Influence of maghemite concentration on magnetic interactions in maghemite/PTT-block-PTMO nanocomposite

Temperature dependence of dc magnetization and ferromagnetic resonance (FMR) of two samples containing γ-Fe₂O₃ (maghemite) magnetic nanoparticles dispersed at low concentration (0.1 and 0.3 wt%) in a nanocomposite based on a poly(ether–ester) multiblock copolymer (PTT-block-PTMO) matrix was investigated. The polymer filler was in a powder form consisting of small-sized magnetic nanoparticles arranged in agglomerates 2–3 μm long and 100 nm thick. The studied samples were characterized by SEM spectroscopy. The SEM showed that the concentration of magnetic nanoparticles was homogenous in both samples The temperature dependence of the dc magnetization revealed that the blocking was about 100 K and the ZFC (zero-field cooling) mode at low magnetic fields uncovered the presence of magnetic interactions between magnetic nanoparticles depending on the properties of the matrix. FMR measurements were carried out in the temperature range 4.2–300 K. An intense resonance absorption line attributed to γ-Fe₂O₃ nanoparticles was recorded with a slightly asymmetric lineshape. At room temperature the resonance line was centered at H_r = 3241(2) and 3253(2) G, with linewidths of ΔH = 1069(1) and 1070(1) G for samples with concentrations of 0.1 and 0.3 wt%, respectively. All FMR parameters showed an anomalous behavior at matrix critical temperatures. It was shown that the difference in concentration of magnetic nanoparticles could be responsible for the observed differences in the thermal behavior of the FMR spectra.     

Mössbauer study of multiphase iron oxide composites

A representative nanocomposite made of ferrimagnetic γ-Fe₂O₃ and antiferromagnetic α-Fe₂O₃ nanoparticles with estimated weight fractions of ～90 and ～10%, respectively, and grown in transparent silica xerogels has been characterized by Mössbauer spectroscopy with respect to the temperature. The changes of the hyperfine parameters of the different subspectral components have been compared with other macroscopic magnetic and structural measurements. This comparison has allowed us to identify two different superparamagnetic transitions, located at ～50 K and at ～250 K, as well as the phases involved in each transition.     

Preparation of optical axial GRIN components through migration of charged amorphous ZrO2 nanoparticles inside an organic–inorganic hybrid matrix by electrophoresis

A novel sol–gel synthesis route for the preparation of a transparent organic–inorganic nanocomposite was developed by combining methacrylic acid (MA) stabilized, amorphous ZrO₂ nanoparticles, which were synthesized by the sol–gel process, with an organic–inorganic dodecandioldimethacrylate (DDDMA)/3-methacryloxypropyl trimethoxysilane (MPTS) hybrid matrix. The average hydrodynamic particle size was determined to be approximately 6 nm by photon correlation spectroscopy. HR-TEM micrographs present irregular shaped zirconia particles with diameters up to 3 nm. Nearly solvent-free nanocomposites with zirconium (Zr) contents up to 15.2 mol% were synthesized and photochemically cured to transparent crack-free bulks. The surface charged nanoparticles in 1-propanol had an electrophoretic mobility of 0.017 (μm cm)/(V s), measured by Laser Doppler Anemometry (LDA) and a refractive index n_e of ⩾1.648 ± 0.007 determined by spectroscopic ellipsometry. After filling the nanocomposite into a linear electrophoresis cell (1 × 1.6 × 0.8 cm³), positively charged high refractive nanoparticles migrated through the low refractive hybrid matrix toward the cathode by the application of an electric potential difference of 2 kV/cm for 96 h. A 67% increase in Zr over a distance of 8 mm between the cathode and anode was observed by high-resolution scanning electron microscopy (HR-SEM) and energy dispersive X-ray spectroscopy (EDXS).     

Magnetic properties of the micro-silica/cement matrix with carbon-coated cobalt nanoparticles and free radical DPPH

Samples of micro-silica/cement containing iron oxide, Fe₂O_3, and doped with carbon-coated cobalt nanoparticles and free radical DPPH were prepared and studied by the magnetic resonance method. The concrete’s main components (silica and cement) produced very complicated FMR/EPR (ferromagnetic and electron paramagnetic resonance) spectra. The temperature dependence of the FMR/EPR spectra was recorded in the 90–300 K temperature range. The cement/micro-silica matrix produced a very broad FMR line originating from iron oxide particles and two EPR lines originating from iron(III) ions in the crystal field of low-symmetry (centered at g_eff ～ 4.3) and from manganese(II) ions (g_eff ～ 2) of hyperfine structure. Additionally, a very narrow line and a very broad EPR/FMR line were registered and, respectively, attributed to DPPH and cobalt nanoparticles. The isolated paramagnetic iron(III) and manganese(II) centers displayed increasing intensity of the EPR spectra with decreasing temperature, while no influence of the magnetic nanoparticles was observed. The intensity of the FMR spectrum of iron oxide decreased strongly and the resonance field was effectively shifted toward low magnetic fields with decreasing temperature. The observed FMR behavior is similar to what was registered for iron oxide magnetic nanoparticles. The introduction of magnetic nanoparticles influenced the EPR spectrum of the free radical DPPH significantly: its intensity decreased above 260 K and increased slightly below this temperature, while the resonance field changed with decreasing temperature. This behavior may be associated with the porous state of cement and/or the reaction of the multi-component magnetic system. The FMR/EPR method could be very useful for the characterization of matrices containing small amounts of magnetic nanoparticles.     

Structural and optical spectroscopy of LiNbO3:Tm nanocrystals embedded in a SiO2 glass matrix

Transparent SiO₂:Li₂O:Nb₂O₅ glass doped with Tm³⁺ has been prepared by the sol–gel method, and heat-treated in air (HT) at temperatures between 500 and 800 °C. X-ray diffraction (XRD) patterns and Raman spectroscopy show SiO₂ and LiNbO₃ phases in samples HT above 650 °C, and a NbTmO₄ phase for T > 750 °C. The XRD SEM analysis show increasing particle size and number with the increase of HT temperature. Intra-4f¹² transitions due to Tm³⁺ ion dispersed in the matrix are observed in samples with T > 650 °C. The luminescence is dominated by the ¹G₄ → ³F₄ (～650 nm), ¹D₂ → ³F₃ (～780 nm), ³H₄ → ³H₆ (～800 nm), ³H₅ → ³H₆ (～1200 nm) and ³H₄ → ³F₄ (～1500 nm) transitions under resonant excitation to the ion levels.     

Pressure-induced suppression of ferromagnetic phase in LaCoO3 nanoparticles

Magnetic and structural properties of nanocrystalline LaCoO₃ with particle size ranging from 25 to 38 nm, prepared by the citrate method, were investigated. All nanoparticles exhibit ferromagnetism below T_C ≈ 85 K. It was found that the unit-cell volume increases monotonically with decreasing particle size and ferromagnetic (FM) moment increases simultaneously with lattice expansion, whereas T_C remains nearly unchanged. It appears that both magnetic and structural properties of LaCoO₃ nanoparticles are size-dependent due to the surface effect. On the other hand, an applied pressure suppresses strongly the FM phase leading to its disappearance at ～11 kbar. Remarkably, the T_C does not change visibly under pressure. Our data reveal that the ferromagnetism in LaCoO₃ nanoparticles, likely related to the intermediate-spin (IS) Co³⁺ state, is simply controlled by the unit-cell volume. Within this scenario, the FM coupled IS states appear/disappear with expanding/compressing the lattice and/or Co–O bonds.     

Low-temperature electron paramagnetic resonance in silver-iron oxide nanoparticles

Water in oil microemulsion technique has been used to obtain silver nanoparticles with a mean size of 7 nm surrounded by a matrix of 2 nm γ-Fe₂O₃ nanoparticles. Magnetic measurements of the sample exhibit a cusp in the ZFC curve at 50 K, which corresponds to the blocking temperature. A detailed study of the thermal evolution of EPR spectra has been performed in the samples. It has been shown that the linewidth, the resonance field and the intensity of the EPR line show different behaviour near the blocking temperature.     

Transport properties near the magneto/structural transition of Tb5Si2Ge2

We report high resolution measurements of the electrical resistivity (ρ, dρ/dT) and thermopower (S, dS/dT) measurements near the magnetic and structural transition of the layered Tb₅Si₂Ge₂ compound, which are fairly close but not fully coupled. The analysis of the transport properties confirms a split magneto/structural transition, with T_S ～ 97 K and T_S ～ 107 K for the structural transition (on cooling and heating respectively; 1st-order transition). The magnetic transition occurs only at T_C ～ 112 K and without hysteresis (2nd-order transition). The magnetic critical behavior of resistivity is analyzed, obtaining an almost classical mean field exponent (α ～ 0.59) for T > T_C. For the structural phase, and below T_S, we obtain a rather different exponent (α ～ 1.06).     

Carbon covered magnetic nickel nanoparticles embedded in PBT-PTMO polymer: Preparation and magnetic properties

Fine particles of a face-centered-cubic phase of Ni covered with a graphite layer were prepared and embedded in a PBT-block-PTMO polymer at a concentration of 0.1 wt%. The mean crystalline size of Ni varied from 8 to 30 nm. A magnetic resonance study of the obtained nanocomposites was carried out in the 4–300 K temperature range using an electron paramagnetic resonance spectrometer. An almost symmetrical and very intense magnetic resonance line was recorded for all the investigated samples. The resonance line was centered at g = 2.253(2) (the resonance field H_r = 3003(1) Gs) and had a peak-to-peak linewidth ΔH_pp = 693(2) Gs at room temperature. The amplitude of the resonance line increased with a temperature increase in the low temperature range (T < 40 K) and in the high temperature range (T > 100 K) but was constant at intermediate temperatures. The resonance field H_r decreased and linewidth ΔH_pp increased as the temperature decreased from room temperature what was similar to the changes observed for other systems of nanoparticles. The thermal gradient of the resonance field, ΔH_r/ΔT, strongly depended on the temperature range. The temperature shift of the resonance field and the linewidth were analyzed in terms of the demagnetizing fields of nonspherical agglomerates. A strong change of linewidth and resonance field was registered below 40 K due to the freezing of the spin system’s dynamical magnetic fluctuations. A comparison was made of the results obtained on the Ni/C with the previous measurements on γ-Fe₂O₃ nanoparticles embedded in a copolymer.     

Synthesis,characterization and application of magnetic-zirconia nanocomposites

In the present study, a magnetic-zirconia nanocomposite was successfully synthesized by a single-step co-precipitation method. The as-prepared nanocomposite was characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, powder X-ray diffraction, and nitrogen sorption measurements. The ultimate material was found to be Fe₃O₄–ZrO₂ nanoparticles with average diameter of 80 nm, high surface area up to 166 m²/g, and strong magnetic property. The application of this new nanocomposite was herein demonstrated by the adsorption of ethyl methylphosphonic acid, a degradation product of nerve agent in water, followed by mass spectrometry detection. Excellent adsorption could be observed, indicating the as-synthesized material was effective to remove phosphonic acid compound from water. Apart from adsorption, the Fe₃O₄–ZrO₂ nanocomposite is promising in various applications such as catalysis and bioseparation.     

Synthesis and characterization of CoFe2O4–PVP nanocomposites

Cobalt ferrite–poly(N-vinyl-2-pyrrolidone) nanocomposites were prepared by drying a dispersion of cobalt ferrite (CoFe₂O₄) nanoparticles and poly(N-vinyl-2-pyrrolidone). Magnetic measurements indicate a superparamagnetic behavior. Zero-field-cooling magnetization experiments at 100 Oe show different trends depending on the CoFe₂O₄ nanoparticles size. For the smaller ones (3.9 nm), the blocking temperatures shift to lower temperatures with increasing concentration; however, this shift is not observed for the larger ones (6.6 nm). These differences can be related to the anisotropy constant of the CoFe₂O₄ nanoparticles and the interparticle dipolar interactions.     

Alteration of the optical properties of poly 9,9′-dioctylfluorene by TiO2 nanocrystalline

A poly 9,9′-dioctylfluorene (PFO)/TiO₂ composite was successfully prepared by adsorbing the polymer on the surface of a TiO₂ nanocrystalline film. The photoluminescence (PL) spectrum of the obtained nanocomposite film showed 28 nm of red-shift, compared to that of the bulk polymer film. The maximum of the PL emission peaks of the pure bulk polymer occurred at 442 nm, while the maximal emission for the nanocomposite film appeared at 470 nm. The interaction between the conjugated polymer chains and the nanopores of the TiO₂ film played a key role in the resulting red-shift.     

Effect of Nb in the nanocrystallization and magnetic properties of FeNbBCu amorphous alloys

The crystallization of melt-spun Fe_79?xNb_5+xB₁₅Cu₁ (x = 0, 2, 4) ribbons has been studied by differential scanning calorimetry and X-ray diffraction. A primary crystallization of bcc-Fe nanoparticles embedded in an amorphous matrix, followed by the precipitation of metastable borides from the residual matrix at higher temperatures is observed. The characteristic temperatures of crystallization events change with Nb concentration. The results obtained from thermal and structural characterization are related to the magnetic properties of the sample. A dependence of the magnetic behavior with the Fe/Nb content in the alloy is also unveiled. The decrease of Nb content in the alloy leads to an enhancement of both the saturation polarization and the Curie temperature due to variations in the exchange coupling between Fe atoms. However, the maximum values of magnetic entropy change do not vary appreciably among the three amorphous alloys. In nanocrystalline samples the amount of the nanocrystalline transformed fraction seems to be the main reason for the change in the saturation polarization of the sample.     

Synthesis of silver-coated magnetite nanoparticles

In this paper we describe the preparation of relatively monodisperse silver-coated Fe₃O₄ nanoparticles by a two-step procedure. Fe₃O₄ nanoparticles of 9 ± 2 nm in size were first prepared in microemulsions. They were subsequently coated with silver using glucose as reducing agent. The presence of a relatively homogeneous coating of ≈2 nm was confirmed by transmission electron microscopy and X-ray diffraction. A preliminary study of the magnetic properties shows a large decrease of the magnetization for the coated magnetite nanoparticles in comparison with the uncoated ones.     

FMR and DSC study of maghemite nanoparticles in PMMA polymer matrix

Nanocomposite samples of poly(methyl methacrylate) (PMMA) polymer with a γ-Fe₂O₃ (maghemite) filler have been synthesised and studied by ferromagnetic resonance (FMR) and differential scanning calorimetry (DSC) methods. Two types of samples have been investigated: containing 5 wt%, 10 wt% of a maghemite filler. DSC measurements have revealed an increase in the glass transition temperature T_g and a decrease in the heat capacity c_p with an increase in the concentration of nanoparticles. A FMR study in the 4–300 K temperature range has shown the presence of an asymmetric spectrum that has been analyzed in terms of two Gaussian-shaped components arising from the assumed magnetic anisotropy of the nanoparticles. The FMR investigation has exposed the temperature range of a superparamagnetic regime (60–290 K) and the blocking temperature of T_B ～ 60 K. In that range a shift in the resonance line δH_r and the linewidth ΔH is related by δH_r ～ (ΔH)ⁿ, where n = 2.79 indicates a fair amount of disorder in the maghemite system. An analysis of the FMR spectra in terms of two component lines has shown the importance of the dipole–dipole interaction for higher concentrations of maghemite and for T > 220 K.     

Nd5Fe64B23Mo4Y4 bulk nanocomposite permanent magnets produced by crystallizing amorphous precursors

The glass forming ability and magnetic properties of Nd₅Fe_68 ? xB₂₃Mo₄Y_x (x = 0, 2, 4, 6) alloys prepared by copper mold casting technique have been studied. Amorphous rods with a diameter of 2 mm were obtained in the Nd₅Fe₆₄B₂₃Mo₄Y₄ alloy. After annealing for 10 min at 1013 K, the Nd₅Fe₆₄B₂₃Mo₄Y₄ alloy showed optimal hard magnetic properties with the coercivity of 764.2 kA/m, remanence of 0.6 T and maximum energy product of 57.3 kJ/m³, respectively. The enhanced magnetic properties can be ascribed to the strong exchange coupling among the magnetically soft α-Fe (25–30 nm), Fe₃B (30–35 nm) and hard Nd₂Fe₁₄B (40–50 nm) grains present in the magnet microstructure. Large size bulk nanocomposite magnets with sound magnetic properties make the Nd–Fe–B–Mo–Y alloy system a promising candidate for industrial applications.     

Preparation of core–shell nanospheres of silica–silver: SiO2@Ag

We prepared SiO₂@Ag core–shell nanospheres: silver nanoparticles (～4 ± 2 nm in diameter) coated silica nanospheres (～50 ± 10 nm in diameter). The preparation route is a modification of the Stöber method, and involves the preparation of homogeneous silica spheres at room temperature, combined with the deposition of silver nanoparticles from Ag⁺ in solution, by using water/ethanol mixtures, tetraethyl-orthosilicate as Si source and silver nitrate as Ag source in a single-pot wet chemical route without an added coupling agent or surface modification, which leads to the formation of core@shell homogeneous nanospheres. We present the preparation and characterization of the SiO₂@Ag core–shell nanospheres and also of bare silica spheres in the absence of silver, and propose a reaction mechanism for the formation of the core–shell structure.     

Epoxide assisted sol–gel synthesis of perovskite-type LaMxFe1−xO3 (M = Ni, Co) nanoparticles

Perovskite-type LaM_xFe_1−xO₃ (M = Ni, Co) nanoparticles were synthesized by a sol–gel method using propylene oxide as a gelation agent. The resulting nanoparticles show a narrow size distribution with particles in the 30–50 nm range. A highly homogeneous wet gel was formed during the hydrolysis and condensation of the precursor salts. This high homogeneity allows a substantial reduction of the calcination temperature and time required for the formation of the perovskite phase as compared with the solid-state and other wet solution routes, reducing drastically the aggregation of the particles during calcination.     

Influence of Ag nanoparticles on luminescent performance of SiO2:Tb3 + nanomaterials

Tb^3 + single-doped SiO₂ (SiO₂:Tb^3 +) and Tb^3 +, Ag co-doped SiO₂ (SiO₂:Tb^3 +–Ag) nanostructured luminescent materials were prepared by a modified Stöber method. Their microstructure and optical property were investigated using scanning electron microscopy, ultraviolet visible absorption and photoluminescence spectrophotometry. Results show that the samples are composed of well-dispersed near-spherical particles with a diameter about 50 nm, the highest fluorescence intensity is obtained when the doping concentration of Tb^3 + is 4.86 mol%, the corresponding internal quantum efficiency is 13.6% and the external quantum efficiency is 8.2%. The experimental results indicate that Ag nanoparticles can greatly enhance the light absorption at 226 nm and the light emission at 543 nm of SiO₂:Tb^3 +–Ag, and the fluorescence lifetime reduces with increasing Ag concentration in SiO₂:Tb^3 +–Ag. Additionally, the present results indicate that fluorescence enhancement is attributed to the local field enhancement and the increased radiative decay rates induced by Ag nanoparticles.     

Extended free radical network from condensed cyanuric chloride with p-phenylenediamine: An EPR/FMR study

A covalent layered network was obtained by condensation of cyanuric chloride with bridging paraphenylenediamine. The local chemical environment of the layered solid can be changed by a redox reaction to obtain new reconstructed derivatives. A blue product was obtained by treating an alcoholic dispersion of the layered solid with ferric nitrate or potassium persulfate, indicating the possible formation of an extended free radical. When iron nitrate was used as oxidant, the temperature-dependent magnetic resonance spectra were measured in the 290–4 K region. The magnetic resonance measurements showed the coexistence at room temperature of two spectra arising from two different magnetic centers: a narrow line centered at g = 2.0038(1) with linewidth of ΔH = 7.42(2) G (free radical) and a broad line centered at g = 2.254(1) with linewidth of ΔH = 1300(5) G (magnetic iron-oxide cluster). A new sample was prepared so that the broader line was more intense. The temperature dependence of the magnetic resonance lines was subject to intense changes in both cases. The integrated intensities decreased with decreasing temperatures in both spectra in the high temperature region. This type of behavior is similar to that of magnetic nanoparticles in non-magnetic matrices. Upon reducing the temperature with the gradient of ΔH_r/ΔT = 1.5(1) G/K, the resonance field of the broad line was shifted towards lower magnetic fields, while the narrow line was shifted towards higher magnetic fields with ΔH_r/ΔT = 0.020(1) G/K. The linewidth of the broader line increased with decreasing temperature, while the narrow line remained almost constant. The magnetic iron-oxide clusters could produce an internal magnetic field acting on free radicals. This field could compel free radicals to form a magnetic ordered state at high temperatures.