Microwave-induced combustion synthesis of Ni–Zn ferrite powder and its characterization

Ni–Zn ferrite powders were successfully synthesized by microwave-induced combustion process. The process takes only a few minutes to obtain calcined Ni–Zn ferrite powders. The resultant powders were investigated by XRD, SEM, VSM, TG/DTA and surface area measurements. The as-received product shows the formation of cubic ferrite with saturation magnetization (M_s)≈23 emu/g, whereas upon annealing at 850°C for 4 h, the saturation magnetization (M_s) increased to ≈52 emu/g.     

Magnetic anisotropy in Ni–Si nanoparticle films produced by ultrashort pulsed laser deposition

Pulsed laser deposition (uPLD) in vacuum by means of subpicosecond laser pulses is a powerful, versatile technique for the production of films constituted by nanoparticles. On impact with the deposition substrate, the nanodrops ejected from the target assume an oblate ellipsoidal shape, solidifying with the major cross-section parallel to the substrate plane. These features and the difficult coalescence among the deposited nanoparticles are peculiar characteristics specific to the films obtained by uPLD. In the case of magnetic nanoparticle films obtained by means of this technique, a magnetization isotropy in the film plane and a hard magnetization axis orthogonal to the film plane are expected. This simple assumption, generated by the specific shape and orientation of the deposited nanoparticles, was not experimentally verified up to now. The present investigation represents the first experimental validation of magnetic anisotropy, determined by the peculiar morphology and topology of the constituent particles, in the uPLD Ni_xSi_100−x nanoparticle films. The in-plane isotropic magnetization behaviour, as well as the presence of a hard magnetization axis perpendicular to the sample surface were demonstrated for all investigated films. The difficult coalescence among the magnetic nanoparticles, even at high Ni volume fractions, is confirmed by the behaviour of the initial magnetization curve, typical for single-domain nanoparticles systems.     

Magnetic and infrared study of CO chemisorption on silica supported nickel-copper alloys

CO adsorption at room temperature on Ni-Cu alloys supported on SiO₂ is studied by two complementary techniques, infra-red spectroscopy and magnetic methods (saturation magnetization). The bond number between CO and the metallic surface calculated from magnetic data decreases from 1.8 to 1 as the Cu content increases. Two bands attributed to CO bonded to Ni are observed (the A band in the 2000–2050 cm^?1 region, and the B band in the 1950–1900 cm^?1 region). A small band assigned to CO bonded to Cu is also detected. As Cu content increases, the intensity of the B band decreases, and a noticeable and continuous frequency shift of the three bands is observed. Experimental results are fully accounted for assuming that: (i) two adsorbed species of CO on Ni, a monodentate and a bridged species (with small amounts of other multicentered species) are formed, as suggested by Eischens and Pliskin; (ii) dilution of Ni by Cu decreases the relative abundance of the bridged (and multicentered) species for some geometric reasons previously invoked by Soma-Noto and Sachtler; (iii) surface complexes are formed between CO and Ni; however Ni remains in its metallic state. The surface complex is sensitive to the electronic environment of the metallic atom, with a frequency shift of the three infra-red bands upon alloying as a consequence.     

Thermal stability of second generation carbosilane dendrimers with peripheral ammonia groups

Spinel nickel zinc ferrite nanowires were successfully prepared in mesoporous silica SBA-15 as a host matrix. The powder was annealed at a range of temperatures (500–900 °C) with heating rate 0.5 °C/min. The required NiZnFe₂O₄ phase was obtained at 700 °C. The specific surface area S_BET data revealed that the surface area of the mesoporous silica after annealing was decreased from 821 to 90 m²/g which indicated that the spinal ferrite fills the channels of mesoporous materials. The one-dimensional spinel nanostructures were characterized by X-ray diffraction, infrared spectroscopy, vibrating sample magnetometer, and transmission electron microscopy before and after a selective removal of the silica template in aqueous solution of NaOH or HF. The presence of SBA-15 lowers the formation temperature of nickel zinc ferrite nanowires compared to the corresponding bulk material. The magnetic properties revealed a high saturation magnetization level (~43 emu/g) for the Ni–Zn nanowires at 900 °C.     

Magnetic properties of Bi-doped Y3Fe5O12 nanoparticles

Bi³⁺ substituted garnet nanoparticles Y_3−xBi_xFe₅O₁₂ (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2 and 1.3) were fabricated by a sol–gel method and their crystalline structures and magnetic properties were investigated by using X-ray diffraction (XRD), IR spectroscopy, thermal gravity analysis–differential thermal analysis (TG-DTA), transmission electron microscope (TEM), Mössbauer spectroscopy and vibrating sample magnetometer (VSM). The XRD patterns of Y_3−xBi_xFe₅O₁₂ have only peaks of the garnet structure. From the results of VSM, it is shown that the saturation magnetization of sample is decreased with increasing the content of Bi ions. Meanwhile, it is observed that with the enhancement of the single magnetic domains surface spin effects, the saturation magnetization is raised as the particle size of samples is increased.     

Magnetic characteristics of MgFe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanoparticles obtained by glycine–nitrate synthesis

The magnetic properties of magnesium–iron spinel (MgFe₂O₄) powdered nanoparticles obtained by glycine–nitrate synthesis are investigated by X-ray phase analysis and the NMR method. According to the results of X-ray phase analysis, the average size of the crystalline part of nanoparticles of the powder under investigation is 45 ± 4 nm. Magnetization J is determined using the formula J = (B/μ0)–H, where B and H are the induction and strength of the magnetic field in the sample, which are measured by the NMR method. The magnetic characteristics of MgFe₂O₄ are as follows: specific saturation magnetization J_sat = 17.52 A m²/kg, specific residual magnetization J_r = 5.73 A m2/kg, coercive force H_c = 4600 A/m, and magnetic moment P_sat = 371 × 10^–20 A m² in the magnetic saturation state and P_r = 121 × 10^–20 A m² in the residual magnetization state.     

The tunable magnetic properties and microstructures of Co–Ni double-substituted M-type Casrla hexaferrites

M-type hexaferrites with Co²⁺ and Ni²⁺ions substituting for Fe³⁺ ions (Ca_0.30Sr_0.35La_0.35Fe_12.0−x(Co_0.5Ni_0.5)_xO₁₉, 0.0 ≤ x ≤ 1.0) were prepared by the traditional solid state method. X-ray diffractometer (XRD), field emission scanning electron microscopy (FE-SEM), physical property measurement system-vibrating sample magnetometer (PPMS-VSM) have been employed to study the microstructures and magnetic properties of hexaferrites. XRD patterns showed that the single magnetoplumbite phase is obtained if Co–Ni content (x) ≤ 0.4 and impurity phases are observed in the structure when Co–Ni content (x) ≥ 0.4. FE-SEM micrographs showed that the hexaferrites with hexagonal platelet-like grains is obtained. The saturation magnetization (M_s), remanent magnetization (M_r), M_r/M_s ratio, magneton number (n_B), coercivity (H_c), magnetic anisotropy field (H_a) and first anisotropy constant (K₁) decrease with increasing Co–Ni content (x) from 0.0 to 1.0. And our reported results with tunable H_c and M_r can be used for recording applications.     

Low temperature M?ssbauer and DC magnetization studies in nano-sized Ni substituted Co�CZn ferrites

Studies in the series of nano ferrites pertaining to the stoichiometry Ni_xCo_0.5???xZn_0.5Fe₂O₄, (0?? x ??0.5) show that contrary to the trend in bulk Co?CZn and Ni?CZn ferrites, the observed saturation magnetization of Ni?CZn ferrite (x = 0.5) is larger than Co?CZn ferrite (x = 0) and are due to finite size effects. This is evident from calculations which show that the magnetic particle sizes are 1.6 nm for Co?CZn ferrite and 2.4 nm for Ni?CZn whereas their average crystallite sizes are 5 to 3 nm respectively. Low temperature Mössbauer spectroscopic studies show that this can be attributed to an increase in an ordered core with the increase in Ni content which is reflected as a corresponding increase in the saturation magnetization.     

Effect of magnetic pulse treatment on the magnetic characteristics of yttrium–iron garnets

The effect weak (10–100 kA m^–1) low-frequency (10–20 Hz) pulsed magnetic fields have on the surface structure and magnetic characteristics of yttrium–iron garnet Y₃Fe₅O₁₂ is studied by means of electron and Mössbauer spectroscopy. A mechanism is proposed for the variation of saturation magnetization in Y₃Fe₅O₁₂ after magnetic pulse treatment. The mechanism is associated with the change in the spin state of iron ions localized in the tetrahedral sublattice.     

Synthesis of nanoporous TiO2 materials using a doubly surfactant system and applying them as useful adsorbents

Hydrothermal and non-hydrothermal nanoporous TiO₂ materials were synthesized via a doubly surfactant route by using cationic cetyltrimethylammonium bromide and anionic sodium dodecyl sulfate surfactants as the molecular template/structure directing agent. Hydrothermal treatment was performed for comparison. The bulk chemical and phase compositions, crystalline structures, particle morphologies, thermal stabilities and surface texturing were determined by means of X-ray powder analysis, SEM and N₂ sorptiometry. The nanoporous TiO₂ materials were found to have a spherical morphology with a diameter range of 50–200 nm and a high surface area (390 m² g^?1). Hydrothermal and non-hydrothermal nanoporous TiO₂ materials were applied for adsorption of heavy metal cations and the toxic organic compound, copper phthalocyanine, from water for evaluation of their adsorption properties. Both nanoporous TiO₂ materials were found to have similar adsorption capacities toward heavy metal cations and CuPc. Both hydrothermal and non-hydrothermal TiO₂ nanoporous materials were found to have very good potential for application as a new adsorbent especially for adsorbing heavy metal cations from wastewaters.     

Magnetic properties and formation of Sr ferrite nanoparticle and Zn,Ti/Ir substituted phases

Strontium hexaferrite nanoparticles are prepared by the chemical sol–gel route. Specific saturation magnetization σ_s and coercive field strength H_c are determined depending on the heat treatment of the gel and iron/strontium ratio in the starting solution. These ultrafine powders with single-domain behavior have specific saturation magnetization σ_s=74 emu/g and coercive field strength H_c=6.4 kOe. Experimental results show that it is necessary to preheat the gel between 400 and 500°C for several hours . It can prevent the formation of intermediate γ-Fe₂O₃ and help to obtain ultrafine strontium ferrite single phase with narrow size distribution at a low annealing temperature. Additionally, the magnetic properties of sol–gel derived strontium ferrite with iron substituted by Zn²⁺, Ti⁴⁺ and Ir⁴⁺ are discussed. For an amount of substitution 0<x⩽0.6, the (Zn, Ti)_x substituted strontium ferrite shows higher values of both coercive field strength and saturation magnetization than the (Zn, Ir)_x substituted phase.     

Finite size and surface effects on the magnetic properties of cobalt ferrite nanoparticles

Cobalt ferrite, CoFe₂O₄, nanoparticles in the size range 2–15 nm have been prepared using a non-aqueous solvothermal method. The magnetic studies indicate a superparamagnetic behavior, showing an increase in the blocking temperatures (ranging from 215 to more than 340 K) with the particle size, D _TEM. Fitting M versus H isotherms to the saturation approach law, the anisotropy constant, K, and the saturation magnetization, M _S, are obtained. For all the samples, it is observed that decreasing the temperature gives rise to an increase in both magnetic properties. These increases are enhanced at low temperatures (below ~160 K) and they are related to surface effects (disordered magnetic moments at the surface). The fit of the saturation magnetization to the T ² law gives larger values of the Bloch constant than expected for the bulk, increasing with decreasing the particle size (larger specific surface area). The saturation magnetization shows a linear dependence with the reciprocal particle size, 1/D _TEM, and a thickness of 3.7 to 5.1 Å was obtained for the non-magnetic or disordered layer at the surface using the dead layer theory. The hysteresis loops show a complex behavior at low temperatures (T ≤ 160 K), observing a large hysteresis at magnetic fields H > ~1000 Oe compared to smaller ones (H ≤ ~1000 Oe). From the temperature dependence of the ac magnetic susceptibility, it can be concluded that the nanoparticles are in magnetic interaction with large values of the interaction parameter T ₀, as deduced by assuming a Vogel–Fulcher dependence of the superparamagnetic relaxation time. Another evidence of the presence of magnetic interactions is the almost nearly constant value below certain temperatures, lower than the blocking temperature T _b, observed in the FC magnetization curves.     

Chemical synthesis of self-assembled Ni nanoparticles and understanding of nonmagnetic layer inside their surface

To chemically synthesize mono-dispersed and self-assembled Ni nanoparticles, it was important to find the best combination of a Ni precursor and a ligand. Our Ni nanoparticles exhibited a face-centered cubic structure and superparamagnetism at room temperature. The value of saturation magnetization for our Ni nanoparticles was largely different from that of bulk Ni. Because of the relationship between the diameter and saturation magnetization per volume, the number of atoms composing the Ni nanoparticle was correlated with magnetization. This result indicated that a magnetic core/shell structure inside a Ni nanoparticle was produced. The nonmagnetic layer, as a magnetic shell of the core/shell structure, was created due to the low crystallinity of Ni nanoparticles and was composed of amorphous Ni‒O states. As a result, antiferromagnetic spins arrayed in the Ni‒O states were broken. Disordered spins were generated, which eventually decreased the total magnetization of the Ni nanoparticles.     

Magnetic properties of nanoparticles of Prussian blue-based molecular magnets M 3[Cr(CN)6]2⋅zH2O (M=Fe, Co, and Ni)

The nanoparticles of Prussian blue-based molecular magnets, M ₃[Cr(CN)₆]₂?zH₂O (where M=Fe, Co, and Ni), prepared by a slow addition (drop by drop) of chemicals using the co-precipitation method, are investigated by means of X-ray diffraction, infra red spectroscopy and dc magnetization measurement techniques. The formation of nanoparticles has been confirmed by scanning electron microscopy, whereas the characteristic peak, observed in the range of 1900–2300 cm^?1 in the infrared spectra, corresponds to the CN stretching frequency of $\mbox{Cr}^{\mathrm{+III}}$ –CN– $M^{\mathrm{+II}}$ , and confirms the formation of Prussian blue compounds. The results, derived from the Rietveld refinement of X-ray diffraction patterns, reveal that all samples are nanocrystalline in nature with a face-centered cubic crystal structure of space group Fm3m. The particle size and the lattice constants decrease with an increasing atomic number of the transition metals (M=Fe, Co and Ni). The magnetization data show a magnetically ordered state of all nanoparticle samples with a low coercivity (except for the Fe₃[Cr(CN)₆]₂?zH₂O) as well as the remanent magnetization. In addition, by varying M with Fe, Co and Ni, the magnetic ordering temperature increases from ～12 to ～28 K, whereas the maximum magnetization and the coercive field decrease from ～14 to ～4.5 μ_B/f.u. and ～554 to ～22 Oe, respectively. The observed magnetization behavior has been discussed in terms of the structural changes due to the decreasing particle size as well as the varying nature of the metal ions.     

Surface modification of Ni and Co metal nanowires through MeV high energy ion irradiation

Nickel (Ni) and cobalt (Co) metal nanowires were fabricated by using an electrochemical deposition method based on an anodic alumina oxide (Al₂O₃) nanoporous template. The electrolyte consisted of NiSO₄ · 6H₂O and H₃BO₃ in distilled water for the fabrication of Ni nanowires, and of CoSO₄ · 7H₂O with H₃BO₃ in distilled water for the fabrication of the Co ones. From SEM and TEM images, the diameter and length of both the Ni and Co nanowires were measured to be ∼ 200 nm and 5–10 μm, respectively. We observed the oxidation layers in nanometer scale on the surface of the Ni and Co nanowires through HR–TEM images. The 3 MeV Cl²⁺ ions were irradiated onto the Ni and Co nanowires with a dose of 1 × 10¹⁵ ions/cm². The surface morphologies of the pristine and the 3 MeV Cl²⁺ ion-irradiated Ni and Co nanowires were compared by means of SEM, AFM, and HR–TEM experiments. The atomic concentrations of the pristine and the 3 MeV Cl²⁺ ion-irradiated Ni and Co nanowires were investigated through XPS experiments. From the results of the HR–TEM and XPS experiments, we observed that the oxidation layers on the surface of the Ni and Co nanowires were reduced through 3 MeV Cl²⁺ ion irradiation.     

Nickel nanoparticles deposited into an activated porous carbon: synthesis,microstructure and magnetic properties

Activated carbons (AC) are commonly used as efficient adsorbents to remove contaminants. The incorporation of a magnetic material into the AC could greatly enhance its manipulation through magnetic separation. However, the composite material will need to have sufficiently saturated magnetization, and as low as possible coercivity to be easily attracted by commercial permanent magnets. In this letter we report on the correlation between microstructure and magnetic behaviour of Ni nanoparticles (NPs) embedded in an amorphous activated porous carbon (Ni–AC). The Ni–AC powders have been synthesized by means of an easy and low‐cost procedure. The addition of sucrose during the preparation process provides effective protection in acid media. This Ni–AC composite has a microstructure composed of crystalline NPs with diameters in the range of 7–25 nm, and exhibits superparamagnetic behaviour at room temperature, with saturation magnetization values around 3 A m² kg^–1 under applied magnetic fields of 200 mT. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of nanoparticles on the magnetic properties of Mn–Zn soft ferrite

In this paper, the effect of nanostructures on the magnetic properties like the specific saturation magnetization (σ_S) and the coercivity (H_C) for Mn_0.4Zn_0.6Fe₂O₄ ferrite prepared by the co-precipitation method has been presented. We have shown by means of X-ray diffraction that the resulting ferrite is made up of nanoparticles, and that the average size of these nanoparticles calculated with the Scherrer formula depends upon the sintering temperature. When the sintering temperature is increased from 500 to 900 °C, the average nanoparticle diameter varies from 19.3 to 36.4 nm. The nanoparticle phase is further confirmed by scanning electron microscopy (SEM). Both results are found to be in good agreement. The magnetic properties are explained on the basis of the single-domain and multi-domain theory.     

Structure,magnetic and microwave properties of FeNi invar nanoparticles obtained by electrical explosion of wire in different preparation conditions

Magnetic nanoparticles (MNPs) of close to invar (Fe_0.635Ni_0.365) composition were prepared by the electrical explosion of wire using different conditions to insure different values of overheating rates. X-ray diffraction, transmission electron microscopy, low temperature nitrogen adsorption, magnetic and microwave measurements were used for the characterization of MNPs. Increase of the energy injected into the wire led to increase of the specific surface (S_sp) of the produced MNPs from 4.6 to 13.5 m²/g. The fabricated MNPs were spherical and weakly aggregated with the average weighted diameter in the range of 54–160 nm depending on the S_sp. The phase composition of FeNi MNPs consists of two solid solutions of Ni in α-phase and γ-phase lattices. The increase of the energy injected into the wire leads to increase of the α-phase from 5 to 10 wt% as the injected energy raised from 0.8 to 2.5 times the sublimation energies of the wire material. Comparative analysis of structure magnetic and microwave properties showed that the obtained MNPs are important magnetic materials with high saturation magnetization and significant zero field microwave absorption which can be expected to lead to important technological applications.     

Preparation and electrochemical performances of nanoporous/cracked cobalt oxide layer for supercapacitors

Nanoporous/cracked structures of cobalt oxide (Co₃O₄) electrodes were successfully fabricated by electroplating of zinc–cobalt onto previously formed TiO₂ nanotubes by anodizing of titanium, leaching of zinc in a concentrated alkaline solution and followed by drying and annealing at 400 °C. The structure and morphology of the obtained Co₃O₄ electrodes were characterized by X-ray diffraction, EDX analysis and scanning electron microscopy. The results showed that the obtained Co₃O₄ electrodes were composed of the nanoporous/cracked structures with an average pore size of about 100 nm. The electrochemical capacitive behaviors of the nanoporous Co₃O₄ electrodes were investigated by cyclic voltammetry, galvanostatic charge–discharge studies and electrochemical impedance spectroscopy in 1 M NaOH solution. The electrochemical data demonstrated that the electrodes display good capacitive behavior with a specific capacitance of 430 F g^?1 at a current density of 1.0 A g^?1 and specific capacitance retention of ca. 80 % after 10 days of being used in electrochemical experiments, indicating to be promising electroactive materials for supercapacitors. Furthermore, in comparison with electrodes prepared by simple cathodic deposition of cobalt onto TiO₂ nanotubes(without dealloying procedure), the impedance studies showed improved performances likely due to nanoporous/cracked structures of electrodes fabricated by dealloying of zinc, which provide fast ion and electron transfer routes and large reaction surface area with the ensued fast reaction kinetics.     

Effects of Fe2+ content in raw materials on Mn–Zn ferrite magnetic properties

The magnetic properties of Mn–Zn ferrite such as initial permeability, saturation magnetization, Curie temperature, resistivity and power loss are affected greatly by the Fe²⁺ content in the raw materials. The experimental results show that low resistivity (ρ) and high eddy current loss (P_e) are induced by the superfluous Fe²⁺ content in the raw materials; the scant Fe²⁺ content in the raw materials will increase hysteresis loss (P_h) and decrease Curie temperature (T_c), saturation magnetization (M_s) and initial permeability (μ_i).