe${}_{13.6}$Zr${}_{0.4}$B ingot and ribbon are characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) for crystal structure determination and morphological studies, respectively. The magnetic properties of the samples have been explored using vibrating sample magnetometer (VSM).

The lattice constants slightly increased due to the difference in the ionic radii of Fe and that of Zr. The bulk density decreased due to smaller molar weight and low density of Zr as compared to that of Fe. Ingot sample shows almost single crystalline phase with larger crystallite sizes whereas ribbon sample shows a mixture of amorphous and crystalline phases with smaller crystallite sizes. The crystallinity of the material was highly affected with high thermal treatments. Magnetic measurements show noticeable variation in magnetic behavior with the change in crystallite size. The sample prepared in ingot type shows soft while ribbon shows hard magnetic behavior.

This paper deals with the problem of the Calculus of Variations for a functional which is the composition of a certain scalar function H with the integral of a vector valued field f, i.e. of the form

We present Finite Volume methods for diffusion equations on generic meshes, that received important coverage in the last decade or so. After introducing the main ideas and construction principles of the methods, we review some literature results, focusing on two important properties of schemes (discrete versions of well-known properties of the continuous equation): coercivity and minimum–maximum principles. Coercivity ensures the stability of the method as well as its convergence under assumptions compatible with real-world applications, whereas minimum–maximum principles are crucial in case of strong anisotropy to obtain physically meaningful approximate solutions.

CoAg films were prepared by ionic liquid electrodeposition to replace conventional aqueous electrodeposition on a copper substrate. Effects of different EMIC concentrations on composition, microstructure and magnetic properties of CoAg films were investigated. With the increase of 1-Ethyl-3-methylimidazolium Chloride (EMIC) concentration, atomic percentage of cobalt in CoAg films increased dramatically but amounts of silver reduced gradually. Almost all of the deposited films were crystalline and contained fcc-Ag(200) and hcp-Co(002) diffraction peaks. With the increase in EMIC concentration, the intensity of Co(002) peaks increased, which led to better crystalline and larger grain size. Dissimilar surface morphology of CoAg films from different EMIC concentration systems could be detected. CoAg films with bigger grain size and rougher surface could be obtained when higher concentration of EMIC was introduced during the electrodeposition. Especially, CoAg films with rice-like structures could be observed when EMIC was 0.45 mol/L. With the increase of EMIC concentration, coercivity of CoAg increased gradually from 140 Oe to 260 Oe.

The magnetic properties and microstructures of electrodeposited Co–Ni–Fe–Gd thin films have been investigated for various concentrations of Mn. Nanocrystalline Co–Ni–Fe–Gd alloy thin films were deposited by electrodeposition method on copper substrate with different concentrations of Mn. Microstructures of the deposited Co–Ni–Fe–Gd films were investigated with scanning electron microscope (SEM) and different morphologies such as one-dimensional flower-like crystallites and two-dimensional plate-like crystallites were observed. The Co–Ni–Fe–Gd alloy films exhibit both soft and hard magnetic properties as the Mn ion concentration is varied and the best soft magnetic character is obtained when Mn concentration is 0.1 M, which may be a good candidate for MEMS and recording media.

We prove the monotonicity inequality for differential operators A and L that occur as coefficients in linear stochastic partial differential equations associated with finite-dimensional Itô processes. We characterize the solutions of such equations. A probabilistic representation is obtained for solutions to a class of evolution equations associated with time dependent, possibly degenerate, second-order elliptic differential operators.

Thin films and powders of Co-ferrite and SiO₂-doped Co-ferrite were fabricated via the sol–gel method. The structural and magnetic properties of the films and powders were investigated with X-Ray Diffractometer (XRD), Vibrating Sample Magnetometer (VSM), Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). VSM measurements showed an enhancement of coercivity by SiO₂-doping for Co-ferrite powders and thin films (coercivity of 3.5 kOe in SiO₂-doped thin films). XRD and SEM investigations revealed a nanostructure of the thin films. Low surface roughness was observed in our AFM study.

Ordered Au/Ni multilayer nanowire arrays have been fabricated into the pores of anodic aluminum oxide (AAO) templates by pulse DC electrodeposition. Structural parameters, such as nanowire length and layer thickness can be controlled by changing the deposition time and the pulse width. The magnetic measurement results indicate that these Au/Ni multilayer nanowire arrays have small coercivity and perpendicular magnetic anisotropy, which the easy axis is parallel to the nanowires. These Au/Ni multilayer structures are expected to be found potential applications in AC inductors or magnetic shielding.

We report the evolution of microstructure, domain structure, and soft magnetic properties of amorphous and nanocrystalline Fe_89-xZr₁₁B_x(x = 0 - 10) alloys. High-resolution electron microscopy observations reveal that the annealed alloys exhibit a two-phase microstructure. Addition of B enhances the ferromagnetic properties of Fe–Zr amorphous phase in the two-phase structured microstructure, resulting in good soft magnetic properties. Large-sized domains with smooth domain walls are observed in the alloys annealed below 873 K, which exhibit excellent magnetic softness. On the other hand, in the alloys annealed above 873 K, small-sized domains with irregular domain walls and domain wall pinning by Fe–Zr compound are seen. The soft magnetic properties in Fe–Zr–B alloys not only depend on mean grain size, but also on the strength of the intergranular magnetic coupling and structural inhomogeneities.

We report the temperature dependent coercivity behavior and the improvement of room temperature soft magnetic properties in interacting amorphous Fe–Mn–Zr–B powder nanoparticles prepared by mechanical alloying of elemental powders. With increasing Mn concentration from 5 to 20 at.% in Fe–Zr–B alloys, the coercivity (H_C) reduces drastically from 170 to 25 Oe. Temperature dependent H_C behavior shows exponential variation of H_C with temperature. Interparticle interactions dominate the low temperature behavior and surface effect on the nanoparticles is more pronounced at low temperatures. Magnetic relaxation behavior shows slow relaxation in the interacting amorphous nanoparticles and the relaxation process is observed to be thermally activated in the present studies.

In this paper, the mechanical behavior of incompressible transversely isotropic materials is modeled using a strain energy density in the framework of Ball’s theory. Based on this profound theory and with respect to physical and mathematical aspects of deformation invariants, a new polyconvex constitutive model is proposed for the mechanical behavior of these materials. From the physical viewpoint, it is assumed that the proposed model is additively decomposed into three parts nominally representing the energy contributions from the matrix, fiber and fiber–matrix interaction where each of the parts should be presented in terms of the invariants consistent with the physics of the deformation. From the mathematical viewpoint, the proposed model satisfies the fundamental postulates on the form of strain energy density, specially polyconvexity and coercivity constraints. Indeed, polyconvexity ensures ellipticity condition, which in turn provides material stability and in combination with coercivity condition, guarantees the existence of the global minimizer of the total energy. In order to evaluate the performance of the proposed strain energy density function, some test data of incompressible transverse materials with pure homogeneous deformations are used. It is shown that there is a good agreement between the test data and the obtained results from the proposed model. At the end, the performance of the proposed model in the prediction of the material behavior is evaluated rather than other models for two representative problems.

Pure nano-Fe₃O₄ and cobalt-doped nano-Fe₃O₄ particles are successfully synthesized by co-precipitation method using tetramethylammonium hydroxide (TMAOH) as alkali. Several key factors that may affect preparation are carefully discussed such as alkali concentration, alkali dosage, reaction temperature, iron salt solution concentration and dispersant agents. Such nano-Fe₃O₄ particles prepared have good dispersibility and a very narrow size distribution with the average diameter about 38 nm, which are proved to be cubic spinel Fe₃O₄ crystal by XRD pattern. It is also found that the addition of PEG-4000 surfactant can improve the dispersibility of nanoparticles. In our work, effects of cobalt dopant concentration on magnetic properties of cobalt-doped nano-Fe₃O₄ are also discussed. The result shows that the coercivity of cobalt-doped nanoparticles changes greatly with the variation of cobalt dopant concentration. The maximum coercivity reaches as high as 1628 Oe, which is very meaningful for preparation of materials with high coercivity.