Synthesis of magnetite core–shell nanoparticles by surface-initiated ring-opening polymerization of l-lactide

Fe₃O₄–polylactide (PLA) core–shell nanoparticles were perpared by surface functionalization of Fe₃O₄ nanoparticles and subsequent surface-initiated ring-opening polymerization of l-lactide. PLA was directly connected onto the magnetic nanoparticles surface through a chemical linkage. Fourier transform infrared (FT-IR) spectra directly provided evidence of the PLA on the surface of the magnetic nanoparticles. Transmission electron microscopy images (TEM) showed that the magnetic nanoparticles were coated by PLA with a 3-nm-thick shell. The amount of grafted polymer determined by thermal gravimetric analysis was ∼13.3% by weight. X-ray diffraction (XRD) patterns of as-prepared core–shell nanoparticles showed the same structure (spinel cubic lattice type) to that of the bare core materials with similar intensity of the corresponding peaks, and that the polymer coating was amorphous. The particles could be stably dispersed in chloroform for several weeks. The prepared Fe₃O₄–PLA core–shell nanoparticles were superparamagnetic behavior with a saturation magnetization value nearly identical to that of the bare magnetic nanoparticles, rendering the Fe₃O₄–PLA nanoparticles for potential applications in both the material technology and biomedical fields.     

One-pot synthesis and characterization of rhodamine derivative-loaded magnetic core–shell nanoparticles

A new method to produce elaborate nanostructure with magnetic and fluorescent properties in one entity is reported in this article. Magnetite (Fe₃O₄) coated with fluorescent silica (SiO₂) shell was produced through the one-pot reaction, in which one reactor was utilized to realize the synthesis of superparamagnetic core of Fe₃O₄, the formation of SiO₂ coating through the condensation and polymerization of tetraethylorthosilicate (TEOS), and the encapsulation of tetramethyl rhodamine isothiocyanate-dextran (TRITC-dextran) within silica shell. Transmission electron microscopy (TEM), energy dispersive X-ray (EDX) analysis, and X-ray diffraction (XRD) were carried out to investigate the core–shell structure. The magnetic core of the core–shell nanoparticles is 60 ± 10 nm in diameter. The thickness of the fluorescent SiO₂ shell is estimated at 15 ± 5 nm. In addition, the fluorescent signal of the SiO₂ shell has been detected by the laser confocal scanning microscopy (LCSM) with emission wavelength (λ_em) at 566 nm. In addition, the magnetic properties of TRITC-dextran loaded silica-coating iron oxide nanoparticles (Fe₃O₄@SiO₂ NPs) were studied. The hysteresis loop of the core–shell NPs measured at room temperature shows that the saturation magnetization (M _s) is not reached even at the field of 70 kOe (7T). Meanwhile, the very low coercivity (H _c) and remanent magnetization (M _r) are 0.375 kOe and 6.6 emu/g, respectively, at room temperature. It indicates that the core–shell particles have the superparamagnetic properties. The measured blocking temperature (T _B) of the TRITC-dextran loaded Fe₃O₄@SiO₂ NPs is about 122.5 K. It is expected that the multifunctional core–shell nanoparticles can be used in bio-imaging.     

Exchange bias behavior of monodisperse Fe3O4/γ-Fe2O3 core/shell nanoparticles

We have carried out systematic studies on well-characterized monodisperse Fe₃O₄/γ-Fe₂O₃ core/shell nanoparticles of 2-30 nm having a very narrow size distribution and possessing a uniquely mono-layer of surface γ-Fe₂O₃. This unique core-shell structure, probably having a disordered magnetic surface state, leads us to three key observations of unusual magnetic properties: i) a very large magnetic exchange anisotropy reaching over 7 × 10⁶ erg/cm³ for the smaller particles, ii) exchange bias behavior in the magnetization data of the core/shell Fe₃O₄/γ-Fe₂O₃ nanoparticles, and iii) the temperature dependence of the coercive field following an unusual exponential behavior.     

Preparation and characterization of magnetic nanoparticles with controlled magnetization

The effect of molar ratio of two hydrated iron salts used as precursors into a (co)precipitation-based synthesis method, on the composition, size and specific saturation magnetization of mixed iron oxides and oxyhydroxides magnetic nanoparticles as reaction products, was studied. The preparation procedure is based on a salt-assisted solid-state chemical reaction. The obtained products are magnetic multiphase components with the mean size ranging from 3 to 10 nm and specific saturation magnetization between 25 and 95.5 emu/g. The specific saturation magnetization modifies in a non-linear manner as the molar ratio of the iron salts varies. Excepting one sample, for which Fe²⁺/Fe³⁺ molar ratio was zero, all magnetic nanoparticles show a ferrofluid-like behaviour in the colloidal form. The small size, ferrofluid-like behaviour, and controlled specific saturation magnetization allow the use of new synthesized nanoparticles in specific biomedical or industrial applications.     

Fabrication of magnetic gold nanorod particles for immunomagnetic separation and SERS application

The preparation and application of rod-shaped core–shell structured Fe₃O₄–Au nanoparticles for immunomagnetic separation and sensing were described for the first time with this study. To synthesize magnetic gold nanorod particles, the seed-mediated synthetic method was carried out and the resulting nanoparticles were characterized with transmission electron microscopy (TEM), ultraviolet visible spectroscopy (UV–Vis), energy-dispersive X-ray (EDX), and X-ray diffraction (XRD). Magnetic properties of the nanoparticles were also examined. Characterization of the magnetic gold nanorod particles has proven that the resulting nanoparticles were composed of Fe₃O₄ core and the gold shell. The rod-shaped gold-coated iron nanoparticles have an average diameter of 16 ± 2 nm and an average length of about 50 ± 5 nm (corresponding aspect ratio of 3). The saturation magnetization value for the magnetic gold nanorod particles was found to be 37 emu/g at 300 K. Rapid and room temperature reaction synthesis of magnetic gold nanorod particles and subsequent surface modification with E. coli antibodies provide immunomagnetic separation and SERS application. The analytical performance of the SERS-based homogenous sandwich immunoassay system with respect to linear range, detection limit, and response time is also presented.     

Magnetic properties of SiO2-coated iron oxide nanoparticles studied by polarized small angle neutron scattering

The structural and magnetic properties of non-coated and SiO₂-coated iron oxide (Fe₃O₄) nanoparticles (NPs) were investigated by a polarized small-angle neutron scattering (P-SANS) method. Measurement of the P-SANS allowed us to obtain nuclear and magnetic scattering cross sections of the NPs under applied magnetic field. The analysis of the scattering intensity provided the structural parameters and the spatial magnetization distribution of the non-coated and the SiO₂ coated core–shell NPs. The measured radius of both NPs and the shell thickness of the core–shell NPs were in consistent with those measured by the transmission electron microscopy. In comparison, the magnetic core radii of both NPs were 0.12–0.6 nm smaller than the nuclear radii, indicating the magnetization reduction in the surface region of core Fe₃O₄ in both NPs. However, the reduced magnetization region, which is the surface spin canting region, of the SiO₂-coated NPs was relatively narrower than that of the non-coated NPs. We suggest that the SiO₂ coating on the Fe₃O₄ NPs may stabilize the spin order of atoms and prohibit the oxidation or defect formation at the surface region of the Fe₃O₄ NPs, and enhance the corresponding magnetization of the Fe₃O₄ NPs by the reduction of the spin canting layer thickness.     

Synthesis and characterization of nanostructured strontium hexaferrite thin films by the sol–gel method

Nanostructured single phase strontium hexaferrite, SrFe₁₂O₁₉, thin films have been synthesized on the (100) silicon substrate using a spin coating sol–gel process. The thin films with various Fe/Sr molar ratios of 8–12 were calcined at different temperatures from 500 to 900 °C. The composition, microstructure and magnetic properties of the SrFe₁₂O₁₉ thin films were characterized using Fourier transform infrared spectroscopy, differential thermal analysis, thermogravimetry, X-ray diffraction, electron microscopy and vibrating sample magnetometer. The results showed that the optimum molar ratio for Fe/Sr was 10 at which the lowest calcination temperature to obtain the single phase strontium hexaferrite thin film was 800 °C. The magnetic measurements revealed that the sample with Fe/Sr molar ratio of 10, exhibited higher saturation magnetization (267.5 emu/cm³) and coercivity (4290 Oe) in comparison with those synthesized under other Fe/Sr molar ratios.     

Synthesis and magnetic properties of barium–calcium hexaferrite particles prepared by sol–gel and microemulsion techniques

The preparation of W-type hexaferrite particles with the composition BaCa₂Fe₁₆O₂₇ by microemulsion and a stearic acid sol–gel method with and without surfactant has been investigated at various sintering temperatures. The structural and magnetic characteristics have been studied by X-ray diffraction (XRD), a vibrating sample magnetometer (VSM), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), differential scanning calorimetric (DSC) and Fourier transform infrared (FTIR) techniques. The effect of sintering temperature on the properties of BaCa₂Fe₁₆O₂₇ hexaferrites has been studied. The value of saturation magnetization (M_s) depends on types of surfactant used. The sample prepared in the presence of polyoxyethylene (20) sorbitan monooleat (Tween 80) shows low saturation magnetization (M_s=15.10 emu/g), whereas the other sample prepared in the presence of a surfactant cetyltrimethylammonium bromide (CTAB) exhibits high saturation magnetization (M_s=24.60 emu/g) compared to the normal sample.     

Microwave synthesis of magnetic Fe3O4 nanoparticles used as a precursor of nanocomposites and ferrofluids

Methods to synthesize magnetic Fe₃O₄ nanoparticles and to modify the surface of particles are presented in the present investigation. Fe₃O₄ magnetic nanoparticles were prepared by the co-precipitation of Fe³⁺ and Fe²⁺, NH₃·H₂O was used as the precipitating agent to adjust the pH value, and the aging of Fe₃O₄ magnetic nanoparticles was accelerated by microwave (MW) irradiation. The obtained Fe₃O₄ magnetic nanoparticles were characterized by Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), X-ray powder diffraction (XRD) and vibrating sample magnetometer (VSM). The average size of Fe₃O₄ crystallites was found to be around 8–9 nm. Thereafter, the surface of Fe₃O₄ magnetic nanoparticles was modified by stearic acid. The resultant sample was characterized by FT-IR, scanning electron microscopy (SEM), XRD, lipophilic degree (LD) and sedimentation test. The FT-IR results indicated that a covalent bond was formed by chemical reaction between the hydroxyl groups on the surface of Fe₃O₄ nanoparticles and carboxyl groups of stearic acid, which changed the polarity of Fe₃O₄ nanoparticles. The dispersion of Fe₃O₄ in organic solvent was greatly improved. Effects of reaction time, reaction temperature and concentration of stearic acid on particle surface modification were investigated. In addition, Fe₃O₄/polystyrene (PS) nanocomposite was synthesized by adding surface modified Fe₃O₄ magnetic nanoparticles into styrene monomer, followed by the radical polymerization. The obtained nanocomposite was tested by thermogravimetry (TG), differential scanning calorimetry (DSC) and XRD. Results revealed that the thermal stability of PS was not significantly changed after adding Fe₃O₄ nanoparticles. The Fe₃O₄ magnetic fluid was characterized using UV–vis spectrophotometer, Gouy magnetic balance and laser particle-size analyzer. The testing results showed that the magnetic fluid had excellent stability, and had susceptibility of 4.46×10⁻⁸ and saturated magnetization of 6.56 emu/g. In addition, the mean size d (0.99) of magnetic Fe₃O₄ nanoparticles in the fluid was 36.19 nm.     

Synthesis of (Mg0.476Mn0.448Zn0.007)(Fe1.997Ti0.002)O4 powder and sintered ferrites by high energy ball-milling process

(Mg_0.476Mn_0.448Zn_0.007)(Fe_1.997Ti_0.002)O₄ nanocrystalline powder prepared by high energy ball-milling process were consolidated by microwave and conventional sintering processes. Phases, microstructure and magnetic properties of the ferrites prepared by different processes were investigated. The (Mg_0.476Mn_0.448Zn_0.007)(Fe_1.997Ti_0.002)O₄ nanocrystalline powder could be prepared by high energy ball-milling process of raw Fe₃O₄, MnO₂, ZnO, TiO₂ and MgO powders. Prefired and microwave sintered ferrites could achieve the maximum density (4.86 g/cm⁻³), the average grain size (15 μm) was larger than that (10 μm) prepared by prefired and conventionally sintered ferrites with pure ferrite phase, and the saturation magnetization (66.77 emu/g) was lower than that of prefired and conventionally sintered ferrites (88.25 emu/g), the remanent magnetization (0.7367 emu/g) was higher than that of prefired and conventionally sintered ferrites (0.0731 emu/g). Although the microwave sintering process could increase the density of ferrites, the saturation magnetization of ferrites was decreased and the remanent magnetization of ferrites was also increased.     

Preparation and magnetic properties of BaFe12O19/Ni0.8Zn0.2Fe2O4 nanocomposite ferrite

Nanocomposite of hard (BaFe₁₂O₁₉)/soft ferrite (Ni_0.8Zn_0.2Fe₂O₄) have been prepared by the sol–gel process. The nanocomposite ferrite are formed when the calcining temperature is above 800 °C. It is found that the magnetic properties strongly depend on the presintering treatment and calcining temperature. The “bee waist” type hysteresis loops for samples disappear when the presintering temperature is 400 °C and the calcination temperature reaches 1100 °C owing to the exchange-coupling interaction. The remanence of BaFe₁₂O₁₉/Ni_0.8Zn_0.2Fe₂O₄ nanocomposite ferrite with the mass ratio of 5:1 is higher than a single phase ferrite. The specific saturation magnetization, remanence magnetization and coercivity are 63 emu/g, 36 emu/g and 2750 G, respectively. The exchange-coupling interaction in the BaFe₁₂O₁₉/Ni_0.8Zn_0.2Fe₂O₄ nanocomposite ferrite is discussed.     

Effect of Fe/Sr mole ratios on the formation and magnetic properties of SrFe12O19 microtubules prepared by sol-gel method

The sol was obtained by sol-gel method. Then, the sol was dripped onto the absorbent cotton template. The gel was obtained after the evaporation of water. Strontium ferrite microtubules were prepared after carrying out calcination process at different temperatures. The phase, morphology and particle diameter and the magnetic properties of samples were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM), respectively. The effects of Fe³⁺/Sr²⁺ mole ratio and calcination temperature on the crystal structure, morphology and magnetic properties of ferrite microtubules were studied. The external diameters of obtained SrFe₁₂O₁₉ microtubules were found to range between 8 and 13 μm; the wall thicknesses ranged between 1 and 2 μm. When the Fe³⁺/Sr²⁺ mole ratio and the calcination temperature were 11.5 and 850 °C, respectively, the coercivity, saturation magnetization and remanent magnetization for the samples were 7115.1 Oe, 70.1 and 42.4 emu/g, respectively. The mechanism of the formation and variation in magnetic properties of the microtubules were explained.     

Multifunctional core–shell nanoparticles: superparamagnetic,mesoporous, and thermosensitive

Multifunctional core–shell composite nanoparticles (NPs) have been developed by the combination of three functionalities into one entity, which is composed of a single Fe₃O₄ NP as the magnetic core, mesoporous silica (mSiO₂) with cavities as the sandwiched layer, and thermosensitive poly(N-isopropylacrylamide-co-acrylamide) (P(NIPAAm-co-AAm)) copolymer as the outer shell. The mSiO₂-coated Fe₃O₄ NPs (Fe₃O₄@mSiO₂) are monodisperse and the particle sizes were varied from 25 to 95 nm by precisely controlling the thickness of mSiO₂-coating layer. The P(NIPAAm-co-AAm) were then grown onto surface-initiator-modified Fe₃O₄@mSiO₂ NPs through free radical polymerization. These core–shell composite NPs (designated as Fe₃O₄@mSiO₂@P(NIPAAm-co-AAm)) were found to be superparamagnetic with high r ₂ relaxivity. To manipulate the phase transition behavior of these thermosensitive polymer-coated NPs for future in vivo applications, the characteristic lower critical solution temperature (LCST) was subtly tuned by adjusting the composition of the monomers to be around the human body temperature (i.e. 37 °C), from ca. 34 to ca. 42 °C. The thermal response of the core–shell composite NPs to the external magnetic field was also demonstrated. Owing to their multiple functionality characteristics, these porous superparamagnetic and thermosensitive NPs may prove valuable for simultaneous magnetic resonance imaging (MRI), temperature-controlled drug release, and temperature-programed magnetic targeting and separation applications.     

Effect of Pb on structural and magnetic properties of Ba-hexaferrite

Lead was doped in barium hexaferrite by co-precipitation method and the Pb-doped Ba-hexaferrite with compositions of Ba_1−xPb_xFe₁₂O₁₉ was investigated for the first time at x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0. The molar ratio (Fe³⁺/Ba²⁺) of the solutions was kept 12 while pH was maintained at 13 by using NaOH (M=5) as precipitating agent. Prepared samples were sintered at 965±5 °C for three hours. Structural and morphological studies were done by X-ray diffractometer (XRD) and the scanning electron microscope (SEM). SEM micrographs confirmed the formation of hexagonal plate like structures and particle size was observed to be increased with the increase in Pb concentration. The hysteresis loops obtained from the magnetometer showed that with the increase in Pb concentration, the coercivity decreased while magnetic induction and remanence increased, which in turn increased the maximum energy product (BH)_max. Lower coercivity and the moderate increase in saturation magnetization obtained from Pb doping makes the material useful for magnetic recording media and other frequency based applications.     

Effect of high magnetic field annealing on the microstructure and magnetic properties of Co-Fe layered double hydroxide

The effects of high magnetic field (10 T) on the products obtained by calcination of Co-Fe LDH precursors at different temperatures were investigated. The XRD results indicated that Fe^III substituted for Co^III in Co₃O₄ to yield Co^IICo^IIIFe^IIIO₄ under the calcination of Co-Fe LDH precursors at 400 °C. The products obtained by magnetic field annealing at 400 °C had a porous plate-like morphology, whereas the products without magnetic field annealing were composed of nanoparticles. It was seen that CoFe₂O₄ phase could be formed at low temperature (about 500 °C) under the magnetic field annealing. The grain size of products obtained by magnetic field annealing at 800 °C was larger than that of zero magnetic field. It was found that the saturation magnetization was significantly enhanced after magnetic field annealing, especially at lower temperature (≤600 °C). The possible reason for the effects on the microstructure and magnetic properties of products obtained by magnetic field annealing was discussed.     

Preparation and characterization of Fe3O4 magnetic composite microspheres covered by a P(MAH-co-MAA) copolymer

A magnetic core–shell-layered polymer microsphere (MPS) was successfully synthesized by a dispersion polymerization route, where the modified Fe₃O₄ nanoparticles (MFN) were used as a core, while poly(maleic anhydride-co-methacrylic acid) P(MAH-co-MAA) as a shell was covered on the surface of the Fe₃O₄ nanoparticles. Environmental scanning electron microscope (ESME) and transmission electron microscope (TEM) measurements indicate that the magnetic P(MAH-co-MAA)/Fe₃O₄ composite microspheres assume sphericity and have a novel core–shell-layered structure. The crystal particle sizes of the unimproved Fe₃O₄ and the MFN samples vary from 8 to 16 nm in diameter, and the average size is about 10.6 nm in diameter. The core–shell magnetic composite microspheres can be adjusted by changing the stirring speed. Since multiple Fe₃O₄ cores were coated with a proper percentage of P(MAH-co-MAA) copolymers, and therefore lower density was acquired for the MPS, which improved sedimentation and dispersion behavior. The saturated magnetization of pure Fe₃O₄ nanoparticles reaches 48.1 emu g⁻¹ and the value for composite nanoparticles was as high as 173.5 emu g⁻¹. The nanoparticles show strong superparamagnetic characteristics and can be expected to be used as a candidate for magnetism-controlled drug release.     

Influence of boron on the giant magnetocaloric effect of La(Fe0.9Si0.1)13

The influences of boron addition on the phase formation, Curie temperature and magnetic entropy change of the NaZn₁₃-type La(Fe_0.9Si_0.1)₁₃ compound have been investigated. Eight boron containing La(Fe_0.9Si_0.1)₁₃B_x samples were prepared with x=0, 0.03, 0.06, 0.1, 0.2, 0.3, 0.5 and 0.6, respectively. Experimental results show that a small amount of B addition in La(Fe_0.9Si_0.1)₁₃ forms the solid solution NaZn₁₃-type structure phase by substituting B for Si or doping B into interstitial position of the lattice, preserves its giant magnetocaloric effects due to their first-order structural/magnetic transition, as well as increase its Curie temperature T_c slightly. The maximum magnetic entropy changes in the magnetic field change of 0–1.6 T are around 20 J kg^–1 K^–1 for the samples with Boron addition less than 0.3, while improving the Curie temperatures by 2 K.     

Synthesis and characterization of permalloy nanostructured films by deposition of laser ablated nanoparticles

A permalloy (Ni₈₁Fe₁₉; at%) microparticle (MP) aerosol was ablated to produce a nanoparticle (NP) aerosol that was then impacted at high velocity onto a substrate to produce porous thick films. The structure of the NPs was analyzed by transmission electron microscopy and the morphologies of the NPs and the nanostructured films were studied using high-resolution transmission electron microscopy and scanning electron microscopy. These analyses showed that the original composition and structure of the MPs were preserved in the NPs and films. The majority of NPs that were produced ranged in size between 2 and 15 nm with some larger particles present. Magnetization-temperature curves showed that the films consisted of a mixture of small superparamagnetic NPs and larger ferromagnetic NPs. A high saturation magnetization of 62.3 emu/g at 300 K was retained in the films indicating that they remained free of significant oxidation.     

A Study on the Rheological and Magnetic Properties of Polymer-Bonded Magnets Using Polycarbonate as the Bonding Material

The effect of three metal oxides on the magnetic properties of polymer bonded magnets (PBMs) was studied. The three PBMs, using polycarbonate (PC) as binder and 5 wt% of Fe₃O₄, Fe₂O₃, or CuO nanoparticles, were prepared by melt extrusion in a twin screw extruder followed by compression molding. Transmission electron microscopic (TEM) images showed a better dispersion for the PC/Fe₃O₄ nanocomposite compared with that of the other nanocomposites. The dynamic intersection frequency (ω_c), which is related to the crossing of the G′ and G^″ curves, showed that there was more homogeneity in the PC/Fe₃O₄ and PC/Fe₂O₃ nanocomposites. The curves of saturation magnetization for the three nanocomposites showed that there was a relationship between the magnetic properties and the homogeneity of the nanoparticles studied by rheometry. Because the magnetic strength of PC/Fe₃O₄ was greater than that of the other nanocomposites, it was concluded that not only the intrinsic magnetic property of the filler was an important factor to increase the magnetic property, but also the homogeneity of the filler within the matrix had an important role.     

Synthesis, structural and magnetic properties of the nanocomposite Fe63B23Nd7Y3Nb3Cr1 magnets

The Fe₆₃B₂₃Nd₇Y₃Nb₃Cr₁ nanocomposite magnets in the form of sheets have been prepared by copper mold casting technique. The phase evolution, crystal structure, microstructural and magnetic properties have been investigated in the as-cast and annealed states. The as-cast sheets show magnetically soft behaviors which become magnetically hard by thermal annealing. The optimal annealed microstructure was composed of nanosize soft magnetic α-Fe (19-29 nm) and hard magnetic Nd₂Fe₁₄B (45-55 nm) grains. The best hard magnetic properties such as intrinsic coercivity, _jH_c of 1119 kA/m, remanence, B_r of 0.44 T, magnetic induction to saturation magnetization ratio, M_r/M_s=0.61 and maximum energy product, (BH)_max of 55 kJ/m³ was obtained after annealing at 680 °C for 15 min. The annealing treatment above 680 °C results in non-ideal phase grains growth, which degrade the magnetic properties.