M.P. Morales

Structural and magnetic properties of uniform magnetite nanoparticles prepared by high temperature decomposition of organic precursors

Magnetite nanoparticles (Fe3O4) of three different sizes below the limit for single domain magnetic behaviour have been obtained by thermal decomposition of an iron precursor in an organic medium in the presence of a surfactant. Good agreement between mean particle size obtained by TEM, crystal size calculated from x-ray diffraction and magnetic diameter calculated from magnetization curves measured at room temperature shows that the samples consist of uniform, crystalline and isolated magnetite nanoparticles with sizes between 5 and 11 nm. High saturation magnetization and high initial susceptibility values have been found, the latter decreasing as the particle size decreases. The main contribution to the anisotropy is magnetocrystalline and shape anisotropy, since surface anisotropy is suppressed by the oleic acid molecules which are covalently bonded to the nanoparticle surface.

The formation of a –Fe 2 O 3 monodispersed particles in solution

Uniform α–Fe 2 O 3 particles of varying axial ratios have been prepared from hydrolyzed ferric chloride solutions at 100 °C. In the absence of phosphate anions, spherical particles were obtained by a mechanism that follows the classical LaMer and Dinegar scheme. However, in the presence of phosphates ellipsoidal particles were observed, with their formation taking place through an aggregation process from smaller primary particles of α–Fe 2 O 3 . It is also shown that all particles are monocrystalline irrespective of their formation mechanism.

Barium ferrite nanoparticles prepared directly by aerosol pyrolysis

BaFe12O19 nanoparticles, 10 nm in diameter, have been obtained by combination of two methods, the citrate precursor and the aerosol pyrolysis technique. For the first time, well-crystallised barium ferrite particles were obtained by pyrolysis of an aerosol, produced by ultrasonic frequency spraying of a barium iron citrate aqueous solution, in a tubular furnace at 1000°C, without further heat treatment. The reason why the hexaferrite phase forms at lower temperatures than by other methods is the nature of the precursor aerosol solution. The particle size was increased up to 100 nm in diameter by heat treatment at 1000°C in an oven. The obtained particles are spherical aggregates of 400 nm, which can be easily disaggregated by grinding in a mortar. Changes in particle size and aggregation state are reflected as changes in the magnetic properties. Saturation magnetisation and coercivity values obtained for the largest particles were similar to those found for commercial pigments, 50 emu/g and 5600 Oe, respectively.

Continuous production of γ-Fe2O3 ultrafine powders by laser pyrolysis

Pure γ-Fe2O3 particles were prepared by a continuous process from cw CO2 laser induced pyrolysis of a 30% solution of iron pentacarbonyl in isopropanol with a yield of about 50% and an average productivity of 0.05 g/h. From TEM the particle size is 5±2 nm with a low degree of aggregation, which agrees with the size obtained from the width of the X-ray diffraction peaks. The nanoparticles seem to be well crystallised according to his infrared spectrum. Moreover, superparamagnetic behaviour was observed at room temperature with a saturation magnetisation value of 30.5 emu/g.

Preparation of uniform γ-Fe2O3 particles with nanometer size by spray pyrolysis

Aerosol droplets of dilute alcoholic solutions of Fe(III) and Fe(II) salts were used to synthesize γ-Fe2O3 powders by spray pyrolysis at 500°C. A wide variety of particle morphologies with sizes ranging from 5 to 60 nm were obtained depending on the nature of the precursor solution. Aggregated particles of γ-Fe2O3 with a medium diameter of 6 nm forming dense spheres were obtained from nitrate solutions. However, γ-Fe2O3 obtained from acetylacetonate solutions gives monodispersed particles of about 5 nm in diameter. The morphology of the maghemite particles derived from Fe(II) ammonium citrate appears as hollow spheres with a medium diameter of 170 nm, whereas γ-Fe2O3 particles with high degree of crystallinity were obtained from Fe(III) chloride solutions. Mossbauer spectra of the samples show the absence of Fe2+ species corroborating the formation of γ-Fe2O3 particles under the conditions studied.

Magnetic nanoparticles with bulklike properties (invited)

The magnetic behavior of Fe3� xO4 nanoparticles synthesized by either high-temperature decomposition of an organic iron precursor or low-temperature coprecipitation in aqueous conditions is compared. Transmission electron microscopy, x-ray absorption spectroscopy, x-ray magnetic circular dichroism, and magnetization measurements show that nanoparticles synthesized by thermal decomposition display high crystal quality and bulklike magnetic and electronic properties, while nanoparticles synthesized by coprecipitation show much poorer crystallinity and particlelike phenomenology, including reduced magnetization, high closure fields, and shifted hysteresis loops. The key role of the crystal quality is thus suggested, because particlelike behavior for particles larger than about 5 nm is observed only when the particles are structurally defective. These conclusions are supported by Monte Carlo simulations. It is also shown that thermal decomposition is capable of producing nanoparticles that, after further stabilization in physiological conditions, are suitable for biomedical applications such as magnetic resonance imaging or biodistribution studies. V C 2011 American Institute of Physics. [doi:10.1063/1.3559504]

Comparative study of ferrofluids based on dextran-coated iron oxide and metal nanoparticles for contrast agents in magnetic resonance imaging

Colloidal suspensions of iron oxide and metal iron nanoparticles prepared by laser pyrolysis have been obtained by coating the particles with dextran in an aqueous media giving rise to biocompatible ferrofluids. The structural characteristics of the powders and the size of the particles and the aggregates in the colloidal suspensions have been analysed and correlated with the magnetic properties of both solids and fluids. For the first time, to our knowledge, a stable ferrofluid based on metal particles (<10?nm) has been obtained with aggregate sizes of ?nm. In comparison to iron oxide based products, this material exhibits higher saturation magnetization (45?emu?g?1) and susceptibilities (4000?emu/g?T). In addition, the nuclear magnetic resonance response of the ferrofluids has been measured in order to gain information about the influence of the crystallochemical and magnetic properties on their relaxation behaviour. The main parameter affected by the presence of the magnetic nanoparticles is the transversal relaxation time T2 and the corresponding relaxivity R2 value that is of the order of 400?(mmol/l)?1?s?1. It has been shown that R2 value increases not only by using iron metal instead of iron oxide but also by increasing the crystal size of the particles. From this study an evaluation of the possibilities of these materials as contrast agents for magnetic resonance imaging has been made.

Magnetite nanoparticles with no surface spin canting

Surface spin canting has been studied for high quality magnetite nanoparticles in terms of size and shape uniformity. Particles were prepared by thermal decomposition of organic precursors in organic media and in the presence of oleic acid. Results are compared to spin canting effect for magnetic iron oxide nanoparticles of similar size prepared by coprecipitation and subsequently coated with silica. Magnetic characterization and Mossbauer spectroscopy at low temperature and in the presence of a magnetic field have been used in this study. Transmission electron microscopy images and x-ray diffractograms show that iron oxide nanoparticles synthesized by thermal decomposition are more uniform than those prepared by coprecipitation, and they have higher crystal order. Magnetic measurements show superparamagnetic behavior for both samples at room temperature but particles synthesized by thermal decomposition shows higher saturation magnetization and lower coercivity at low temperature. The imaginary part of t...

Structural and magnetic transformation of monodispersed iron oxide particles in a reducing atmosphere

Uniform metal iron ellipsoidal particles of around 200 nm in length were obtained by reduction and passivation of alumina-coated α–Fe2O3 (hematite) particles under different conditions of temperature and hydrogen flow rate. The monodispersed hematite particles were prepared by the controlled hydrolysis of ferric sulfate and further coated with a homogeneous thin layer of Al2O3 by careful selection of the experimental conditions, mainly pH and aluminum salt concentration. The reduction mechanism of α–Fe2O3 into α–Fe was followed by x-ray and electron diffraction, and also by the measurements of the irreversible magnetic susceptibility. The transformation was found to be topotactic with the [001] direction of hematite particles, which lies along the long axis of the particles, becoming the [111] direction of magnetite and finally the [111] direction of metal iron. Temperature and hydrogen flow rate during the reduction have been found to be important parameters, which determine not only the degree of reduct...

Synthesis of Monodispersed Magnetite Particles From Different Organometallic Precursors

Magnetite (Fe3O4) nanoparticles with very narrow particle size distribution can be obtained by decomposition of an organometallic compound in the presence of oleic acid. It has been shown that the carboxylic acid catalyzes the reaction leading to decreased decomposition temperatures and, consequently, as the surfactant concentration increases, smaller particles are formed. Precursors such as Fe(CO)5, iron acetylacetonate, and iron-oleate complexes, previously formed from an iron salt, have been used for the preparation of magnetite particles. Although the particles are magnetite in all cases, the size, shape, and distribution of nanoparticles differ depending on the precursor and consequently the samples show different magnetic behavior. Besides, a different mechanism of formation of the nanoparticles is expected and related to the decomposition rate of the precursor