M?ssbauer and Magnetic Studies of Co0.5Mn0.5Fe2O4 and Mn0.1Mg0.2Co0.7 Fe2O4 Nanoferrites

We report the magnetic properties of Mn_0.5Co_0.5Fe₂O₄ and Mn_0.1Mg_0.2Co_0.7Fe₂O₄ nanoferrites. The compounds were synthesized by a glycol-thermal method with average particle sizes of about 13?nm and 8?nm, respectively. The M?ssbauer measurements were done at 300?K. The distribution of cations between tetrahedral?(A) and octahedral?(B) sites is investigated. The M?ssbauer spectra indicate ferrimagnetic behavior of the compound. Field cooled (FC) and zero field cooled (ZFC) magnetizations were performed by a Superconducting Quantum Interference Device (SQUID) magnetometer from 4?C380?K. Variation of the magnetizations with the applied fields (up to 50 kOe) were recorded at isothermal temperatures 4, 50, 100, 200 and 300?K. An increase in FC magnetization is observed with increasing applied field. This is explained based on superparamagnetic behavior of the particles.     

Superparamagnetic properties of carbon-encapsulated Ni nanoparticle assemblies

Carbon-encapsulated Ni nanoparticles [Ni(C)] were synthesized using a modified arc-discharge reactor under methane atmosphere. The average particle size was revealed to be typically 10.5 nm with a spherical shape. The intimate and contiguous carbon fringe around these Ni nanoparticles is good evidence for complete encapsulation by carbon shell layers. Superparamagnetic property studies indicate that the blocking temperature (TB) is around 115 K at 1000 Oe applied field. Below TB, the temperature dependence of the coercivity is given by Hc = Hci[1 -(T/TB)1/2], with Hci approximately 500 Oe. Above TB, the magnetization M(H, T) can be described by the classical Langevin function L using the relationship M/Ms(T = 0) = coth(microH/kT)-kT/microH. The particle size can be inferred from the Langevin fit (particle moment mu) and the blocking temperature theory (TB), with values slightly larger than the high-resolution transmission electron microscopy observations. It is suggested that these assemblies of carbon-encapsulated Ni nanoparticles have typical single-domain, field-dependent superparamagnetic relaxation properties.     

Size Effects on the Magnetic Properties of ZnFe2O4 Nanoparticles

We report structural and magnetic measurements on ZnFe₂O₄ nanoparticles obtained through coprecipitation chemical method. The Rietveld analysis of X-ray patterns reveals that (i) our samples are single phase, (ii) the average particle size increases with synthesis temperature, and (iii) the cationic disorder increases with decreasing of the mean particle size. The Zero-Field-Cooled (ZFC) and Field-Cooled (FC) magnetization measurements show that the blocking temperature increases with increasing of the particle size and, to the sample grown at T=850?°C, it is possible to observe both Néel temperature to larger particles and blocking effects to smaller particles. Finally, we have observed that the coercive field does not decay with the square root of temperature following the Néel relaxation and Bean–Livingston approaches.     

Effect of Nickel Doping on the Magnetotransport Properties of?Sm0.55Sr0.45MnO3 Manganites

We studied the effects of nickel (Ni) doping on the magneto-transport properties of Sm_0.55Sr_0.45MnO₃ manganites near the metal-insulator transition. Various concentrations of Ni-doped Sm_0.55Sr_0.45MnO₃ samples up to 10% were prepared (Ni was partially substituted at the Mn-site). The temperature dependence of resistivity and magnetoresistance were measured as a function of Ni concentrations at various applied magnetic fields. We observed a nonlinear reduction of the metal-insulator transition temperature (MIT) with increasing concentration of Ni, 5% of Ni was sufficient to completely suppress the insulator-metal transition. Moreover, we observed dramatic increases of the resistance of the doped material with an increasing Ni-doping (5% of Ni increases R by more than 1000 times). The resistivity peaks at various magnetic fields collapses on themselves at the high temperature ends above the MIT. We also performed magnetization versus temperature measurements on both Ni-free the Ni-doped samples for FC and ZFC states. The FC and ZFC curves rapidly decrease to paramagnetic state at 175 K and 130 K for ZFC and FC states, respectively. For other Ni-doped samples, we observed a reduction in the paramagnetic transition temperature with increasing Ni concentration.     

Magnetic Properties of FeAs Single Crystal

Magnetic measurements have been performed on FeAs single crystals and polycrystalline samples. Magnetic measurements revealed a very sharp spin-glass magnetic transition in self flux grown single crystal. The bifurcation point between zero field cooled (ZFC) and field cooled (FC) magnetization occurs at 130 K. Susceptibility of flux grown single crystals revealed ferromagnetic-type behavior in the temperature range 4–300 K, while single crystals grown by iodine transport vapor were non-magnetic revealed features associated with antiferromagnetic and spin wave characteristics. The shift in Raman lines reveled sharper peaks for the crystals grown by vapor transport method than that obtained for the flux grown crystals. The difference in the spectrum may indicate sharper distribution in the bonding strength between Fe and As ions in crystals grown by the iodine vapor transport method.     

Coaxial metal-silicide Ni2Si/C54-TiSi2 nanowires

One-dimensional metal silicide nanowires are excellent candidates for interconnect and contact materials in future integrated circuits devices. Novel core-shell Ni(2)Si/C54-TiSi(2) nanowires, 2 μm in length, were grown controllably via a solid-liquid-solid growth mechanism. Their interesting ferromagnetic behaviors and excellent electrical properties have been studied in detail. The coercivities (Hcs) of the core-shell Ni(2)Si/C54-TiSi(2) nanowires was determined to be 200 and 50 Oe at 4 and 300 K, respectively, and the resistivity was measured to be as low as 31 μΩ-cm. The shift of the hysteresis loop with the temperature in zero field cooled (ZFC) and field cooled (FC) studies was found. ZFC and FC curves converge near room temperature at 314 K. The favorable ferromagnetic and electrical properties indicate that the unique core-shell nanowires can be used in penetrative ferromagnetic devices at room temperature simultaneously as a future interconnection in integrated circuits.     

The hydrothermal synthesis of super-paramagnetic barium hexaferrite particles

Superparamagnetic Ba-hexaferrite nanoparticles were prepared using modified hydrothermal synthesis. The precursor and hydroxide [OH⁻] concentrations were optimized and the synthesis temperature and time were drastically reduced. The size and the morphology of synthesized nanoparticles was confirmed by transmission electron microscopy (TEM) images. The crystal structure of the nanoparticles was characterized by X-ray diffraction data. Powders synthesized at 160 °C exhibit a bimodal particle size distribution while those synthesized at T_S = 150 °C show a monomodal particle size distribution. Zero-field-cooling (ZFC) and field-cooling (FC) magnetization measurements were performed using a superconducting quantum interference device magnetometer from 2 to 300 K to investigate the magnetic properties of nanoparticles. The FC/ZFC magnetization measurements showed a typical superparamagnetic behavior. The synthesized superparamagnetic particles exhibit a disc-like shape, in average 11 nm wide and 3 nm thick with a room temperature magnetization of approximately 10 Am²/kg at 5 T.     

Preparation of monodisperse Ni nanoparticles and their assembly into 3D nanoparticle superlattices

Monodisperse Ni nanoparticles with sizes varying from 4.8 to 11.3 nm are prepared via a one-pot reaction that involves the reduction of nickel(II) acetylacetonate in oleylamine in the presence of trioctylphosphine and 1,2-hexadecanediol. Reaction parameters such as temperature and the concentration of capping agent and metal precursor are critical for the adjustment of particle size. The decrease of crystallinity is observed for the samples with smaller particle sizes, which significantly affects the magnetic properties. Three-dimensional (3D) superlattices that are composed of Ni nanoparticles with different sizes are obtained on different substrates by a facile self-assembly process, and are characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and small-angle X-ray diffraction (SAXRD). The Ni nanoparticle superlattices formed on carbon-coated TEM copper grids exhibit a dominant hexagonal close-packed (hcp) symmetry, although local fcc packing is also occasionally observed. The formation of 3D nanoparticle superlattice structures on Si substrates is confirmed from the SAXRD measurements. The method revealed in this study for the preparation of 3D superlattices composed of Ni nanoparticles with tunable sizes offers the potential to explore their interesting collective properties for multiple applications.     

XRD, Magnetic and M?ssbauer Spectral Studies of?Ag x Ni1?x Fe2O4 Ferrite Nanoparticles

Nanophase Ag _x Ni_1?x Fe₂O₄ (x=0,0.2) ferrites were prepared by glycothermal method. The NiFe₂O₄ (x=0) nanosized sample was also produced by high-energy ball milling for comparison of properties. Structural investigations of the samples were carried out by X-ray diffraction. The experiment reveals that pure Ag-Ni ferrite materials with grain sizes of about 8 nm can be obtained after annealing at relatively low temperature of about 500?°C. The nanosized compounds produced by glycothermal reaction indicate superparamagnetic behavior. A higher value of coercive field (910 Oe) is observed in x=0 milled sample with similar particle size. The zero field cooled (ZFC) and field cooled (FC) magnetization measurements reveals spin glass like behavior of the nanosized compounds.     

Single crystalline nickel nanorods inside carbon nanotubes: growth behavior, structure, and magnetic properties

Nickel nanorods with diameters ranging from 5 to 10 nm, encapsulated inside the carbon nanotubes, are prepared using microwave plasma chemical vapor deposition. High-resolution transmission electron microscopy (HRTEM) studies reveal the perfect crystalline nature of the rods with d-spacing closely matching the (111) interplanar spacing of Ni. The (111) planes of the Ni nanorods are always aligned at 39.6 degrees with respect to the graphite planes of the nanotubes. The cosine component of the d-spacing along the direction of the graphite planes is found to be 1.6 A; exactly half the d-spacing between the graphite planes. The electron diffraction pattern shows clear spots corresponding to Ni structure. The field cooled and zero field cooled magnetization data reveal the reversibility of the magnetization of the Ni nanorods and show a blocking temperature of 195 K, which correspond to energy barrier of 0.4 eV/(V).     

Structure and Magnetic Properties of La0.66Sr0.33MnO3 Thin Films Derived Using Sol-Gel Technique

In this paper the results of a study of the structural and magnetic properties of La_0.66Sr_0.33MnO₃ (LSMO) polycrystalline films grown on glass substrate using Sol-Gel technique are presented. The samples were structurally characterized using X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques. The average grain size range of 30 nm has been obtained from XRD investigations of granular LSMO samples. Zero Field Cooling (ZFC) and Field Cooling (FC) magnetization measurements have been performed and magnetic hysteresis loops of LSMO were recorded at various temperatures. The temperature dependences of the magnetization of LSMO films recorded in ZFC and FC regimes exhibited considerable difference between the curves. The blocking and the ferromagnetic phase transition temperatures of about 262?K and 300?K, respectively, were observed from magnetization measurements.     

Effect of grafted polymer species on particle monolayer structure at the air-water interface

We have studied poly(methyl methacrylate)-grafted(PMMA) particle monolayer systems at the air-water interface. In previous papers, we reported that PMMA chains grafted from particles (silica particle and polystyrene latex) were extended on water surfaces. Through observing deposited particle monolayers on substrates using SEM, we have confirmed that PMMA of large molecular weights were either dispersed or arrayed in structure with long inter-particle distances approximately 500 nm. In contrast, low molecular weight PMMA were observed to aggregate upon deposition. We speculated that the difference in morphology in deposited particle monolayers would be attributed to the affinity between the grafted polymer and the substrate. To examine the effect of this affinity three new polymer-grafted silica particles were synthesized with a fairly high graft density of about 0.14 approximately 0.43 nm(-2). As well as PMMA-grafted silica particles (SiO2-PMMA), poly(2-hydroxyethyl methacrylate) and poly(t-butyl methacrylate)--grafted silica particles (SiO2-PHEMA and SiO2-PtBuMA) were also prepared and subjected to pi-A isotherm measurements and SEM observations. These pi-A isotherms indicated that polymer-grafted silica formed monolayer at the air-water interface, and the onset area of increasing surface pressure suggests that the polymer chains are extended on a water surface. However, the morphology of the deposited monolayer is highly dependent on polymer species: SiO2-PHEMA showed that the dispersed particle monolayer structure was independent of grafted molecular weight while SiO2-tBuMA showed an aggregated structure that was also independent of grafted moleculer weight. SiO2-PMMA showed intermediate tendencies: dispersed structure was observed with high grafted molecular weight and aggregated structure was observed with low grafted molecule weight. The morphology on glass substrate would be explaiened by hydrophilic interaction between grafted polymer and hydrophilic glass substrate.     

Polymer nanocomposites exhibiting magnetically tunable microwave properties

Polymer nanocomposites (PNCs) have been synthesized using Rogers polymer and CoFe?O? nanoparticles (CFO NPs). X-ray diffraction (XRD) confirms the inverse spinel crystal structure of CFO NPs and transmission electron microscopy (TEM) images show the uniform dispersion of nanoparticles (10 nm ± 1) into the polymer matrix. Magnetic measurements indicate superparamagnetic response near room temperature for all PNCs. A blocking temperature T(B)~298 K was observed and does not vary for different loading fractions of CFO NPs for the PNCs. The saturation magnetization (M(s)) was found to be 11 emu g?1 for 30 wt% CFO, increasing to 32 emu g?1 for the 80 wt% CFO loaded PNC. A large value of coercivity (H(c) = 19 kOe) is also observed at 10 K and is not affected by varying CFO loading. Microwave measurements show significant absorption in the 80 wt% CFO loading PNC and the quality factor shows a strong enhancement with applied magnetic field.     

胶乳共混法天然橡胶/二氧化硅纳米复合材料的制备及性能

采用胶乳共混法制备天然橡胶/二氧化硅(NR/SiO2)纳米复合材料。先用硅烷偶联剂KH-570对纳米二氧化硅进行改性,再经乳液聚合接枝上聚甲基丙烯酸甲酯(PMMA)得到PMMA-SiO2粒子,最后将其与用MMA改性的天然胶乳(NR-PMMA)共混制得NR/SiO2纳米复合材料。采用红外光谱仪、透射电镜、扫描电镜、热重分析仪、橡胶拉伸测试机对样品进行了表征。实验结果表明,PMMA成功地接枝于SiO2表面,PMMA-SiO2在橡胶基体中分散均匀,平均粒径在60nm～80nm之间,复合材料的拉伸强度比纯的NR提高了35%,定伸应力也有显著提高。     

Synthesis of silica nanowires by active oxidation of silicon substrates

Amorphous silica nanowires have been produced by thermal annealing of Si/SiO2/Ni substrate structures at 900 degrees C under an atmosphere of hexamethyldisilazane (HMDS) and hydrogen (H2). The wires have diameter ranging from 35 to 55 nm, which are controlled by the Ni particle size. It is demonstrated that the growth occurs through vapor-liquid-solid mechanisms, and it is proposed that the vapor source is volatile SiO generated from the etching of the Si substrate through active oxidation reactions. The role of the HMDS-H2 atmosphere in promoting such reactions is discussed.     

Surface spin-glass freezing in interacting core-shell NiO nanoparticles

Magnetization and AC susceptibility measurements have been performed on ～3?nm NiO nanoparticles in powder form. The results indicate that the structure of the particles can be considered as consisting of an antiferromagnetically ordered core, with an uncompensated magnetic moment, and a magnetically disordered surface shell. The core magnetic moments block progressively with decreasing temperature, according to the distribution of their anisotropy energy barriers, as shown by a broad maximum of the low field zero-field-cooled magnetization (M(ZFC)) and in the in-phase component χ' of the AC susceptibility, centred at ～70?K. On the other hand, surface spins thermally fluctuate and freeze in a disordered spin-glass-like state at much lower temperature, as shown by a peak in M(ZFC) (at 17?K, for H = 50?Oe) and in χ'. The temperature of the high temperature χ' peak changes with frequency according to the Arrhenius law; instead, for the low temperature maximum a power law dependence of the relaxation time was found, τ = τ(0)(T(g)/(T(ν)-T(g)))(α), where α = 8, like in spin glasses, τ(0) = 10(-12)?s and T(g) = 15.9?K. The low temperature surface spin freezing is accompanied by a strong enhancement of magnetic anisotropy, as shown by the rapid increase of coercivity and high field susceptibility. Monte Carlo simulations for core/shell antiferromagnetic particles, with an antiferromagnetic core and a disordered shell, reproduce the qualitative behaviour of the temperature dependence of the coercivity. Interparticle interactions lead to a shift to a high temperature of the distribution of the core moment blocking temperature and to a reduction of magnetization dynamics.     

Photolithographic fabrication of gated self-aligned parallel electron beam emitters with a single-stranded carbon nanotube

In this paper we report on the development of a photolithographic process to fabricate a gated-emitter array with single-stranded carbon nanotubes (CNTs) self-aligned to the center of the emitter gate using plasma-enhanced chemical vapor deposition (PECVD). Si tips are formed on a silicon wafer by anisotropic etching of Si using SiO(2) as a mask. Deposition of a SiO(2) insulating layer and Cr-W electrode layers creates protrusions above the Si tips. This wafer is polished, and the Cr-W on the tips is removed. Etching of the SiO(2) using hydrofluoric acid is performed to expose the gated Si tip. Incorporation of a novel diffusion process produces single-stranded CNTs by depositing a thin Ni layer on the Si tips and thermally diffusing the Ni layer to yield a catalyst particle for single-stranded CNT growth. The large surface to volume ratio at the apex of the Si tip allows a Ni particle to remain to act as a catalyst to grow a single-stranded CNT for fabricating the CNT based emitter structure. Diffusion of the Ni is carried out in situ during the heating phase of the PECVD CNT growth process at 600?°C. The diameters of the observed CNTs are on the order of 20?nm. The field emission characteristics of the gated field emitters are evaluated. The measured turn-on voltage of the gated emitter is 5?V.     

The Role of Carbon Content: A Comparison of the Nickel Particle Size and Magnetic Property of Nickel/Polysiloxane-Derived Silicon Oxycarbide

A facile and novel processable method to synthesize the Ni nanoparticles (Ni NPs) by tailoring their size in the matrix of the silicon oxycarbide (SiOC) ceramic system is reported. This method is based on polymer-derived ceramics (PDCs), instead of the conventional powder route. The specific structural characteristics and magnetic properties of the various Ni NPs/SiOC composites as a function of carbon content are systematically investigated. The magnetic properties are experimentally investigated as a function of NP size and measurement temperature. It is demonstrated that the change in the size of Ni NPs (average from ≈4 to ≈ 19 nm) determines the magnetic nature of superparamagnetism. Zero-field-cooled (ZFC) and field-cooled (FC) magnetization studies under magnetic fields of 100 Oe are performed. The saturated M versus H (M–H) loops (saturation magnetization) increase and the coercivity decreases with the size reduction of Ni NPs. It is an indicator of the presence of superparamagnetic behavior and single-domain NP for ceramic materials.     

Microstructure of nanostructured Fe40Ni38Mo4B18 alloy

Nanostructured Fe40Ni38Mo4B18 specimens with a wide range of different nanostructures can be produced by annealing the amorphous alloy in the temperature range from 700 K to 1060 K. The nanostructured Fe40Ni38Mo4B18 specimens prepared between 700 K and 760 K have a microstructure of the 4–9 nm fcc (Fe, Ni) solid solution crystallites embedded in the amorphous matrix. The nanostructured Fe40Ni38Mo4B18 specimens obtained between 780 K and 910 K show a polycrystalline microstructure of the major cubic (Fe, Ni, Mo)23B6 phase with grain sizes from 20 nm to 40 nm and the minor fcc (Fe, Ni) solid solution crystallites of about 10 nm. The nanostructured Fe40Ni38Mo4B18 specimens produced between 930 K and 1060 K represent a polycrystalline microstructure of the 12–25 nm fcc (Fe, Ni) solid solution crystallites and the 45–240 nm cubic (Fe, Ni, Mo)23B6 crystallites.