Magnetic properties of monodisperse NiHx nanoparticles and comparison to those of monodisperse Ni nanoparticles

We produced, for the first time, monodisperse NiH(x) nanoparticles with particle diameters of 7.0 nm and investigated their magnetic properties. We also produced monodisperse Ni nanoparticles with nearly the same particle diameters as those of NiH(x) nanoparticles as a comparison. The magnetic properties of NiH(x) nanoparticles were quite different from those of Ni nanoparticles. We observed two compositional phases in NiH(x) nanoparticles, similar to bulk material: one is the nearly pure Ni phase with the blocking temperature (T(B)) of 11 K and the other is the hydride phase. We observed T(B) of 40 K in Ni nanoparticles.     

Preparation of zirconium diboride ultrafine hollow spheres by a combined sol–gel and boro/carbothermal reduction technique

In this work, we introduce a modified novel silica sol–gel process to synthesize hexagonal close-packed (hcp) and face-centered cubic (fcc) nickel (Ni) nanoparticles supported on amorphous carbon and silica matrix. The supporting of amorphous carbon and silica can prevent the Ni nanoparticles from aggregating and being oxided which would result in the loss of their magnetism and dispersibility. The phase structure of the Ni nanoparticles which were obtained from the gels pyrolyzed from 250 to 350 °C is hcp structure, whereas that of the Ni nanoparticles pyrolyzed at 750 °C is fcc structure. The grain sizes of the hcp Ni nanoparticles calcined at 250 °C range from 5 to 20 nm in diameter, and that of the fcc Ni nanoparticles calcined at 750 °C range in 7–35 nm. The studies of magnetic properties of the hcp and fcc Ni nanoparticles show that both have quite different magnetic behaviors.     

Size and surface effects on the magnetic properties of NiO nanoparticles

NiO nanoparticles (NPs) were prepared by a sol-gel process using the citrate route. The sol-gel parameters were tuned to obtain samples with different average particle sizes, ranging from 12 to 70 nm. Magnetic characterization revealed an increase in the blocking temperature with the diameter of the NPs and an increase in the effective magnetic anisotropy (K(eff)) with decreasing particle size. The magnetic moment per particle was calculated for all samples using the susceptibility value at T = 300 K. The number of uncompensated spins per NP was found to be proportional to n (n(S)≡ total number of spins), indicating that they are randomly distributed on the NP surface. For small diameters (<30 nm) the surface anisotropy constant was estimated, using, for NiO NPs, a recent model describing the evolution of K(eff) with particle size. Hysteretic loops performed at low temperatures after field cooling displayed loop shifts (～6.5 kOe in the field axis and ～0.18 emu g(-1) vertically), coercive field enhancement (H(C)≈ 4.8 kOe) and training effects for the smaller NPs. The sample with NPs of larger diameters presented magnetic properties close to those of bulk NiO.     

Magnetic properties of monodispersed Ni/NiO core-shell nanoparticles

We have recently developed a method to fabricate monodispersed Ni/NiO core-shell nanoparticles by pulsed laser ablation. In this report, the size-dependent magnetic properties of monodispersed Ni/NiO core-shell nanoparticles were investigated. These nanoparticles were formed in two steps. The first was to fabricate a series of monodispersed Ni nanoparticles of 5 to 20 nm in diameter using a combination of laser ablation and size classification by a low-pressure differential mobility analyzer (DMA). The second step was to oxidize the surfaces of the Ni particles in situ to form core-shell structures. A superconducting quantum interference device (SQUID) magnetometer was used to measure the magnetic properties of nanostructured films prepared by depositing the nanoparticles at room temperature. Ferromagnetism was observed in the magnetic hysteresis loop of the nanostructured films composed of core-shell nanoparticles with core diameters smaller than the superparamagnetic limit, which suggests the spin of Ni core was weakly exchange coupled with antiferromagnetic NiO shell. In contrast, smaller nanoparticles with core diameters of 3.0 nm exhibited superparamagnetism. The drastic change in the hysteresis loops between field-deposited and zero-field-deposited samples was attributable to the strong anisotropy that developed during the magnetic-field-assisted nanostructuring process.     

Monte Carlo simulations of segregation in Pt-Ni catalyst nanoparticles

We have investigated the segregation of Pt atoms in the surfaces of Pt-Ni nanoparticles, using modified embedded atom method potentials and the Monte Carlo method. The nanoparticles are constructed with disordered fcc configurations at two fixed overall concentrations (50 at. % Pt and 75 at. % Pt). We use octahedral and cubo-octahedral nanoparticles terminated by {111} and {100} facets to examine the extent of the Pt segregation to the nanoparticle surfaces at T=600 K. The model particles contain between 586 and 4033 atoms (particle size ranging from 2.5 to 5 nm). Our results imply that a complete {100}-facet reconstruction could make the cubo-octahendral Pt-Ni nanoparticles most energetically favorable. We predict that at 600 K due to segregation the equilibrium cubo-octahedral Pt50Ni50 nanoparticles with fewer than 1289 atoms and Pt75Ni25 nanoparticles with fewer than 4033 atoms would achieve a surface-sandwich structure, in which the Pt atoms are enriched in the outermost and third atomic shells while the Ni atoms are enriched in the second atomic shell. We also find that, due to an order-disorder transition, the Pt50Ni50 cubo-octahedral nanoparticles containing more than 2406 atoms would form a core-shell structure with a Pt-enriched surface and a Pt-deficient homogenous core.     

Large Magnetocaloric Effect,Moment, and Coercivity Enhancement after Coating Ni Nanoparticles with Ag

We observe a large magnetocaloric effect in monodisperse Ni and Ni_coreAg_shell nanoparticles in the superparamagnetic region. The organically passivated Ni nanospheres show a large magnetic entropy change of 0.9 J kg^?1 K for a 3 T magnetic field change. In comparison to the surfactant‐coated Ni nanoparticles, the Ni_coreAg_shell nanoparticles show an enhanced coercivity, magnetization, and magnetocaloric effect (1.3 kg K for a 3 T magnetic field change). The coercivity at 10 K increases from 360 Oe for Ni nanoparticles to nearly 610 Oe for Ni_coreAg_shell particles. This large enhancement is attributed to the enhanced inter‐particle interaction, which is mediated by the metallic shell, over the relatively weaker dipolar interaction in the surfactant‐coated Ni nanoparticles, and to modification of the surface spin structure.     

Obtaining gadolinium nanoparticles and studying their properties in a helium flow

A method for obtaining Gd nanoparticles with diameters of 89 to 18 nm upon metal evaporation both in a flow of pure helium and with the addition of 0.5% of oxygen is described. It is found that the addition of O₂ does not affect the size of the particles, their structure, or the Curie temperature, though the magnetization is reduced. Particles with sizes of 18 nm have cubic lattice symmetry (fcc) and remain paramagnetic below T_c; with an increase in the size of nanoparticles, the proportion of the hexagonal (hcp) phase, which coincides with the gadolinium structure, also grows, and below T_c such particles become ferromagnetic. Oxygen impurities seem to have no effect on magnetic and structural transitions in nanoparticles.     

Molecular simulation of cross-nucleation between polymorphs

Recent experiments report that an early nucleating crystalline structure (or polymorph) may nucleate another polymorph. We use molecular dynamics simulations to model this phenomenon known as cross-nucleation. We study the onset of crystallization in a liquid of Lennard-Jones particles cooled at a temperature 22% below the melting temperature. We show that growth proceeds through the successive cross-nucleation of the metastable hexagonal close-packed (hcp) polymorph on the stable face-centered cubic (fcc) polymorph and of the stable fcc polymorph on the metastable hcp polymorph. This finding is in agreement with the experimental results which demonstrated that the cross-nucleation of a stable polymorph on a metastable polymorph is just as likely as the cross-nucleation of a metastable polymorph on a stable polymorph. We then extend our findings established in the case of the homogeneous crystal nucleation to a situation of practical interest, i.e., when a seed of the stable polymorph is used. By studying the crystal growth from the (111) plane of a perfect fcc crystal, we show that, again, growth proceeds through the cross-nucleation of the hcp and fcc structures.     

Stabilization of hexagonal close-packed metallic nickel for alumina-supported systems prepared from Ni(II) glycinate

The decomposition in flowing argon of the neutral complex [Ni^II(glycinate)₂(H₂O)₂] leads to a mixture of face-centered cubic (fcc) and hexagonal close-packed (hcp) metallic nickel. The latter is the main phase when the Ni(II) complex is supported on alumina. Unlike most hexagonal Ni phases described earlier, and similar to hexagonal Ni₃C, the unit cell parameters (a=0.2493 and ) lead to Ni-Ni distances equal to those encountered in fcc Ni. TEM shows that the nanoparticles are protected by graphite layers, whose elimination by heating in hydrogen results in transformation to the fcc phase and crystal growth. Magnetic measurements provide evidence of the coexistence of superparamagnetic and ferromagnetic nanoparticles. This result is in line with the broad size distribution observed by TEM and is interpreted on the basis of the metallic character of hcp Ni particles.     

NixPb1-x nanowire arrays: effects of annealing

Ni(x)Pb(1-x) nanowire arrays were successfully fabricated by AC electrodeposition within the nanopores of ordered porous alumina films prepared by a two-step anodization. Transmission electron microscopy analyses showed that the Ni-Pb nanoarrays are polycrystalline with dimension uniformity around 20 nm in diameter and lengths up to several micrometers. X-ray diffraction results revealed that the face-centered-cubic (fcc) Ni and fcc Pb peaks are detected when the Ni component (x) is below 0.71, indicating that the Ni(x)Pb(1-x) nanoarrays do not form metastable phase alloy. Hysteresis loops determined by vibrating sample magnetometer indicated that the Ni(x)Pb(1-x) nanoarrays obtained possess obvious magnetic anisotropy, and the perpendicular coercivity was lower than that of pure Ni nanowries before and after annealing. Annealing under magnetic field was carried out to examine the effect of a magnetic field on magnetic properties using an electromagnet field up to 0.3 T.     

Synthesis of face-centered tetragonal FePt nanoparticles and granular films from Pt@Fe2O3 core-shell nanoparticles

This paper describes a new approach for making face-centered tetragonal (fct) FePt nanoparticles with a diameter of 17 nm and granular films from Pt@Fe2O3 core-shell nanoparticle precursors. The core-shell nanoparticles were converted to fct FePt through a reduction and alloy formation process at enhanced temperatures. The Fe and Pt elemental analysis was conducted on both individual nanoparticles and granular films using energy-dispersive X-ray (EDX) spectroscopy. Our convergent evidence from selected area electron diffraction (SAED), powder X-ray diffraction (PXRD), and EDX analysis indicates that the final products are fct FePt alloys. The fct FePt films have coercivities of 8.0-9.1 kOe at 5 K and 7.0 kOe at 300 K measured by a SQUID magnetometer. These values depend on the conversion temperatures of Pt@Fe2O3 nanoparticles. Unlike the previously synthesized disordered face-centered cubic (fcc) FePt nanoparticles with diameters of 4-6 nm (Sun, S. H.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989), the FePt nanoparticles presented in this work not only possess the preferred fct phase but also are in a size range that is expected to be ferromagnetic and have high coercivity, which is important to the practical applications in ultrahigh density data storage media and magnetic nano devices.     

Applied magnetic field rejects the coating of ferromagnetic carbon from the surface of ferromagnetic cobalt: RAPET of CoZr2(acac)2(OiPr)8

We present the results of the RAPET (reaction under autogenic pressure at elevated temperatures) dissociation of CoZr(2)(acac)(2)(O(i)Pr)(8) at 700 degrees C in a closed Swagelok cell under an applied magnetic field of 10 T. It produces a mixture of carbon-coated and noncoated metastable ZrO(2) nanoparticles, bare metallic Co nanoparticles, and bare carbon. The same reaction in the absence of a magnetic field produces spherical Co and ZrO(2) particles in sizes ranging from 11 to 16 nm and exhibiting, at room temperature, metastable phases: fcc for cobalt and a tetragonal phase for zirconia. The metastable phases of Co and ZrO(2) are manifested because of a carbon shell of approximately 4 nm thickness anchored to their surfaces. The effect of an applied magnetic field to synthesize morphologically different, but structurally the same, products is the key topic of the present paper.     

Preparation and characterization of ultrafine co and ni particles in a polymer matrix

A facile route for in situ synthesis of Co and Ni nanoparticles in a preorganized polyacrylamide gel is reported. Metal-polymer composites were prepared by gamma-irradiation at room temperature. The Co nanoparticles were roughly 3-5 nm in size and were stable in the polymer matrix in the presence of air. The presence of Co and Ni nanoparticles was established by their ability to transfer an electron to methyl viologen {paraquat: 1,1'-dimethyl 4,4'-dipyridinium dichloride; MV(2+) (Cl(-))(2)}. The Co and Ni nanoparticles were probed for their magnetic characteristics by a superconducting quantum interferometer device (SQUID) magnetometer and display a low superparamagnetic blocking temperature T(B) of about 13 and 10 K, respectively. The field-dependent magnetic behavior below T(B) displays the standard features corresponding to superparamagnetism, as expected for very small Co and Ni crystallites. This also suggests that particles are polycrystalline in nature.     

Synthesis and Soft Magnetic Properties of Mg-doped Ni Nanoparticles

Mg-doped Ni nanoparticles with good soft magnetic properties were prepared with the sol-gel method and were sintered at 400, 500, 600, and 900℃ in argon atmosphere, respectively. The structure and magnetic properties of the samples were studied by means of X-ray diffraction, TEM, and VSM magnetometers. X-Ray powder diffraction results show that Ni-Mg solid solution was formed with the single phase of face-centered cubic（fcc） structure. The particle size became larger with the increase of temperature. When the sintering temperature was 400 °C, the particle size was 6.3 nm, whereas it was 46.2 nm at 900 °C. Both the saturation magnetization（Ms） and the coercivity were enhanced with the increase of the particle size. The Ms values of the samples ranged from 18.965 to 46.766 emu/g and the coercivity ranged from 1051.3568 to 9145.0848 A/m.     

Characterization of palladium nanoparticles by using X-ray reflectivity, EXAFS, and electron microscopy

We compared the characteristics of dodecanethiolate palladium nanoparticles synthesized by two different techniques, a one-phase method and a two-phase method. From transmission electron microscopy (TEM), we determined that the particle sizes were 46 +/- 10 angstroms and 20 +/- 5 angstroms for the one- and two-phase particles, respectively. Electron diffraction confirmed that their structure was face-centered cubic (fcc). The lattice constant a0 was 3.98 +/- 0.01 angstroms and 3.90 +/- 0.01 angstroms for the one- and two-phase particles, respectively. High-resolution TEM (HRTEM) showed that the one-phase particles had an ordered core surrounded by a disordered shell structure, while the two-phase particles appeared to be crystalline throughout. The particles were also analyzed with extended X-ray absorption fine structure (EXAFS). A cuboctahedral fcc model was used to fit the data, which implied particle sizes of less than 10 angstroms for both the one- and two-phase particles. The discrepancy between the two techniques was attributed to the presence of a disordered phase, which we presumed was composed of Pd-S compounds. Compared with the bulk palladium, lattice expansion was observed in both one- and two-phase particles by electron diffraction, HRTEM, and EXAFS. At the air/water interface, a uniform film that produced surface pressure/area isotherms could only be obtained from the two-phase particles. The one-phase particles did not wet the water surface. X-ray reflectivity data indicated that the Langmuir monolayer of the two-phase particles was only 13 angstroms thick. TEM revealed the diameter of the particles in this layer to be 23 angstroms; hence the particles assumed an oblate structure after spreading. EXAFS examination of a stack of 750 Langmuir monolayers indicated far fewer Pd-S compounds, which may have dissolved in the water. The data were consistent with a model of a monolayer of truncated cuboctahedron Pd particles that were 7 angstroms thick and 19 angstroms in diameter.     

Alignment of carbon nanotubes under low magnetic fields through attachment of magnetic nanoparticles

The alignment of multiwalled carbon nanotubes (MWNTs) has been accomplished through deposition of uniform layers of magnetite/maghemite nanoparticles (diameter = 6-10 nm) and use of an external magnetic field. The coating of CNTs with magnetic nanoparticles was performed by combining the polymer wrapping and layer-by-layer (LbL) assembly techniques. The particle-coated MWNTs are superparamagnetic and can be aligned at room temperature on any substrate by deposition from an aqueous solution in an external field B = 0.2 T. The volume magnetization of the particle coated MWNTs is found to be enhanced by 17% compared to the pure particles in a powder indicating that either the adsorption process onto the CNTs changes the particle magnetization, or the MWNTs carry an intrinsic magnetization due to remaining Ni used as a catalyst for the growth process.     

A simple solvothermal synthesis of MFe2O4 (M=Mn, Co and Ni) nanoparticles

Nanoparticles of MFe₂O₄ (M=Mn, Co and Ni), with diameters ranging from 5 to 10 nm, have been obtained through a solvothermal method. In this synthesis, an alcohol (benzyl alcohol or hexanol) is used as both a solvent and a ligand; it is not necessary, therefore, to add a surfactant, simplifying the preparation of the dispersed particles. We have studied the influence of the synthetic conditions (temperature, time of synthesis and nature of solvent) on the quality of the obtained ferrites and on their particle size. In this last aspect, we have to highlight that the solvent plays an important role on the particle size, obtaining the smallest diameters when hexanol was used as a solvent. In addition, the magnetic properties of the obtained compounds have been studied at room temperature (RT). These compounds show a superparamagnetic behaviour, as was expected for single domain nanoparticles, and good magnetization values. The maxima magnetization values of the MFe₂O₄ samples are quite high for such small nanoparticles; this is closely related to the high crystallinity of the particles obtained by the solvothermal method.     

Magnetic nanoparticles with hybrid shape

Co₅₀Ni₅₀ particles with very unusual shapes, resembling dumbbells or diabolos, are obtained by reducing mixtures of cobalt and nickel acetates in sodium hydroxide solution in 1,2-propane diol. These particles consist of a central column richer in cobalt than the overall composition, capped with two terminal platelets that are richer in nickel. These hybrid shapes are the result of a two-step growth mechanism due to a difference in reactivity of the two metal ions. The sodium hydroxide concentration controls the length and diameter of the column, in the ranges 50–250 nm and 5–15 nm, respectively, and the diameter of the platelets in the range 25–50 nm. The X-ray diffraction patterns show a mixture of hcp and fcc phases in various proportions depending on the particle shape. High-resolution electron microscopy shows that the hcp phase is located mainly in the central column and the fcc phase mainly in the terminal platelets. The particles are ferromagnetic at room temperature. When the volume fraction of the central column is high enough and the hcp phase is predominant and not much faulted, high coercivity (up to 1900 Oe) is observed.     

Laser-assisted synthesis of superparamagnetic Fe@Au core-shell nanoparticles

A novel method combining wet chemistry for synthesis of an Fe core, 532 nm laser irradiation of Fe nanoparticles and Au powder in liquid medium for deposition of an Au shell, and sequential magnetic extraction/acid washing for purification has been developed to fabricate oxidation-resistant Fe@Au magnetic core-shell nanoparticles. The nanoparticles have been extensively characterized at various stages during and up to several months after completion of the synthesis by a suite of electron microscopy techniques (HRTEM, HAADF STEM, EDX), X-ray diffraction (XRD), UV-vis spectroscopy, inductively coupled plasma atomic emission spectroscopy, and magnetometry. The surface plasmon resonance of the Fe@Au nanoparticles is red shifted and much broadened as compared with that of pure colloidal nano-gold, which is explained to be predominantly a shell-thickness effect. The Au shell consists of partially fused approximately 3-nm-diameter fcc Au nanoparticles (lattice interplanar distance, d = 2.36 A). The 18-nm-diameter magnetic core is bcc Fe single domain (d = 2.03 A). The nanoparticles are superparamagnetic at room temperature (300 K) with a blocking temperature, T(b), of approximately 170 K. After 4 months of shelf storage in normal laboratory conditions, their mass magnetization per Fe content was measured to be 210 emu/g, approximately 96% of the Fe bulk value.     

Green synthesis of colloidal silver nanoparticles using natural rubber latex extracted from Hevea brasiliensis

Colloidal silver nanoparticles were synthesized by an easy green method using thermal treatment of aqueous solutions of silver nitrate and natural rubber latex (NRL) extracted from Hevea brasiliensis. The UV–Vis spectra detected the characteristic surface plasmonic absorption band around 435 nm. Both NRL and AgNO₃ contents in the reaction medium have influence in the Ag nanoparticles formation. Lower AgNO₃ concentration led to decreased particle size. The silver nanoparticles presented diameters ranging from 2 nm to 100 nm and had spherical shape. The selected area electron diffraction (SAED) patterns indicated that the silver nanoparticles have face centered cubic (fcc) crystalline structure. FTIR spectra suggest that reduction of the silver ions are facilitated by their interaction with the amine groups from ammonia, which is used for conservation of the NRL, whereas the stability of the particles results from cis-isoprene binding onto the surface of nanoparticles. Therefore natural rubber latex extracted from H. brasiliensis can be employed in the preparation of stable aqueous dispersions of silver nanoparticles acting as a dispersing and/or capping agent. Moreover, this work provides a new method for the synthesis of silver nanoparticles that is simple, easy to perform, pollutant free and inexpensive.