Numerical study of exchange bias in antiferromagnetic/ferromagnetic core/shell nanoparticles with non-magnetic defects

We perform Monte Carlo simulations for an antiferromagnetic/ferromagnetic core/shell nanoparticle with a doubly inverted structure. We investigate the dependence of the exchange bias field and coercivity on the magnetic dilution of the shell-interface and shell part. It is demonstrated that exchange bias and coercivity can exhibit monotonic or non-monotonic behavior depending on the location of the non-magnetic components. Also, temperature dependence of the exchange bias and coercivity of the system are studied for a particular defect concentration value. Our results provide an alternative way for tunning the magnetic properties of doubly inverted nanoparticles.     

Tunning exchange bias in inverted antiferromagnetic/ferromagnetic core/shell nanoparticles by binary alloy shells

A detailed investigation of exchange bias properties of an inverted nanoparticle with an antiferromagnetic core and a ferromagnetic binary alloy shell of the type $B_x{C}_{1?x}$ is presented by benefiting from Monte Carlo simulations. Exchange bias displays a non-monotonic behavior with the varying value of the concentration the type-B magnetic components, x. Coercivity exhibits a monotonic or a non-monotonic variation with x depending on the relative strength between unlike magnetic components in the shell. Particular attention has also been given to determine the effects of the cooling field process on the magnetic properties of the nanoparticle. Numerical results obtained in this work present a different physical mechanism and an alternative way for tuning the exchange bias and coercivity of bimagnetic core/shell nanoparticles.     

Sonochemical rate enhanced by a new nanomagnetic embedded core/shell nanoparticles and catalytic performance in the multicomponent synthesis of pyridoimidazoisoquinolines

A sonochemical approach for the one-pot three-component synthesis of pyridoimidazoisoquinolines via by using phthalaldehyde, trimethylsilylcyanide and aminopyridines the presence of a catalytic amount of a new nanomagnetic catalyst Fe₃O₄@SiO₂-CO-C₆H₄-NH₂ is described. The characterization of the nanocatalyst and the product was done by various methods, such as FT-IR, SEM, EDX, TGA/DTA, NMR, MS and CHN analyses. This is the first design, preparation, characterization and application of the present core/shell nanomaterial and also the first ultrasound irradiated synthesis of the biologically and pharmaceutically important fused polycyclic compounds in ethanol as a green solvent. This novel protocol offers several advantages such as high yields, short reaction times, environmentally-friendly reaction media, easily isolation of the products, simple preparation and recoverability of the nanocatalyst by an external magnet and reusing several times without significant decrease in catalytic activity.     

A facile one-pot ultrasound assisted for an efficient synthesis of benzo[g]chromenes using Fe3O4/polyethylene glycol (PEG) core/shell nanoparticles

In this research, a general synthetic method for the synthesis of benzo[g]chromenes has been developed using Fe₃O₄/polyethylene glycol (PEG) core/shell nanoparticle under ultrasonic irradiations. Compared to the conventional methods, ultrasound procedure showed several advantages including mild reaction conditions, high yield products, short reaction times, reusability of the catalyst and little catalyst loading.     

Mechanisms of effective gold shell on Fe3O4 core nanoparticles formation using sonochemistry method

Sonochemical synthesis (sonochemistry) is one of the most effective techniques of breaking down large clusters of nanoparticles (NPs) into smaller clusters or even individual NPs, which ensures their dispersibility (stability) in a solution over a long duration. This paper demonstrates the potential of sonochemistry becoming a valuable tool for the deposition of gold (Au) shell on iron oxide nanoparticles (Fe₃O₄ NPs) by explaining the underlying complex processes that control the deposition mechanism. This review summarizes the principles of the sonochemistry method and highlights the resulting phenomenon of acoustic cavitation and its associated physical, chemical and thermal effects. The effect of sonochemistry on the deposition of Au NPs on the Fe₃O₄ surface of various sizes is presented and discussed. A Vibra-Cell ultrasonic solid horn with tip size, frequency, power output of ½ inch, 20 kHz and 750 W respectively was used in core@shell synthesis. The sonochemical process was shown to affect the surface and structure of Fe₃O₄ NPs via acoustic cavitation, which prevents the agglomeration of clusters in a solution, resulting in a more stable dispersion. Deciphering the mechanism that governs the formation of Au shell on Fe₃O₄ core NPs has emphasized the potential of sonication in enhancing the chemical activity in solutions.     

Effects of pressure on maghemite nanoparticles with a core/shell structure

The magnetic property and intraparticle structure of the γ phase of Fe₂O₃ (maghemite) nanoparticles with a diameter (D) of 5.1±0.5 nm were investigated through AC and DC magnetic measurements and powder X-ray diffraction (XRD) measurements at pressures (P) up to 27.7 kbar. Maghemite originally exhibits ferrimagnetic ordering below 918 K, and has an inverse-spinel structure with vacancies. Maghemite nanoparticles studied here consist of a core with structural periodicity and a disordered shell without the periodicity, and core shows superparamagnetism. The DC and AC susceptibilities reveal that the anisotropy energy barrier (ΔE/k_B) and the effective value of the core moment decrease against the initial pressure (P≤3.8 kbar), recovering at P≥3.8 kbar. The change of ΔE/k_B with P is qualitatively identical with that of the core moment, suggesting a down-and-up fluctuation of the number of Fe³⁺ ions constituting the core at the pressure threshold of about 4 kbar. This phenomenon was confirmed by the analysis of the XRD measurement using Scherrer’s formula. The core volume decreased for P≤2.5 kbar, whereas at higher pressure the core was restructured. For 2.5≤P≤10.7 kbar, the volume shrinkage of particle hardly occurs. There, ΔE/k_B is approximately proportional to the volume associated to the ordered fraction of the nanoparticles as seen from XRD, V_core. From this dependence it is possible to separate the core/shell contribution to ΔE/k_B and estimate core and surface anisotropy constants. As for the structural experiments, similar experimental data have been obtained for D=12.8±3.2 nm as well.     

Optical properties of hybrid plasmonic nanofluid based on core/shell nanoparticles

The plasmonic effect is used in nanofluid to help capture and absorb sunlight. The optical absorption is significantly enhanced as plasmonic effect excited. To obtain an enhanced absorption in a broad band, the hybrid plasmonic nanofluid is developed. It is composed of core/shell nanoparticles of different sizes. The overall absorption of hybrid nanofluid is examined. Compared to the nanofluid of single particle size, the hybrid nanofluid exhibits a broadband absorption. As particle size increases, the plasmon resonance peak is shifted to longer wavelength. The variation in the sizes of core/shell nanoparticles can broaden the absorption spectrum. In the near-infrared region, the proportion of different size particles has an obvious influence. With the increase of proportion of larger particles, the absorption band is broadened. Since the suspended nanoparticles have different sizes, the particle distribution in base fluid also has an effect on absorption of light. The large particle in upper has a broadband absorption, however, less energy can be transmitted to lower after the absorption of upper particles. The contribution from the particles in lower is relatively weak.     

Synthesis of robust water-soluble ZnS:Mn/SiO2 core/shell nanoparticles

Water-soluble Mn doped ZnS (ZnS:Mn) nanocrystals synthesized by using 3-mercaptopropionic acid (MPA) as stabilizer were homogeneously coated with a dense silica shell through a multi-step procedure. First, 3-mercaptopropyl triethoxy silane (MPS) was used to replace MPA on the particle surface to form a vitreophilic layer for further silica deposition under optimal experimental conditions. Then a two-step silica deposition was performed to form the final water-soluble ZnS:Mn/SiO₂ core/shell nanoparticles. The as-prepared core/shell nanoparticles show little change in fluorescence intensity in a wide range of pH value.     

Facile synthesis and characterization of glass/cobalt core/shell composite spheres with tunable shell morphologies

Polyhedral cobalt microcrystals assembled on hollow glass spheres are successfully synthesized by a facile and easy-control hydrothermal reduction process, and thus hierarchical glass/cobalt core/shell composite hollow spheres are fabricated with low-density (0.96 g cm⁻³). By properly tuning the process conditions and the component of the reaction solution, a series of composite spheres with gradient in morphologies of the shell layer can be prepared. Based on a series of contrast experiments, the probable formation mechanism of the core/shell hierarchical structures is proposed. The magnetic properties of the products are studied and the results demonstrate that the composite spheres present ferromagnetic properties related to the special shell morphologies. The composite hollow spheres thus obtained may have some promising applications in the fields of low-density magnetic materials, conduction, and catalysis, etc. This work provides an additional strategy to prepared core/shell composite spheres with tailored shell morphology and magnetic properties.     

Fluorescence study of the core/shell interface in polyelectrolyte micelles. Binding of fluorescent surfactants in the interfacial region

Properties of the interfacial region between the nonpolar core and the polar shell in polystyreneblock-poly (methacrylic acid) micelles were studied by fluorescence techniques using 5-(N-octadecanoyl) aminofluorescein (OAF) as a probe for microfluidity and local pH. The block copolymer used was tagged between blocks by one 9, 10-diphenylanthracene (DPA) group, which allowed us to study binding of OAF at the interface by means of nonradiative energy transfer between DPA and OAF. A shift in the pK _a of OAF and appreciable changes in anisotropy and quenching efficiency due to immobilization of the fluorophore head-group in hydrophobic poly(methacrylic acid) domains were observed after binding of the probe at the interface.     

Deposition and characterization of Cu(In,Ga)Se2 thin films from the ink of sonochemically prepared CIGSe nanoparticles

CuIn_1-xGa_xSe₂ (CIGSe) nanoparticles were synthesized via a sonochemical method at different Ga content (x?=?0.15, 0.30 and 0.45) and dispersed in a low toxic isobutanol to form the nanoparticle-ink. Thin films of CIGSe were grown by a facile, non-vacuum and inexpensive “nanoparticle-ink based air-spray coating” method. Effects of Ga-content and annealing time on the physical properties of CIGSe thin films were investigated. The elemental composition, crystalline structure, surface morphology and optical properties of the films were explored by various characterization methods. XRD studies of as-synthesized films showed a tetragonal structure with (112) orientation. After annealing, the CIGSe films showed an improvement in the intensity ratio of I(220)/I(112) for the annealing time of 60?min. The morphological studies of annealed CIGSe films showed plank-like larger grains of size ～1–2?µm. The films deposited at different gallium content, x?=?0.15, 0.30 and 0.45 showed PL peak maxima at 954, 1049 and 1168?nm, respectively. The present method is capable of producing CIGSe absorbers by a greener route in large scales at lower cost.     

Effects of crystalline grain size and packing ratio of self-forming core/shell nanoparticles on magnetic properties at up to GHz bands

Self-forming core/shell nanoparticles of magnetic metal/oxide with crystalline grain size of less than 40 nm were synthesized. The nanoparticles were highly concentrated in an insulating matrix to fabricate a nanocomposite, whose magnetic properties were investigated. The crystalline grain size of the nanoparticles strongly influenced the magnetic anisotropy field, magnetic coercivity, relative permeability, and loss factor (tan δ=μ″/μ′) at high frequency. The packing ratio of the magnetic metallic phase in the nanocomposite also influenced those properties. High permeability with low tan δ of less than 1.5% at up to 1 GHz was obtained in the case of the nanoparticles with crystalline grain size of around 15 nm with large packing ratio of the nanoparticles.     

Surface‐enhanced Raman scattering studies of carbon nanotubes using Ag‐core Au‐shell nanoparticles

Surface‐enhanced Raman scattering from carbon nanotube bundles adsorbed with plasmon‐tunable Ag‐core Au‐shell nanoparticles (Ag@Au nps) was carried out for the first time. By utilizing nanoparticles whose plasmon resonance peak (541, 642 nm) closely matches the commonly used Raman excitation sources (532, 632.81 nm), we can observe a large enhancement in the Raman signatures of carbon nanotubes. We obtain greater enhancement in the Raman signal for the above case when compared to nanotubes adsorbed with conventional Ag, Au or other ‘off resonant’ Ag@Au nps. The power‐dependent SERS experiment on single‐walled nanotubes (SWNTs) with resonant Ag@Au nps reveals a linear behavior between the G‐band intensity and the photon flux density, which is in agreement with the vibrational pumping model of SERS. The observed enhancement by resonance matching is pronounced for carbon nanotubes and may lead to insights into understanding nanotube–nanoparticle interaction. Copyright © 2009 John Wiley & Sons, Ltd.     

Effect of the shell on the transport properties of poly(glycerol) and Poly(ethylene imine) nanoparticles

Dendritic core–shell architectures containing poly (glycerol) and poly (ethylene imine) cores and poly(lactide) shell (PG-PLA and PEI-PLA respectively) were synthesized. Analogous of these core–shell architectures containing the same cores but poly (L-lactide) shell (PG-PLLA and PEI-PLLA, respectively) were also synthesized. In this work PG and PEI were used as macroinitiator for ring opening polymerization of the lactid and L-lactide monomers. Different molar ratios of monomer to end functional groups of PG ([LA]/[OH]) and PEI ([LA]/[NHn] (n = 1 or 2)) were used to prepare the core–shell architectures with different shell thickness. These core–shell architectures were able to encapsulate and transport the small guest molecules. Their transport capacity (TC) depended on the type and thickness of the shells. TC of core–shell architectures containing PLLA shell was higher than that for their analogs containing PLA shell. The diameter of core–shell architectures was between 20–80 nm. The rate of release of guest molecules from chloroform solution of nanocarriers to water phase was investigated and it depended on the type of the core, shell and solvent.     

Sonochemical syntheses of a nano-sized copper(II) supramolecule as a precursor for the synthesis of copper(II) oxide nanoparticles

Nanoparticles of a three-dimensional supramolecular, [Cu(L)₂(H₂O)₂] (1), (L⁻ = 1H-1,2,4-triazole-3-carboxylate), have been synthesized by a sonochemical process and characterized by scanning electron microscopy, X-ray powder diffraction, IR spectroscopy and elemental analyses. The thermal stability of compound 1 both its bulk and nano-size has been studied by thermal gravimetric (TG) and differential thermal analyses (DTA) and compared each other. Concentration of initial reagents effects on size and morphology of nano-structured compound 1, have been studied. Calcination of the nano-sized compound 1 at 600 °C under air atmosphere yields CuO nanoparticles.     

Modeling of exchange bias in the antiferromagnetic (core)/ferromagnetic (shell) nanoparticles with specialized shapes

Zero-field-cooled (ZFC) and field-cooled (FC) hysteresis loops of egg- and ellipsoid-shaped nanoparticles with inverted ferromagnetic (FM)-antiferromagnetic (AFM) core-shell morphologies are simulated using a modified Monte Carlo method, which takes into account both the thermal fluctuations and energy barriers during the rotation of spin. Pronounced exchange bias (EB) fields and reduced coercivities are obtained in the FC hysteresis loops. The analysis of the microscopic spin configurations allows us to conclude that the magnetization reversal occurs by means of the nucleation process during both the ZFC and FC hysteresis branches. The nucleation takes place in the form of “sparks” resulting from the energy competition and the morphology of the nanoparticle. The appearance of EB in the FC hysteresis loops is only dependent on that the movements of “sparks” driven by magnetic field at both branches of hysteresis loops are not along the same axis, which is independent of the strength of AFM anisotropy. The tilt of “spark” movement with respect to the symmetric axis implies the existence of additional unidirectional anisotropy at the AFM/FM interfaces as a consequence of the surplus magnetization in the AFM core, which is the commonly accepted origin of EB. Our simulations allow us to clarify the microscopic mechanisms of the observed EB behavior, not accessible in experiments.     

Facile sonochemical synthesis of tellurium and tellurium dioxide nanoparticles: Reducing Te(IV) to Te via ultrasonic irradiation in methanol

In this study, tellurium (Te) nanostructures were synthesized via a facile sonochemical method by reducing TeCl₄ to Te under ultrasonic irradiation in methanol. Moreover, by carrying out the reaction in an alkaline environment TeO₂ nanoparticles were produced. According to our knowledge, it is the first time that without secondary reducing agent Te⁴⁺ are reduced to Te. Also, the effects of ultrasonic power, irradiation time, solvent, and surfactant on the morphology and particle size of Te nanostructures were investigated. The obtained products were characterized by XRD, SEM, EDS, FT-IR and DRS.     

Formation and strain distribution of Ni/NiO core/shell magnetic nanoparticles fabricated by pulsed laser deposition

Nucleation and growth lead to substantial strain in nanoparticles embedded in a host matrix.The distribution of strain field plays an important role in the physical properties of nanoparticles.Magnetic Ni/NiO core/shell nanoparticles embedded in the amorphous Al2O3 matrix were fabricated by pulsed laser deposition.The results from a high-resolution transmission electron microscope also revealed that the core/shell nanoparticles consist of a single crystal Ni core with a faced-centered cubic structure(Space ...     

壳核型磁性纳米纤维素微球的超声制备及表征

以纳米级的Fe3 O4液体作为磁核 ,在非水体系的纤维素DMAc(N N二甲基乙酰胺 ) /LiCl溶液中 ,使用包埋法 ,在超声波的辅助下制备得到了纳米尺度的壳核型磁性纤维素微球 .利用FT IR、XRD及AFM/MFM (原子力显微镜 /磁场力显微镜 )对得到的磁性微球进行了表征 ,证实该微球由磁性的核与纤维素的壳组成 ,微球大小为 30～ 5 0nm ,且具有良好的分散性 .并研究了超声条件对磁性微球尺寸的影响 .     

Sonochemical synthesis and characterization of three nano zinc(II) coordination polymers; Precursors for preparation of zinc(II) oxide nanoparticles

Nanostructures of three Zinc(II) coordination polymers, [Zn(NNO)₂(H₂O)₄]_n (1), [Zn(PNNO)₂(H₂O)₂]_n (2) and [Zn(H₂O)₆]·(INNO)₂ (3) {NNO: Nicotinic acid N-oxide, PNNO: Picolinic acid N-oxide and INNO: Isonicotinic acid N-oxide}, have been synthesized by a sonochemical process and reaction of ligands with Zn(CH₃COO)₂. The Zinc(II) oxide nano-particles have been synthesized from thermolysis of [Zn(NNO)₂(H₂O)₄]_n (1), [Zn(PNNO)₂(H₂O)₂]_n (2) and [Zn(H₂O)₆]·(INNO)₂ (3) at two different methods (with surfactant and without surfactant) and two temperatures (200 and 600 °C). The ZnO nanoparticles were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Comparison of the SEM images of ZnO nano-particles at two different methods and temperatures shows that higher temperature results in an increasing of agglomeration and thus small and spherical ZnO particles with good separation were produced by thermolysis of compounds at 200 °C and by use of surfactant.