Temperature dependence of magnetic properties of NiFe2O4 nanoparticles embeded in SiO2 matrix

Spinel ferrite NiFe₂O₄ nanoparticles (?25 nm) in SiO₂ matrix were prepared by sol–gel method. The phase and average crystallite size of the samples were determined by X-ray diffraction method and the particle size distributions were studied by a transmission electron microscope. Magnetic properties of the samples were investigated with different ferrite particle sizes and at various temperatures down to 10 K. Superparamagnetic properties were observed at room temperature when the particle size is less than 10 nm.In superparamagnetic state, the field dependence of magnetization follows Langevin function which was originally developed for paramagnetism. The effective anisotropy constant K_eff is found to increase significantly with the decrease in particle volume and an order of magnitude higher than that of the bulk samples when the particle size is below 5 nm due to the dominance of surface anisotropy. In case of nanosized systems, the effect of size reduction on the law of approach to saturation has also been studied in detail.     

Temperature dependence of electron magnetic resonance and magnetization in NiO nanorods

For NiO nanorods of 5 nm diameter prepared by sol-gel technique, variations of the magnetization M with temperature T (5-370 K) and magnetic field H up to 55 kOe are reported. Also, temperature variations of the EMR (electron magnetic resonance) parameters (intensity I₀, linewidth ΔH and resonance field H_r) of an observed line due to uncompensated spins are followed for The M vs. H and T variations yield a blocking above which the data fits modified Langevin function with magnetic moment μ_p?1240 μ_B/particle. For the EMR line, I₀ decreases rapidly for T<T_B, and the line broadens and shifts to lower H with lowering T, following the lineshift δH_r=(ΔH)ⁿ with n?2.8. This is close to the value of n=3 expected for randomly oriented particles.     

Amine-terminated water-dispersible FePt nanoparticles

Chemically disordered face-centered cubic FePt nanoparticles (NPs) were synthesized via pyrolysis of iron(III)ethoxide and platinum(II)acetylacetonate. The surface ligands of these NPs were then exchanged from oleic acid to 2-aminoethanethiol (AET). The AET-capped FePt NPs were found to be well dispersed in water when pH<8, and the zeta potential was more than +30 mV when pH?7.     

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)     

Particle size effect on phase and magnetic properties of polymer-coated magnetic nanoparticles

Polymer-coated magnetic nanoparticles are hi-tech materials with ample applications in the field of biomedicine for the treatment of cancer and targeted drug delivery. In this study, magnetic nanoparticles were synthesized by chemical reduction of FeCl₂ solution with sodium borohydride and coated with amine-terminated polyethylene glycol (aPEG). By varying the concentration of the reactants, the particle size and the crystallinity of the particles were varied. The particle size was found to increase from 6 to 20 nm and the structure becomes amorphous-like with increase in the molar concentration of the reactant. The magnetization at 1 T field (M_1T) for all samples is > 45 emu/g while the coercivity is in the range of 100-350 Oe. When the ethanol-suspended particles are subjected to an alternating magnetic field of 4 Oe at 500 kHz, the temperature is increased to a maximum normalized temperature (3.8 °C/mg) with decreasing particle size.     

Superparamagnetic FePt nanoparticles as excellent MRI contrast agents

Chemically disordered face-centered cubic (fcc) FePt nanoparticles (NPs) with a mean diameter of 9 nm were synthesized via pyrolysis of iron(III) ethoxide and platinum(II) acetylacetonate. The surface ligands of these NPs were then exchanged from oleic acid to tetramethylammonium hydroxide (TMAOH) to measure the longitudinal (T₁) and transverse (T₂) proton relaxation times of aqueous dispersion of FePt NPs. Magnetic resonance relaxometry reveals that TMAOH-capped FePt NPs have a higher T₂-shortening effect than conventional superparamagnetic iron oxide NPs, indicating that fcc-phase FePt NPs might be superior negative contrast agents for magnetic resonance imaging.     

Synthesis and magnetic properties of cobalt ferrite (CoFe2O4) nanoparticles prepared by wet chemical route

Magnetic nanoparticles of cobalt ferrite have been synthesized by wet chemical method using stable ferric and cobalt salts with oleic acid as the surfactant. X-ray Diffraction (XRD) and Transmission Electron Microscope (TEM) confirmed the formation of single-phase cobalt ferrite nanoparticles in the range 15–48 nm depending on the annealing temperature and time. The size of the particles increases with annealing temperature and time while the coercivity goes through a maximum, peaking at around 28 nm. A very large coercivity (10.5 kOe) is observed on cooling down to 77 K while typical blocking effects are observed below about 260 K. The high field moment is observed to be small for smaller particles and approaches the bulk value for large particles.     

Synthesis and magnetic characterization of nickel ferrite nanoparticles prepared by co-precipitation route

Magnetic nanoparticles of nickel ferrite (NiFe₂O₄) have been synthesized by co-precipitation route using stable ferric and nickel salts with sodium hydroxide as the precipitating agent and oleic acid as the surfactant. X-ray diffraction (XRD) and transmission electron microscope (TEM) analyses confirmed the formation of single-phase nickel ferrite nanoparticles in the range 8-28 nm depending upon the annealing temperature of the samples during the synthesis. The size of the particles (d) was observed to be increasing linearly with annealing temperature of the sample while the coercivity with particle size goes through a maximum, peaking at ∼11 nm and then decreases for larger particles. Typical blocking effects were observed below ∼225 K for all the prepared samples. The superparamagnetic blocking temperature (T_B) was found to be increasing with increasing particle size that has been attributed to the increased effective anisotropy energy of the nanoparticles. The saturation moment of all the samples was found much below the bulk value of nickel ferrite that has been attributed to the disordered surface spins or dead/inert layer in these nanoparticles.     

Intrinsic magnetic properties of In2O3 and transition metal-doped-In2O3

This paper reports on the influence of the sintering temperature and atmosphere and transition-metal doping on the magnetic properties of nanocrystalline and bulk In₂O₃. Undoped nanocrystalline In₂O₃ is diamagnetic whatever the sintering temperature and atmosphere. All single-phase transition-metal-doped In₂O₃ samples are paramagnetic, with a paramagnetic effective moment originating from weakly interacting transition metal ions. No trace of ferromagnetism has been detected even with samples sintered under argon, except extrinsic ferromagnetism for samples with magnetic dopant concentrations exceeding the solubility limit.     

Effect of anions on the phase stability of γ-FeOOH nanoparticles and the magnetic properties of gamma-ferric oxide derived from lepidocrocite

The effect of anions such as Cl⁻, SO₄²⁻, and HPO₄²⁻ on the phase stability of FeOOH (α or γ) during precipitation is investigated. Oxidation of Fe(OH)₂·xH₂O from FeCl₂ solution with high Cl⁻ concentration ([Cl⁻]/[Fe]=R_Cl≥8) or (NH₄)₂Fe(SO₄)₂ (FAS) with [HPO₄²⁻]/[Fe]=R_P≥0.02 yields phase-pure γ-FeOOH. In the medium ranges of R_Cl and R_P, mixed phases of α-FeOOH and γ-FeOOH are obtained. Replacement of OH⁻ by Cl⁻ with the bridging cations or strongly bonded HPO₄²⁻ ions in the matrix of the intermediate phase (Fe_x²⁺Fe_y³⁺(OH)_2x+2y−nz·xH₂O(A)_zⁿ⁻, where A is anions such as Cl⁻, SO₄²⁻, HPO₄²⁻, etc.), promoted the lower density γ-FeOOH. However, the particles are less developed and have poor crystallinity as evidenced from transmission electron microscope and thermogravimetry-differential thermal analysis of the precipitates. Whereas, monophasic, uniformly sized, nano-lath shaped particles with high aspect ratio >10 are obtained when morphology-controlling cation additives such as Pt⁴⁺, Pd²⁺ or Rh³⁺ are present in FeCl₂ (R_Cl≥8) solution. Preferential adsorption of additives on (0k0) and (h00) planes limits the growth in the perpendicular directions leading to high aspect ratios. The effect of these additives are suppressed by the phosphate ion, a strong complexing ligand, giving rise to fibrous aggregate with the length of individual particles as small as 10-30 nm. While most of the Cl⁻ ion is removed from the final precipitates on washing, phosphate remained as HPO₄²⁻ as evidenced from IR absorption spectra. Maghemite obtained by dehydroxylating γ-FeOOH contains randomly distributed micropores bringing in the relaxation effects of spins on the surface atoms as deciphered from Mössbauer spectroscopy. This leads to the low σ_s (44-48 emu/g) and H_c (120-130 Oe) for γ-Fe₂O_3−δ particles. Whereas nearly pore-free single crystalline particles obtained by reduction followed by reoxidation has high value of σ_s (73 emu/g) and H_c (320 Oe), which decreases to 30 emu/g and 75 Oe, respectively, for nanoparticles obtained from phosphate stabilized lepidocrocite. The mobility of iron ions and counter mobility of vacancies during the topotactic transformation of γ-FeOOH to magnetite to γ-Fe₂O_3−δ renders the particles pore-free.     

A study of nanosized Ni substituted Co–Zn ferrite prepared by coprecipitation

Nanoparticles of Ni_0.25Co_0.25Zn_0.5Fe₂O₄ with different particle sizes were synthesized by chemical coprecipitation technique. The prepared nanoparticles were characterized and studied by X-ray diffraction, room temperature ⁵⁷Fe Mössbauer spectroscopy and DC magnetization. Increase in annealing temperature leads to inversion of cations from their normal configurations which results in an increase in the hyperfine fields at both tetrahedral A and octahedral B sites. The decrease in saturation magnetization values with an increase in annealing temperature can be attributed to the variation in proportions of superparamagnetic phase and also the redistribution of cations in these ferrite nanoparticles. Variation of coercivity with annealing temperatures can be explained in terms of the size effect.     

High-frequency magnetic properties and attenuation characteristics for barium ferrite composites

High-frequency magnetic properties and attenuation characteristics for barium-ferrite/epoxy composites have been studied. The methods for increasing μ′ and μ″ and controlling f_R, including ion substitution, doping of small amount of oxides, effect of damping, as well as the modification of particle sizes and shapes, are introduced. The results show that the composites are potential candidates for use as electromagnetic (EM) attenuation materials with low reflectivity and broad bandwidth at 2-18 GHz.     

Magnetic properties and microstructure of cobalt nanoparticles in a polymer film

Superparamagnetic properties of self-aggregated cobalt nanoparticles in the perfluorinated sulfo-cation membrane (MF-4SK) prepared by ion-exchange method were investigated by transmission electron microscopy (TEM) and superconducting quantum interference device (SQUID) magnetometry at various temperatures. Our experimental results show that cobalt nanoparticles in MF-4SK exhibit superparamagnetic properties above the blocking temperature (T_B), which varies from ∼80 to ∼300 K depending on the cobalt concentration at 100 Oe applied field. The average particle radius of 3.8 nm inferred from Langevin function fit for the concentration of 7.8×10¹⁹ cobalt atoms per 1 g of polymer film is in good agreement with TEM observation. This experimental evidence suggests that cobalt nanoparticles in the polymer film obey a single-domain theory. The results are discussed in the light of current theory for the superparamagnetic behavior of magnetic nanoparticles.     

Paramagnetic susceptibility of ferrite and cementite obtained from ab initio calculations

Paramagnetic susceptibility of BCC ferrite (α$\mathrm{\alpha }$-Fe) and of cementite (Fe₃C), was calculated in a temperature range above the Curie point, using a density functional-based method in conjunction with statistical approximations. The electronic structure and the magnetic energy of both systems were calculated using the full potential linear augmented plane wave (FPLAPW) method as implemented in the Wien2k Code. The temperature effect was captured by introducing the Boltzmann statistical distribution to describe the orientation perturbation of the magnetic moments caused by thermal agitation. The magnetic moment was calculated in the temperature range from 1045 to 1175 K for alpha-iron and from 540 to 640 K for cementite. The modeling was performed for applied magnetic fields ranging from 1–2 T. The main assumption was that the effect of temperature and that of the applied magnetic field can be decoupled and, consequently, be treated separately. The calculated moments were used to estimate the paramagnetic susceptibility according to the linear relation χ=M/B$\chi =M/B$ well established for paramagnetic systems. The calculated values were compared with published experimental measurements for iron and with values obtained from the Curie–Weiss law for cementite.     

Magnetization of sol-gel prepared zinc ferrite nanoparticles: Effects of inversion and particle size

Nanoparticles of ZnFe₂O₄ have been prepared by using sol-gel method in two different mediums (acidic and basic) in order to observe the influence of the medium on the magnetic properties of the obtained nanoparticles. X-ray diffraction and Mössbauer studies of these samples show the presence of single-phase spinel structure. The average size of the particles as determined by X-ray diffraction increases with the annealing temperature from 18 to 52 nm. With the increase in particle size, magnetization decreases while the magnetization blocking temperature increases. Magnetization studies show that the samples prepared in basic medium have more ferrimagnetic nature as compared to those prepared in acidic medium. We understand this increase in magnetization as reflective of the increased degree of inversion (transfer of Fe³⁺ ions from octahedral to tetrahedral sites) in the particles of smaller size unit cells. From lattice parameter calculations on different particles it is determined that inversion is more favorable in the particles prepared in a basic medium than in the acidic medium due to the smaller cell size in the former.     

Equilibrium magnetic properties of dipolar interacting ferromagnetic nanoparticles

We use Monte Carlo simulations to study the influence of dipolar interaction on the equilibrium magnetic properties of monodisperse single-domain ferromagnetic nanoparticles. Low field magnetizations simulated in zero field cooling (ZFC)/field cooling (FC) procedures and field-dependent magnetization curves above the blocking temperatures show strong dependence on the concentration and the spatial arrangement (cubic or random) of the magnetic particles. The field-dependent magnetizations can not be simply described by the T^* model at relative low temperatures due to the interplay between anisotropy and dipolar interactions, as well as the spatial arrangement effect.     

Interaction and size effects in magnetic nanoparticles

Magnetic nanoparticles dispersed as a ferrofluid with volumetric concentrations in the range 0.4 to 10% and sizes ranging from 59–77 Å have been studied via magnetic measurements at room and low temperatures. Remanence measurements have been used to determine switching and coupling effects. Particle size and concentration effects have been investigated and we find that the samples with higher concentrations have larger coercivities than expected due to coupling effects. Interactions have been found to be demagnetising overall as expected for dipolar interacting systems. Surface effects become evident below 10 K when thermal effects are weak.     

Magnetic properties of nanostructured MnZn ferrite

Mn_0.5Zn_0.5Fe₂O₄ nanoparticles (10-30 nm) have been prepared via mechanochemical processing, using a mixture of two single-phase ferrites, MnFe₂O₄ and ZnFe₂O₄. SQUID measurements (field-cooled magnetization curves and hysteresis loops) were performed to follow the mechanically induced evolution of the MnFe₂O₄/ZnFe₂O₄ mixture submitted to the high-energy milling process. The resulting single MnZn nanoferrite phase was characterized by SQUID (M-H curve), Faraday balance (M-T curve) and transmission electron microscopy. The magnetic characteristics of the mechanosynthesized material were compared with those of bulk Mn_0.5Zn_0.5Fe₂O₄. It was found that the saturation magnetization of nanostructured Mn_0.5Zn_0.5Fe₂O₄ (87.2 emu/g) is lower than that of the bulk Mn_0.5Zn_0.5Fe₂O₄, but, the Néel temperature of the sample (583 K) is higher than that of the bulk Mn_0.5Zn_0.5Fe₂O₄.     

Magnetic behavior of core–shell particles

We have prepared composite magnetic core–shell particles using the process of soap-free emulsion polymerization and the co-precipitation method. The shell of the synthesized composite sphere is cobalt ferrite (CoFe₂O₄) nanoparticles and the core consists of poly(styrene-co-methacrylic acid) polymer. The mean crystallite sizes of the coated CoFe₂O₄ nanoparticles were controlled in the range of 2.4–6.7 nm by the concentration of [NH₄⁺] and heated temperature. The magnetic properties of the core–shell spherical particles can go from superparamagnetic to ferromagnetic behavior depending on the crystalline sizes of CoFe₂O₄.