Papers by Leandro Socolovsky
Journal of Magnetism and Magnetic Materials, Jan 1, 2011
The existence of metastable hexaferrite is reported. Synthesis of strontium hexaferrite, SrFe12O1... more The existence of metastable hexaferrite is reported. Synthesis of strontium hexaferrite, SrFe12O19, at 400 °C was realized under controlled oxygen atmosphere. Such technique allows obtaining of SrFe12O19 at lower temperatures than those by traditional methods (above 800 °C). Phase transformation occurred during a measurement of magnetization vs. temperature (heating up to 625 °C). The heat treatment induces a change from SrFe12O19 to γ-Fe2O3 (as the main phase), and SrFeO2.74 to Sr2Fe2O5. Together with these phase transformations, an increment in the amount of SrCO3 is detected. Magnetic study of the samples, before and after the heating, supports the structural analysis conclusions.► SrFe12O19, as the majority phase, is obtained by heat treatment at 400 °C under oxygen flux. ► Hexaferrite nanoparticles have crystalline imperfections that make it an unstable phase. ► Together with these phase transformations, an increment in the amount of SrCO3 is detected. ► To our knowledge, this is the first report of the existence of metastable SrFe12O19.
Journal of Alloys and Compounds, Jan 1, 2010
One of the current issues at the basis of the understanding of novel materials is the degree of t... more One of the current issues at the basis of the understanding of novel materials is the degree of the role played by spatial inhomogeneities due to subtle phase separations. To clarify this picture here we compare the plain glass network response of transition metal granular films with different metal fractions against what is known for conducting oxides. Films for Nix(SiO2)1−x (x = 1.0, 0.84, 0.75, 0.61, 0.54, 0.28) were studied by temperature dependent far infrared measurements. While for pure Ni the spectrum shows a flat high reflectivity, those for x ∼ 0.84 and ∼0.75 have a Drude component, vibrational modes mostly carrier screened, and a long tail that extents toward near infrared. This is associated with hopping electron conductivity and strong electron–phonon interactions. The relative reduction of the number of carriers in Ni0.75(SiO2)0.25 allows less screened phonon bands on the top of a continuum and a wide and overdamped oscillator at mid-infrared frequencies. Ni0.54(SiO2)0.46 and Ni0.28(SiO2)0.72 have well defined vibrational bands and a sharp threshold at ∼1450 cm−1. It is most remarkable that a distinctive resonant peak at ∼1250 cm−1 found for p-polarized angle dependent specular reflectivity. It originates in an electron cloud traced to electrons that are not able to overcome the metal–dielectric interface that, beating against the positive background, generates the electric dipole. Overall, we conclude that the spectra are analogous to those regularly found in conducting oxides where with a suitable percolating network polarons are formed.
Journal of Applied Physics, Jan 1, 2011
This paper reports the structural and magnetic properties of Ag-CoFe2O4 colloidal dimer nanoparti... more 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
Physical Review B, Jan 1, 2005
Nanostructure of granular Co-SiO 2 thin films modified by thermal treatment and its relationship ... more Nanostructure of granular Co-SiO 2 thin films modified by thermal treatment and its relationship with the giant Hall effect. Leandro M. Socolovsky 1 * , Cristiano LP Oliveira 2,3 , Juliano C. Denardin 1 , Marcelo Knobel 1 , and ...
Solid State Communications, Jan 1, 2007
We report the infrared specular reflectivity of Cox(SiO2)1−x (x∼0.85x∼0.85, 0.55, 0.38) films on ... more We report the infrared specular reflectivity of Cox(SiO2)1−x (x∼0.85x∼0.85, 0.55, 0.38) films on SiO2 glass spanning from a metal-like to insulating behavior. While films for x∼0.85x∼0.85 show carrier metallic shielding and hopping conductivity, for x∼0.65x∼0.65 and lower concentrations, the nanoparticles’ number and size promote a localization edge near the highest longitudinal optical frequency. Such an edge is associated with a reflectivity minimum and a higher frequency band connoting strong electron–phonon interactions, carrier phonon assisted hopping, and polaron formation. Optical conductivity fits with current polaron models provide grounds toward a microscopic understanding of transport properties in these as-prepared granular films.
Journal of Applied Physics, Jan 1, 2006
Granular TMx(SiO2)1-x (TM=Co,Fe,Ni) thin films were thermally treated at different temperatures a... more 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.
Journal of Applied Physics, Jan 1, 2009
We report on near normal infrared reflectivity spectra of ~550 nm thick films made of cosputtered... more 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
Journal of Physics D-applied Physics, Jan 1, 2010
Physica B-condensed Matter, Jan 1, 2004
The alloy Fe65Ni20Nb6B9 was obtained from the elemental constituents in a high-energy planetary b... more The alloy Fe65Ni20Nb6B9 was obtained from the elemental constituents in a high-energy planetary ball mill and subsequently thermally treated at 873 and 900 K in order to obtain the equilibrium phases. The as-prepared nanocrystalline alloy consists primarily of metastable BCC α-Fe(Ni) nanocrystals while the treated ones consist of a mixture of BCC (ferromagnetic) and FCC (paramagnetic at room temperature) phases. Hysteresis loops at 5 and 300 K present low remanence and coercivity. As-prepared sample exhibits the best soft magnetic properties. In all samples, the susceptibility curves suggest magnetic collective (long-range order) behavior with a maximum between 70 and 90 K. This feature is caused by nanometric-sized magnetic particles. The huge difference between blocking and irreversibility temperatures in the field cooling and zero-field cooling scans indicates a wide grain-size distribution.
IEEE Transactions on Magnetics, Jan 1, 2003
In situ measurements of resistance were made on cosputtered Co0.35[SiO2]0.65 granular films durin... more In situ measurements of resistance were made on cosputtered Co0.35[SiO2]0.65 granular films during annealing. The aim is to control the thermal treatment parameters and map the microstructural changes of the samples, with the respective magnetotransport response. Results of transmission electron microscopy, dc magnetization, and temperature dependence of resistivity, after annealing, show a clear evolution in the nanostructure of the samples, with increasing average Co grain sizes and wider dispersion.
IEEE Transactions on Magnetics, Jan 1, 2003
Size-controlled Fe nanoparticles have been obtained by thermal decomposition of Fe(CO)5 in organi... more Size-controlled Fe nanoparticles have been obtained by thermal decomposition of Fe(CO)5 in organic solution, in the presence of ligand molecules. Samples with different sizes, in the 6-12 nm range have been characterized by high-resolution transmission electron microscopy, X-ray diffraction, Mössbauer spectroscopy, and magnetization measurements. All samples present narrow size distributions and superparamagnetic behavior. The exposition to air leads to the formation of a polycrystalline phase, mainly Fe3O4.
Journal of Magnetism and Magnetic Materials, Jan 1, 2007
In this work the magnetocaloric effect in a Fe63.5Cr10Si13.5B9CuNb3 soft magnetic nanocrystalline... more In this work the magnetocaloric effect in a Fe63.5Cr10Si13.5B9CuNb3 soft magnetic nanocrystalline alloy is analysed. High resolution transmission electron microscocopy indicates the precipitation of the desired nanocrystalline structure (grains around 10 nm in size surrounded by a residual amorphous phase) upon suitable treatments of the initial amorphous sample. The temperature dependence of the magnetic entropy variation, ΔSM(T)ΔSM(T), calculated from the magnetization curves displays a maximum negative value around the Curie temperature of the residual amorphous phase (TC,a≈180TC,a≈180 K). The dependence of ΔSM(T)ΔSM(T) on the applied field and its evolution with the nanocrystalline volume fraction indicates the main contribution of the residual amorphous phase to the magnetic entropy change. Some specific low temperature magnetization features (spin freezing) are also analysed in terms of ΔSM(T)ΔSM(T).
Physica B-condensed Matter, Jan 1, 2004
We have investigated the mechanically induced self-propagating reaction between Fe2O3 and Al. In ... more We have investigated the mechanically induced self-propagating reaction between Fe2O3 and Al. In this reaction the final phases, Al2O3 and Fe, are formed by an in situ chemical reaction. The evolution of the system was characterized as a function of the milling time using X-ray diffraction, magnetometry (300 and 5 K) and Mössbauer spectroscopy (300 and 77 K). After reaction, Fe particles in a crystalline Al2O3 matrix have been formed. At larger milling times, a rather wide Fe grain size distribution of around 20 nm was obtained according to the diffraction patterns. Mössbauer spectra were constituted of both paramagnetic and ferromagnetic contributions. The former was attributed to small grains of superparamagnetic Fe (within the Mössbauer characteristic time τM≈10−8 s) and FeAl2O4, whereas the latter was associated to larger Fe grains. Saturation magnetization, coercivity field and remanent magnetization rapidly reached a stationary value with the milling time. A correlation between the microstructural characteristics of the sample and its magnetic properties before and after reaction is described.
Journal of Magnetism and Magnetic Materials, Jan 1, 2005
The magnetic behavior of superparamagnetic Co nanoparticles (2–4 nm in diameter) dispersed in an ... more The magnetic behavior of superparamagnetic Co nanoparticles (2–4 nm in diameter) dispersed in an amorphous, insulating SiO2 matrix was studied. Conventional fittings of magnetization curves present mean magnetic moments which diminish with decrease in temperature. In order to treat this anomalous behavior, we have applied the interacting superparamagnetic model (ISP). Mean diameters obtained from transmission electron microscopy (TEM) were compared with values obtained applying ISP model.
Journal of Applied Physics, Jan 1, 2007
Nanotechnology, Jan 1, 2005
Page 1. Dipolar interaction and size effects in powder samples of colloidal iron oxide nanopartic... more Page 1. Dipolar interaction and size effects in powder samples of colloidal iron oxide nanoparticles This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2005 Nanotechnology 16 S285 (http://iopscience.iop.org/0957-4484/16/5/028) ...
Physical Review B, Jan 1, 2005
Physical Review B, Jan 1, 2005
ABSTRACT
In this work we report on the study of the magnetic properties of 2D arrays and 3D dispersions of... more In this work we report on the study of the magnetic properties of 2D arrays and 3D dispersions of colloidal iron oxide nanoparticles prepared by Langmuir–Blodgett technique and by dilution in paraffin wax solid solution, respectively. The influence of magnetic inter-particle coupling on the superparamagnetic relaxation behavior was investigated by means of DC magnetization measurements. A quan-
titative analysis of the field dependence of the blocking temperature and the role of the interparticle coupling is presented. We explain our results using a phenomenological model based on the random anisotropy and micromagnetic theories that account for particle coupling effects in the superparamagnetic properties.
Field dependence of the optical transmission of a polyaspartic-coated magnetite magnetic fluid di... more Field dependence of the optical transmission of a polyaspartic-coated magnetite magnetic fluid dispersed in water was investigated at different particle volume fractions. The particle size distribution of the sample was obtained from the analysis of the transmission electron microscopy pictures. The transmissivity decreased increasing the magnetic field until a critical field is achieved. Above this value, the opposite effect was observed. Indeed, the critical field decreases the higher the particle volume fraction being in qualitative similarity with phase separation behavior. However, the origin of the effect is attributed to the precipitation of field-induced nanoparticle chains. These phenomena might be useful on obtaining one-dimensional nanoparticle arrangements.
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Papers by Leandro Socolovsky
titative analysis of the field dependence of the blocking temperature and the role of the interparticle coupling is presented. We explain our results using a phenomenological model based on the random anisotropy and micromagnetic theories that account for particle coupling effects in the superparamagnetic properties.
titative analysis of the field dependence of the blocking temperature and the role of the interparticle coupling is presented. We explain our results using a phenomenological model based on the random anisotropy and micromagnetic theories that account for particle coupling effects in the superparamagnetic properties.