L.M. Socolovsky

Chemical Synthesis and Structural Characterization of Highly Disordered Ni Colloidal Nanoparticles

This work focuses on synthetic methods to produce monodisperse Ni colloidal nanoparticles (NPs), in the 4-16 nm size range, and their structural characterization. Narrow size distribution nanoparticles were obtained by high-temperature reduction of a nickel salt and the production of tunable sizes of the Ni NPs was improved compared to other methods previously described. The as-synthesized nanoparticles exhibited spherical shape and highly disordered structure, as it could be assigned by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). Annealing at high temperature in organic solvent resulted in an increase of nanoparticle atomic ordering; in this case, the XRD pattern showed an fcc-like structure. Complementary data obtained by X-ray absorption spectroscopy confirmed the complex structure of these nanoparticles. Temperature dependence of the magnetic susceptibility of these highly disordered Ni NPs showed the magnetic behavior cannot be described by the conventional superparamagnetic theory, claiming the importance of the internal structure in the magnetic behavior of such nanomaterials.

Dipolar interaction and size effects in powder samples of colloidal iron oxide nanoparticles

Dipole–dipole interactions in nanostructured materials deeply affect their magnetic properties, and detailed studies are still required to fully understand them. In this work, the dependence of magnetic properties on particle size has been evaluated in powder samples of Fe oxide nanoparticles produced by colloidal methods. Zero-field-cooled and field-cooled magnetization curves and magnetization versus applied field data have been analysed by taking into account dipolar interactions through a correction to the classical superparamagnetic model. Morphological and magnetic data were in very good agreement, which has allowed us to quantify relevant physical parameters, such as the anisotropy constant, magnetic moment, and interacting volume for our system.

Blocking phenomena in granular magnetic alloys through magnetization, Hall effect, and magnetoresistance experiments

Magnetization and magnetotransport were measured in CoxAg1−x granular composites as a function of temperature and applied magnetic field. A transition from blocked to superparamagnetic behavior with increasing temperatures can be observed in magnetization, giant magnetoresistance and the extraordinary Hall effect measurements. However, the blocking temperature determined from magnetotransport measurements is systematically lower than the one estimated from magnetic measurements. This is due to the selective magnetic scattering, which is enhanced for smaller particles, while the magnetization probes the whole particle size distribution.

Size dependence on the ordering process in colloidal FePt nanoparticles

An alternative method to study the effects of annealing process on colloidal FePt nanoparticles (2–4 nm) has been achieved. Annealing experiments at temperatures between 773 and 1073 K under inert atmosphere flux were performed in powder samples with excess of surfactant molecules on nanoparticle surface. Transmission electron microscopy, x-ray diffraction and magnetic measurements were performed to evidence the evolution of the chemically disordered fcc to chemically ordered face-centered tetragonal phase transformation. Magnetization measurements under zero-field-cooling and field-cooling (MZFC−MFC) conditions, and hysteresis loops are extremely sensitive to the particle size distribution and were strongly affected by the annealing treatment.

Nanostructure and giant Hall effect in TMx(SiO2)1−x (TM=Co,Fe,Ni) granular system

Granular TMx(SiO2)1−x (TM=Co,Fe,Ni) thin films were thermally treated at different temperatures and their magnetotransport and structural properties were studied. Hall resistivity decreases with thermal annealing. Structure was analyzed based on small angle x-ray scattering results. A model of polydisperse system of hard spheres was used for obtaining structural parameters. Analysis reveals that a volume fraction of transition-metal atoms (less than 29%) are forming nanospheres. Changes in giant Hall effect upon annealing can depend on a particular combination of nanoparticle diameter, interparticle distance, and size distribution.

Magnetotransport, magnetic, and structural properties of TM–SiO2 (TM=Fe, Co, Ni) granular alloys

TM{sub x}(SiO{sub 2}){sub 1-x} (transition metals, or TM=Fe, Co, Ni) thin films were prepared in a wide concentration range with the purpose of studying the giant Hall effect (GHE). The structure was studied using transmission electron microscopy (TEM), X-ray diffraction (XRD), and small angle X-ray scattering (SAXS). Magnetic, transport, and magnetotransport properties were investigated by means of magnetization, resistivity, and Hall effect measurements. TEM images show nanometer-sized spherical structures embedded in an SiO{sub 2} amorphous matrix, with typical sizes ranging from 2 to 7 nm when TM volume concentration x is increased. SAXS measurements show a broadened peak. XRD measurements show that the system consists of amorphous SiO{sub 2} and a crystalline peak, corresponding to small TM crystallites. The observed magnetic properties are strongly dependent on x, and clearly display an evolution resulting from the progressive increase in the mean particle size. Above the percolation threshold region all samples display GHE. Relationships between structure and magnetotransport properties are discussed.

Structural, magnetic and Mossbauer characterization of controlled-size iron-iron oxide nanoparticles obtained by chemical methods

In this paper, structural, magnetic and Mossbauer characterization of controlled-size iron-iron oxide nanoparticles obtained by chemical methods.

Synthesis and Characterization of Iron Oxyhydroxide Nanowires

A very simple method of synthesis of goethite, α-FeOOH, nanowire is reported. To fabricate the nanowires, an anodized alumina nanoporous template (AAO) is used. AAO has pores with an average diameter of 60 nm. The synthesis is based on a self-combustion reaction of the chemical precursor (Fe(NO3)3 saturated solution) which occurs inside the nanopores. The geometry of AAO determines the morphology of the nanowires and the confinement conditions in which the heat treatment determines the composition of the nanostructure. The nanowires are characterized using scanning electron microscopy, high resolution transmission electron microscopy and magnetometry [magnetization versus applied field (M versus H)]. TEM analysis indicates that nanowires are composed of several α-FeOOH single crystals. The nanowires have a clear magnetic oriented structure.

Synthesis of Ag–CoFe2O4 dimer colloidal nanoparticles and enhancement of their magnetic response

This paper reports the structural and magnetic properties of Ag–CoFe2O4 colloidal dimer nanoparticles (NPs) synthesized using a two-step solution-phase route. Ag NPs were used as seeds to grow Ag–CoFe2O4 dimer NPs using thermal decomposition of metallic precursor. By means of temperature and field dependent dc magnetization measurements, it is found that the silver due to its interface with CoFe2O4 particles leads to thermal stabilization of the dimer NPs superior as compared to CoFe2O4 alone. Our results show enhancement of the magnetic anisotropy and a large coercivity at 2 K for dimer NPs, which could be ascribed to interface effect between Ag and CoFe2O4 components and the related structural defects.

Electron dynamics in films made of transition metal nanograins embedded in SiO2: Infrared reflectivity and nanoplasma infrared resonance

We report on near normal infrared reflectivity spectra of ~550 nm thick films made of cosputtered transition metal nanograins and SiO2 in a wide range of metal fractions. Co0.85(SiO2)0.15,with conductivity well above the percolation threshold has a frequency and temperature behavior according to what it is find in conducting metal oxides. The electron scattering rate displays an unique relaxation time characteristic of single type of carriers experiencing strong electron-phonon interactions. Using small polaron fits we identify those phonons as glass vibrational modes. Ni0.61(SiO2)0.39, with a metal fraction closer to the percolation threshold, undergoes a metal-non metal transition at ~77 K. Here, as it is suggested by the scattering rate nearly quadratic dependence, we broadly identify two relaxation times (two carrier contributions) associated to a Drude mode and a mid-infrared overdamped band, respectively. Disorder induced, the mid-infrared contribution drives the phase transition by thermal electron localization. Co0.51(SiO2)0.49 has the reflectivity of an insulator with a distinctive band at ~1450cm\^{-1} originating in electron promotion, localization, and defect induced polaron formation. Angle dependent oblique reflectivity of globally insulating Co0.38(SiO2)0.62, Fe0.34(SiO2)0.66, and Ni0.28(SiO2)0.72, reveals a remarkable resonance at that band threshold. We understand this as due to the excitation by normal to the film electric fields of defect localized electrons in the metallic nanoparticles