Ni Nanocrystallites Research Articles

Dependence of exchange bias on the field-cooled remanent magnetic state in Ni/NiO nanogranular samples

The magnitude of the exchange bias (EB) effect in nanogranular Ni/NiO samples (with Ni content between about 4 and 69 wt% and mean size of the Ni crystallites of the order of 10 nm) has been found to be strictly related to the increase in the remanent magnetization measured after field-cooling, with respect to the value after zero-field-cooling, normalized to the saturation magnetization. This allows describing the EB mechanism in terms of the fraction of the magnetic moments of the Ni nanocrystallites that irreversibly has aligned in the field direction during field-cooling, due to the exchange anisotropy, and that is effectively involved in the loop shift. Hence, the possibility of tuning EB by controlling the field-cooled remanent magnetic state is shown.

Tailoring of the Magnetic Properties of Co, Fe and Ni Nanocrystallites

Ferromagnetic or superparamagnetic nanocrystallites, stabilized in carbon matrix, were prepared by the procedure comprising formation of the appropriate metal acrylamide complex, followed by frontal polymerization and pyrolysis of the polymer. The pyrolysis products were in a form of beads, which contained in their volume randomly distributed nanocrystallites. The nanocrystallites had various composition, magnetic properties and sizes ranging from few to tens of nanometers, depending on the element type. Application of this procedure stabilizes the nanostructure and enables processing of nanoparticles within a narrow window of sizes. The magnetic parameters depend on the crystallite size, determined by their chemical composition. The nanocrystallites pyrolysed at 773 K exhibited ferromagnetic properties for Co and Fe, and superparamagnetic behaviour for the Ni. The nanocrystallites can potentially be applied as sensors for tagging the biological substances or for targeted drug delivery.

Magnetic characteristics of thin Ni films electrodeposited on n-Si(1 1 1)

The nanocrystalline Ni films were grown on n-Si(1 1 1) substrate by pulsed electrodeposition in non-aqueous NiCl 2 + methanol solution. The frequency of potential pulse was modulated during the deposition of Ni onto Si substrates. When the frequency varies from 20 to 900 Hz, the average size of Ni nanocrystallites varied in the ranges from 48 to 130 nm. In these cases, all Ni films have grown through a three-dimensional instantaneous nucleation followed by diffusion-limited growth. From X-ray diffraction measurement, it has been found that Ni(1 1 1) grows preferentially on the Si(1 1 1) substrates. The magnetic hysteresis loops for as-deposited films were measured by using VSM. As the angle θ between film plane and applied magnetic field varies from 0 to 90, the coercivity ( H c ) and squareness ( S) obtained from the magnetic hysteresis loops showed an opposite behavior. With the increase in θ , H c increased but S decreased near linearly. We have also investigated the variation of H c as a function of Ni nanocrystallite’s size. From VSM measurement, we could observe that the coercivities for the magnetic field applied perpendicular and parallel to the film plane increase up to the average size of 86 nm but begin to decrease over this size.

Tailoring the exchange bias of Ni/NiO nanogranular samples by the structure control

The exchange bias (EB) effect has been studied in Ni/NiO nanogranular samples obtained by annealing in H 2, at selected temperatures (200≤ T ann≤300 °C), NiO powder previously milled for 5, 10, 20 and 30 h. Both the as-milled NiO powders and the Ni/NiO samples have been analyzed by X-ray diffraction and the exchange bias properties have been investigated in the 5–200 K temperature range. The structure and the composition of the Ni/NiO samples can be satisfactorily controlled during the synthesis procedure by varying both T ann and the milling time of the precursor NiO powders. In particular, by increasing this last parameter, the mean grain size of the NiO phase reduces down to the final value of 16 nm and the microstrain increases, which is consistent with an enhancement of the structural disorder. The structure of the milled NiO matrix strongly affects the process of nucleation and growth of the Ni nanocrystallites induced by the H 2 treatments, so that, T ann being equal, the amount and the mean grain size D Ni of the Ni phase vary substantially in samples having different milling times. Such features of the Ni phase determine the extent of the Ni/NiO interface and consequently the magnitude of the exchange field H ex: the highest value (∼940 Oe) has been measured at T=5 K in a sample containing ∼7 wt% Ni and with D Ni=19 nm. However, in Ni/NiO samples with very different structural characteristics and different values of H ex at T=5 K, the EB effect vanishes at the same temperature (∼200 K) and the same thermal dependence of H ex is observed. We consider that the evolution of the EB effect with temperature is ultimately determined by the microstructure of the Ni/NiO interface, which cannot be substantially modified by changing the synthesis parameters, milling time and T ann.

Nanocrystalline glass-coated FeNiMoB microwires

The evolution of the structure of glass-coated Fe40Ni38Mo4B16 amorphous microwire with thermal treatments and its interplay with magnetism has been studied. As shown by x-ray diffraction, a primary crystallization process resulted into formation of γ-(Fe,Ni) nanocrystallites embedded in a residual amorphous matrix. The evolution of the saturation magnetization and the switching field after different thermal treatment was studied. Amorphous glass-coated microwires based on FeNi exhibit magnetic bistability even in the nanocrystalline state. This is explained by the high magnetoelastic anisotropy, which is also responsible for magnetic hardening after annealing at the temperatures above 670 K.

AN ELECTROCHEMICAL INVESTIGATION OF NANOCRYSTALLINE Mg2Ni0.75Nb0.25 COMPOUND SYNTHESIZED BY MECHANICAL ALLOYING

The effects of mechanical alloying on the microstructure of binary and ternary powder mixtures with stoichiometric compositions of Mg 2 Ni and Mg 2 Ni 0.75 Nb 0.25 were studied, respectively. Also, the electrode properties of the milled products were investigated in 6M KOH solution. X-ray diffraction and scanning electron microscopy of the milled products showed the formation of Mg 2 Ni -based nanocrystallites after 15 and 10h of milling using the initial binary and ternary mixtures, respectively. It was found that partial substitution of Nb for Ni has beneficial effect on the formation kinetic of nanocrystalline Mg 2 Ni . Longer milling times resulted in the formation of an amorphous phase. A relatively high discharge capacity of 350mAhg-1 was measured for the electrode made up of the ternary milled product after 20h. Also, the ternary milled product showed longer discharge life. This ternary electrode showed a microstructure consisting of Mg 2 Ni nanocrystallites and an amorphous phase. Nb substitution for Ni was found to be beneficial to electrode properties of the milled products.

Quantum interference effects and magnetic scattering in the electrical resistivity of Ni nanocrystallites in TiN matrix

The electrical resistivity, ρ of self-assembled, epitaxial nickel nanocrystallites in titanium nitride (TiN) matrix, was measured in fields from 0to4T between 4.2 and 300K. At lower temperatures, ρ(T) conclusively shows −T dependence below the observed minima at Tmin unlike −lnT in two dimensional multilayers. In this three dimensional system, the origin of the minima lays in quantum interference effects. The ρ(T) data well above Tmin, interpreted in terms of electron-phonon and electron-magnon scatterings, show that the temperature dependence of the composite Ni∕TiN sample mainly comes from that of Ni and the residual resistivity from TiN.

Microstructures of electroless Ni–P alloy deposits and their transformation sequences during post-deposition annealing

Ni–P deposits of amorphous, nanocrystalline, and mixed structures were prepared by electroless deposition. The three deposits were hypoeutectic Ni–P alloys with different P concentrations. The overall transformation sequences of the deposits during post-deposition annealing were investigated using differential scanning calorimetry, x-ray diffraction, and transmission electron microscopy. It was found that there existed three heat-release peaks in a mixed-structure deposit during annealing. The first peak came from the precipitation of Ni nanocrystallites from an amorphous matrix, the second peak resulted from the decomposition of the retained amorphous matrix into Ni + Ni3P having a composition close to the eutectic point, and the third peak, newly found in hypoeutectic Ni–P alloys, was assumed to be caused by both grain growth and the precipitation of Ni3P from as-deposited supersaturated Ni(P) nanocrystals interspersed within the amorphous matrix. By comparing the transformation sequences of the amorphous deposit with that of the nanocrystalline deposits, it was concluded that the transformation sequence of the mixed-structure deposit was a superimposition of those of both the amorphous and nanocrystalline deposits.

Structure and magnetic properties of CoNiP nanowire arrays embedded in AAO template

Ternary CoNiP nanowire (NW) arrays have been synthesized by electrochemical deposition inside the nanochannels of anodic aluminum oxide (AAO) template. The CoNiP NWs deposited at room temperature present soft magnetic properties, with both parallel and perpendicular coercivities less than 500 Oe. In contrast, as the electrolyte temperature ( T elc) increases from 323 to 343 K, the NWs exhibit hard magnetic properties with coercivities in the range of 1000–2500 Oe. This dramatic increase in coercivities can be attributed to the domain wall pinning that is related to the formation of Ni and Co nanocrystallites and the increase of P content. The parallel coercivity (i.e. the applied field perpendicular to the membrane surface) maximum as high as 2500 Oe with squareness ratio up to 0.8 is achieved at the electrolyte temperature of 328 K. It has been demonstrated that the parallel coercivity of CoNiP NWs can be tuned in a wide range of 200–2500 Oe by controlling the electrolyte temperature, providing an easy way to control magnetic properties and thereby for their integration with magnetic-micro-electro-mechanical systems (MEMS).

Ce5Mg41–xNi nanocomposites for electrochemical hydrogen storage

The Ce(5)Mg(41) hydrogen storage alloy was ball-milled with Ni powder, leading to the formation of a structure of inlaid metallic Ni nanocrystallites dispersed throughout a Ce-Mg amorphous alloy matrix. This structure was identified to have a positive effect on improving electrochemical hydrogen storage capacity. The ball-milled Ce(5)Mg(41) + 200 wt% Ni composite shows the highest initial discharge capacity (1046 mA h g(-1)) and high-rate dischargeability (HRD). Electrochemical impedance spectra, cyclic voltammograms and anodic polarization measurements show that the high discharge capacity and HRD was due to high hydrogen diffusivity and low reaction resistance. With a further increase in Ni content, the discharge capacity and HRD decreases, as a result of the excessively high reaction resistance due to the presence of an excessive oxide film of Ni(OH)(2). However, the cycling stability improves with the increase of Ni content in the nanocomposite.

The growth of nickel nanoparticles on conductive polymer composite electrodes

Uniformly sized and dispersed nickel core-shell nanoparticles on polypyrrole have been grown by electrodeposition. Two principal types of Ni particle morphologies were observed: small Ni nanocrystallites ranging in diameter from 7 to 12 nm, and larger spherical Ni clusters of 100 nm to over a micron in size. The type of Ni nanoparticles deposited depends sensitively on the preparation of the conducting polymer substrate. The optimal deposition conditions for the small Ni nanocrystallites were determined. TEM diffraction and XPS analysis show that these nanoparticles are predominantly NiO.

The enhanced magnetic properties of Ni nanocrystallites induced by the coupling between Ni/NiO interfaces

A composite Ni/NiO ultrafine and homogeneous nanocrystallites (NCs) system has been prepared by vacuum thermolysis of coprecipitated product from NiCl 2 and NaOH aqueous solution in the presence of polyvinylpyrrolidone (PVP). It is interesting to note that high magnetic properties have been achieved in this investigated system, including an exchange bias field H E of 180.4 kA/m and a high coercivity H C of 477.9 kA/m at 4.2 K which is one order larger than the bulk Ni or 10 nm pure Ni crystallite systems. These results can be attributed to the antiferromagnetic (AFM) pining effect from AFM NiO to ferromagnetic (FM) Ni.

Extraordinary Hall effect in self-assembled epitaxial Ni nanocrystallites embedded in a TiN matrix

Nanocrystalline nickel particles embedded in crystalline titanium nitride (TiN) matrices were prepared using pulsed laser deposition technique where Ni nanocrystallites are self-assembled and epitaxially grown. The electrical transport and the magnetotransport properties, especially the Hall effect, in such a sample have been investigated in the temperature range from 4.2 to 290 K. This work is mainly focused on the scaling behavior of the extraordinary Hall constant, ${R}_{s}$. The analysis shows that the usual scaling law ${R}_{s}\ensuremath{\sim}{\ensuremath{\rho}}^{n}$ is not markedly different from that of the bulk, even though the particles are reduced to nanoscale size (10--20 nm). However, the current density through the Ni nanoparticles had to be properly corrected. The exponent of the scaling law $(n\ensuremath{\approx}1.72)$ is found to be very similar to that of bulk Ni $(n\ensuremath{\approx}1.68)$, both above 100 K, implying thereby that the origin of the extraordinary Hall effect (side-jump) still remains unaltered.

Control of catalyst particle crystallographic orientation in vertically aligned carbon nanofiber synthesis

Vertically aligned carbon nanofibers (VACNF) have been synthesized where the crystallographic orientation of the initial catalyst film was preserved in the nanoparticle that remained at the nanofiber tip after growth. A substantial percentage of catalyst particles (75%), amounting to approximately 200 million nanofibers over a 100 mm Si wafer substrate, exhibited a sixfold symmetry attributed to a cubic Ni(1 1 1)∥Si(0 0 1) orientation relationship which was verified by X-ray diffraction studies. The Ni catalyst films were prepared by rf-magnetron sputtering under substrate bias conditions to yield a single (1 1 1) film texture. The total energy of the Ni thin film was estimated by calculating the sum of the surface free energy and strain energy. The total film energy was minimized by the evolution of the plane of lowest surface free energy, the (1 1 1) texture. This result was in agreement with X-ray diffraction measurements. The preferred orientation present in the Ni catalyst film prior to nanofiber growth was preserved in the Ni catalyst particles throughout the VACNF growth process. The Ni catalyst particles at the nanofiber tips were not pure single crystals but rather consisted of a mosaic structure of Ni nanocrystallites embedded within Ni catalyst nanoparticles (200–400 nm). The tip-located nanoparticles exhibited a faceted, crystal morphology with the faceting transferred to the underlying carbon nanofiber during the growth process. The possibility of precisely and accurately controlling VACNF growth velocity over macroscopic wafer dimensions with uniformly aligned catalyst particles is discussed.

Martensitic transformation and its effects on permeability-frequency spectra of FeNiZrBCu alloys

Effects of Ni addition on the microstructure and the dynamic magnetic properties of Nanoperm alloys were studied. It is shown that for the Ni-addition Nanoperm alloys, there exists a critical austenitic grain size, below which γ-(Fe, Ni) (austenitic) phase becomes more stable than α-(Fe, Ni) (martensitic) phase. The Ni addition in Nanoperm alloys may make the non-ferromagnetic γ-(Fe, Ni) nanocrystallites form between the ferromagnetic α-(Fe, Ni) nanocrystallites, which results in the interruption of exchange coupling between ferromagnetic crystallites and the anisotropy cannot be effectively averaged out. In addition, the formation of the non-ferromagnetic nanocrystallites has pinning effect on domain wall motion. These results may explain why the Ni addition may drastically deteriorate the soft magnetic properties of Nanoperm alloys. The effects of the Ni addition on the nanocrystalline materials and the coarse-grained materials are completely different.

Synthesis and One-Dimensional Self-Assembly of Acicular Nickel Nanocrystallites under Magnetic Fields

Polycrystalline nickel wires with an average length of 10 μm and diameter of about 200 nm were prepared at 70 °C by a hydrothermal process with a 0.25 T magnetic field applied. Studies show that the magnetic field induced one-dimensional assembly of acicular Ni nanocrystallites with dimensions of 200 nm in length and 10−30 nm in diameter, leading to the formation of the polycrystalline nickel wires. Magnetic measurements show saturation magnetization (Ms) of the sample prepared in a 0.25 T external magnetic field is higher than that of the sample synthesized without an external magnetic field applied. It is suggested that the one-dimensional self-assembly of acicular nickel nanocrystallites with their magnetic easy axes [111] aligned along the magnetic line of force results in the improvement of the magnetic properties.

Martensitic transformation and its effects on permeability-frequency spectra of FeNiZrBCu alloys

Effects of Ni addition on the microstructure and the dynamic magnetic properties of Nanoperm alloys were studied. It is shown that for the Ni-addition Nanoperm alloys, there exists a critical austenitic grain size, below which γ-(Fe, Ni) (austenitic) phase becomes more stable than α-(Fe, Ni) (martensitic) phase. The Ni addition in Nanoperm alloys may make the non-ferromagnetic γ-(Fe, Ni) nanocrystallites form between the ferromagnetic α-(Fe, Ni) nanocrystallites, which results in the interruption of exchange coupling between ferromagnetic crystallites and the anisotropy cannot be effectively averaged out. In addition, the formation of the non-ferromagnetic nanocrystallites has pinning effect on domain wall motion. These results may explain why the Ni addition may drastically deteriorate the soft magnetic properties of Nanoperm alloys. The effects of the Ni addition on the nanocrystalline materials and the coarse-grained materials are completely different.

Effective synthesis of carbon-coated Co and Ni nanocrystalliteswith improved magnetic properties by AC arc discharge under anN2 atmosphere

Carbon-coated Co and Ni nanocrystallites were effectively synthesized by an AC arc discharge methodunder an N2 atmosphere with yields over 80%. The metal content of as-made Co and Ni samples wasdetermined by a thermogravimetric method and it was found that the metal contentincreased by raising the metal loading ratio in the electrodes. The morphology,structure and magnetic properties of the Co and Ni samples were investigatedby transmission electron microscope, x-ray powder diffractometer and vibratingsample magnetometer, respectively. In addition to the carbon-coated Co andNi nanocrystallites, a small number of carbon nanotubes were also observedin the as-made Co and Ni samples. The values of saturation magnetization(Ms)for the sample containing 26.2 wt% of Co and the sample containing 37.6 wt% of Ni prepared underN2 gas were 21.7 and14.0 emu g−1, respectively.However, Ms for 28.8 wt% of Co and 40.9 wt% of Ni prepared under He gas were 16.8 and9.0 emu g−1,respectively. This suggested that the carbon-coated Co and Ni nanocrystallites prepared underN2 gas had largervalues of Ms than that of those prepared under He gas.

Growth and magnetism of Ni nanoparticles in Ni/Al 2O 3/Si or Si 3N 4 multilayers

We report on the fabrication of Ni/Al 2O 3/Si and textured Ni/Al 2O 3/Si 3N 4 multilayers containing Ni nanoparticles that exhibit significantly improved results. The secondary phases arising from thermal reaction between Ni and Si can be remarkably suppressed with increasing layers of Al 2O 3 and deposition of Ni/Al 2O 3 multilayers on Si 3N 4 substrates. Atomic force microscopy shows the formation of large as well as nanoclusters of Ni when grown on Si, whereas textured Ni nanoparticles are formed on Si 3N 4 substrates. The magnetization measurements on Ni/Al 2O 3/Si containing a single buffer layer of Al 2O 3 shows higher coercivity field with magnetic nanowire-like behavior, whereas with several Al 2O 3 alternate layers almost a superparamagnetic-like behavior is observed. However, significantly improved magnetic hysteresis was observed in textured Ni/Al 2O 3/Si 3N 4 multilayers due to preferred alignment of Ni nanocrystallites.

Preparation of carbon-coated Co and Ni nanocrystallites by a modified AC arc discharge method

A modified AC arc discharge method was used to prepare carbon-coated cobalt and nickel nanocrystallites under He atmosphere. The metal content of the magnetic nanomaterials were remarkably enhanced by increasing metal loading ratio and modifying experimental apparatus. The powder was separated from the soot by magnetic separation and acid (HCl) treatment. Structure and morphology were examined by powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). The metal content was determined by thermogravimetric analysis (TG). The magnetic properties of the powder were measured by a vibrating sample magnetometer (VSM). The size of the wrapped metal particles was between 10 and 100 nm. XRD profiles showed peaks associated with major peaks of graphite and single phase of fcc β-Co and cubic Ni with no evidence of metal carbides. With the Co metal content ranging from 28.8 to 57.5 wt.%, the corresponding B s ranged from 210.6 to 758.2 G. The B s of the carbon-coated Ni powder ranged from 90.7 to 205.0 G while the Ni metal content ranged from 34.6 to 64.5 wt.%.