Daniel Cornejo - Academia.edu (original) (raw)

Papers by Daniel Cornejo

Journal of Superconductivity and Novel Magnetism, 2013

This work describes the study on the magnetization reversal for two different spin valves: the fi... more This work describes the study on the magnetization reversal for two different spin valves: the first one is characterized by in-plane magnetic anisotropy, and the second by a perpendicular anisotropy. Magnetoresistance-firstorder-reversal-curve analysis was used in order to map the magnetoresistance of each valve as a function of the coercive fields and the interaction fields of the particles in the layers. For the valve with in-plane anisotropy, it was possible to identify an exchange-spring behavior in the pinned layer induced by the contact with the AF layer, and meanfield effects produced by interparticle interactions. Also, it was determined that the rotation of the magnetization of the pinned layer cannot happen by coherent rotation. For the second valve, it was possible to identify the distribution of coercivities and interaction fields of each layer, as a function the different magnetic orientations that they may have.

Chemical Communications, 2013

We describe a fast and simple method to prepare composite films of magnetite nanoparticles and xa... more We describe a fast and simple method to prepare composite films of magnetite nanoparticles and xanthan networks. The particles are distributed close to hybrid film surface, generating a coercivity of 27 ± 2 Oe at 300 K. The proliferation of fibroblast cells on the hybrid composites was successful, particularly when an external magnetic field was applied.

Physica B: Condensed Matter, 2000

Recent studies of demagnetization processes in permanent magnets have used two di!erent methods f... more Recent studies of demagnetization processes in permanent magnets have used two di!erent methods for determining the reversible magnetization, often arriving at con#icting results. In the present work, enhanced-remanence SmCo "lms have been prepared by DC magnetron sputtering. The evolution of the reversible (M ) and irreversible (M ) magnetization along the initial magnetization curve and the demagnetization curve of the "lms was determined using the two methods. In one case, the DCD de"nition of the reversible magnetization was used to measure the quantity from a set of recoil curves. Alternatively, the moving Preisach model was used to separate the magnetization into its components M and M . We discuss the di!erent behaviors encountered for the reversible magnetization and the inter-relation function "(RM /RM ) & and interpret the di!erences.

Se realizó un estudio del proceso de inversión de la magnetización a 300 K en un material de ferr... more Se realizó un estudio del proceso de inversión de la magnetización a 300 K en un material de ferrita de cobalto co-precipitada. La evolución de las componentes reversible M rev e irreversible M irr de la magnetización se determinó por el Método de Magnetización Isotérmica Remanente (MIR) que consiste en medir un conjunto de curvas de retorno a partir de diferentes puntos de la curva de magnetización inicial y por el Método de Desmagnetización de Corriente Directa (DCD) que consiste en medir un conjunto de curvas de retorno a partir de diferentes puntos de la curva mayor de desmagnetización. Estudios recientes han demostrado que existe una interrelación entre M rev y M irr expresada fenomenológicamente por la ecuación constitutiva dM rev = χ i rev dH+ηdM irr , donde χ i rev es la susceptibilidad intrínseca reversible, H i es el campo interno y η(H i ,H irr )= (∂M rev /∂M irr ) H i es la denominada función de interrelación. En esta ecuación constitutiva el primer término se encuentra asociado con la rotación reversible del vector de magnetización y el segundo término se debe a cambios en la cantidad y el movimiento de las paredes de dominio. En este trabajo se realizó una determinación experimental completa del comportamiento de M rev (H i ,M irr ) y η(H i ,M irr ) Las curvas M rev (H i ,M irr ) para H i constante muestran un máximo bien definido (mínimo) para la curva de desmagnetización característico del movimiento de las paredes de dominio dentro de los granos del material. Por otro lado, el comportamiento experimental de R(H i ,M irr ) sugiere que el mecanismo dominante para la inversión de la magnetización en este material es el movimiento de las paredes de los dominios sujetas a anclaje.

... respectivamente. 2. Através da micrografia do pó foi possível observar a formação de aglomera... more ... respectivamente. 2. Através da micrografia do pó foi possível observar a formação de aglomerados moles (constituídos por forças fracas de Van der Waals) de nanopartículas. ... Mater. 164 (1996) 255-260. 15. K. Suresh, NRS Kumar, KC Patil, Advan. Mater. Comm. 3 [3] (1991) ...

SmCo films with a Cr underlayer have been investigated as potentially attractive candidates for h... more SmCo films with a Cr underlayer have been investigated as potentially attractive candidates for high density recording media. Magnetron sputtering was used here to produce Cr/SmCo x /Cr films on Si ͑100͒ substrates. The magnetic films were deposited at a substrate temperature of 350°C and an Ar pressure of 5 mTorr. Cr underlayers were deposited both at 25°C and at 350°C and exhibited different textures. Layer thicknesses were evaluated using Rutherford backscattering spectroscopy, while SmCo grain size and underlayer texture were determined from a Rietveld analysis of x-ray diffraction data. The magnetic properties were measured with a vibrating sample magnetometer. The resulting films had in-plane coercivities in the range 4-8 kOe. For a SmCo layer with a thickness of 200 nm, the Cr underlayer thickness was varied in the range 75-300 nm to study its effect on intergranular coupling in the films. For thermally demagnetized samples, both magnetizing and demagnetizing remanence curves were measured and used to evaluate switching field distributions and ␦m plots. The underlayer texture was found to affect magnetic properties more strongly than underlayer thickness.

Astrophysical Journal, 2011

The atmospheric parameters of the components of the 16 Cygni binary system, in which the secondar... more The atmospheric parameters of the components of the 16 Cygni binary system, in which the secondary has a gas giant planet detected, are measured accurately using high-quality observational data. Abundances relative to solar are obtained for 25 elements with a mean error of σ([X/H]) = 0.023 dex. The fact that 16 Cyg A has about four times more lithium than 16 Cyg B is normal considering the slightly different masses of the stars. The abundance patterns of 16 Cyg A and B, relative to iron, are typical of that observed in most of the so-called solar twin stars, with the exception of the heavy elements (Z > 30), which can, however, be explained by Galactic chemical evolution. Differential (A-B) abundances are measured with even higher precision (σ(Δ[X/H]) = 0.018 dex, on average). We find that 16 Cyg A is more metal-rich than 16 Cyg B by Δ[M/H] = +0.041 ± 0.007 dex. On an element-to-element basis, no correlation between the A-B abundance differences and dust condensation temperature (T C) is detected. Based on these results, we conclude that if the process of planet formation around 16 Cyg B is responsible for the observed abundance pattern, the formation of gas giants produces a constant downward shift in the photospheric abundance of metals, without a T C correlation. The latter would be produced by the formation of terrestrial planets instead, as suggested by other recent works on precise elemental abundances. Nevertheless, a scenario consistent with these observations requires the convective envelopes of ~= 1 M sun stars to reach their present-day sizes about three times quicker than predicted by standard stellar evolution models.

An evaluation was made of the microstructure and magnetic properties of Ni0.5-xCuxZn0,5Fe2O4 ferr... more An evaluation was made of the microstructure and magnetic properties of Ni0.5-xCuxZn0,5Fe2O4 ferrites with x = 0.1, 0.2, 0.3 and 0.4 prepared by combustion reaction with particle sizes 23 to 29 nm. The resulting powders were uniaxially compacted at 385 MPa into pellets and sintered at 1000 ºC/2 h. The samples were characterized by apparent density and porosity measurements, X-ray diffraction, scanning electron microscopy and M-H magnetic measurements. The X-ray diffraction revealed crystalline inverse spinel as the major phase in all the specimens and a small amount of hematite (a-Fe2O3) as the secondary phase in the x = 0.1 and 0.4 samples. An increase in the amount of copper caused a slight increase in grain size (0.65 to 0.68 µm), a decrease in the apparent porosity (33.7% to 6.6%) and a reduction in saturation magnetization from 69 to 54 emu/g.

Chromium-doped nickel nanoferrite powder was prepared by combustion reaction, using microwave ene... more Chromium-doped nickel nanoferrite powder was prepared by combustion reaction, using microwave energy as the external heating source, and the influence of the Cr3+concentration on the NiCrxFe2-xO4 system was investigated. The powder, which was prepared according to the concept of propellant chemistry, was heated in a microwave oven under a power of 980 W. The resulting material was characterized by XRD, BET, SEM, helium picnometry and magnetic measurements. The results indicate that it was possible to obtain chromium-doped nickel ferrite nanopowders and that raising the chromium concentration caused an increase in particle size (from 15 nm to 55 nm), and a reduction in the magnetic parameters.

Journal of Magnetism and Magnetic Materials, 2007

Fe polycrystalline nanowires were electrodeposited in nanoporous anodized alumina membranes. Thei... more Fe polycrystalline nanowires were electrodeposited in nanoporous anodized alumina membranes. Their magnetic properties were studied at 10 K. The behavior of the isothermal remanence MrMr and the demagnetization remanence MdMd was determined. The corresponding ΔMΔM plot revealed the character demagnetizing of the dominant interactions. A simple analysis suggests that curling is the reversal magnetization mode in these samples. The forms of the Mrev(Mirr)HiMrev(Mirr)Hi curves were different from those encountered until now, and it seems that these forms are associated with the curling reversal mode.

Materials Science Forum, 2012

Research in Mn-Zn ferrites have been growing in recent years, mainly because of the properties in... more Research in Mn-Zn ferrites have been growing in recent years, mainly because of the properties in high frequency application such as high saturation magnetization, low core losses, which are sensitive to the structure, processing conditions, such as sintering temperature, time and atmosphere. This study aims to evaluate the microstructure and magnetic characteristics Mn 0.65 Zn 0.35 Fe 2 O 4 ferrite sintered in N 2 atmosphere and to microwave energy at 1200 o C/2h. The samples were compacted by uniaxial pressing at 200 MPa and sintered in a microwave oven with a heating rate of 50 o C/min and sintering time of 30 min. For samples sintered in N 2 atmosphere was used heating rate of 5 o C/min with sintering time of 2 hours. The samples were characterized by apparent and bulk density, XRD, SEM and magnetic measurements. The results indicate the formation single phase of Mn-Zn ferrite for sample sintering in N 2 atmosphere, and to sintering by microwave oven observed the phase ferrite Mn-Zn with trace of secondary phase hematite. The density and saturation magnetization were 3.14 and 3.12 g/cm 3 , 5 and 83 emug -1 for the samples sintered by microwave energy and N 2 atmosphere respectively.

Materials Science Forum, 2014

This work involved a study of the reproducibility of the process of combustion synthesis to produ... more This work involved a study of the reproducibility of the process of combustion synthesis to produce Ni-Zn ferrites. The structural, morphological and magnetic characteristics of the samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and magnetometry using an alternating gradient magnetometer (AGM). The XRD diffractograms of the samples indicated that they are monophasic, crystalline, with crystallite sizes ranging from 21 to 38 nm, and have a homogeneous morphology consisting of agglomerates of spherical particles. The samples behaved as soft magnetic materials, with magnetization levels ranging from 37 to 47 emu·g -1 .

Materials Science Forum, 2014

This study proposes to evaluate the influence of the variation of sintering temperature on micros... more This study proposes to evaluate the influence of the variation of sintering temperature on microstructural characteristics and magnetic ferrite Ni 0,5 Zn 0,5 Fe 2 O 4 sintered by microwave energy. The samples were sintered at 900, 1000, 1100 and 1200 o C for exposure time of 10 minutes, with rate 50 o C/minutes and characterized by density and porosity, X-ray diffraction, scanning electron microscopy and magnetic measurements. The results indicate that the values of density and apparent porosity were 4.2, 4.5, 4.4 and 4.5 g/cm 3 and 3.4, 2.1, 2.2 and 2.4% for the sintering temperatures of 900, 1000, 1100 and 1200 o C respectively. The formation of the ferrite phase Ni 0,5 Zn 0,5 Fe 2 O 4 been identified for all conditions of sintering, with grain sizes of 52, 62, 71 and 58nm and saturation magnetization values of 63, 68, 69 and 27 emu/g to temperatures sintering 900,

Advanced Powder Technology Vii, 2010

This paper describes the synthesis of Ni-Zn nanoferrites by combustion reaction using microwave e... more This paper describes the synthesis of Ni-Zn nanoferrites by combustion reaction using microwave energy as a heating source, and evaluates the performance of these materials as absorbers of electromagnetic energy at frequencies between 4 -12 GHz. The influence of the synthesis conditions on the structure, morphology and absorption characteristics was investigated. The powders were characterized by DRX, BET, AGM and reflectivity measurements in the frequency bands of 8 to 12 GHz. The XRD results show the formation of Ni-Zn ferrite phase and Fe 2 O 3 and Ni as secondary phases. The crystallite sizes ranged from 32 to 42 nm. The parameters of exposure time and power of the microwave oven changed the final characteristics of the resulting powders. The morphology of all the powders consisted of soft nanoparticle agglomerates. The best saturation magnetization and attenuation results were 70 emu/g and -4.1 dB in the frequency of 10 GHZ.

Advanced Powder Technology Viii, Pts 1 and 2, 2012

This paper proposes to assess the sintering of Ni-Zn ferrites synthesized by combustion reaction ... more This paper proposes to assess the sintering of Ni-Zn ferrites synthesized by combustion reaction in advance to procure materials for applications as soft magnetic devices. The samples used for sintering have different morphological characteristics and were previously synthesized by microwave energy. The samples were uniaxially pressed and sintered in a microwave oven at 1200 o C/2h with a heating rate of 5 o C/min, and characterized by XRD, SEM and magnetic measurements. The results show that for all samples have the formation of Ni-Zn phase and traces hematite as secondary phase. The resulting microstructure after sintering was different and was influenced by previous morphological characteristics of the synthesized samples. As for the magnetic parameters, all samples were characteristic of soft magnetic material with saturation magnetization between 57 and 62 emu.g -1 , indicating are promising materials for the fabrication of soft magnetic devices.

Advanced Powder Technology Viii, Pts 1 and 2, 2012

This paper assesses the sintering in a conventional oven and microwave oven of Ni-Zn ferrite. The... more This paper assesses the sintering in a conventional oven and microwave oven of Ni-Zn ferrite. The samples were previously synthesized by combustion reaction in the microwave, and then sintered in a conventional oven and microwave oven 1200 o C/2h with a heating rate of 5 o C/min. The samples were characterized by: XRD, SEM and magnetic measurements by AGM. The results indicate the formation of inverse spinel crystalline phase of Ni-Zn ferrite, with inter-and intergranular porosity and average grain size of 0.15 µm and 0.30 µm, respectively for the samples sintered in a conventional oven and microwave oven. The sample sintered in conventional furnace showed a pattern characteristic of superparamagnetic materials with saturation magnetization of 7.6 emu.g -1 , while in the microwave sintered sample showed a typical behavior of soft magnetic materials with saturation magnetization of 70 emu.g -1 .

Magnetics International Conference, 1998

A series of densified exchange-enhanced SmFeCo magnets was produced by mechanical alloying, follo... more A series of densified exchange-enhanced SmFeCo magnets was produced by mechanical alloying, followed by a heat treatment. The nanocrystalline phases and the grain size distribution of the samples were characterized from x-ray diffraction patterns. Magnetic measurements in terms of different models were analyzed for the temperature behavior of the magnetization and coercivity. One analytical formulation was derived from the random anisotropy model predicting that with increasing M r /M s ratio a decreasing coercivity occurs. Another numerical approach, which is based on a micromagnetic calculation using finite element techniques was also applied. Attempts were made to improve the existing model derived from the finite element technique. The nucleation model was found to be unsatisfactory for describing the temperature-dependent coercivity.

Journal of Magnetism and Magnetic Materials, 2008

Ni-Zn ferrites have been widely used in components for high-frequency range applications due to t... more Ni-Zn ferrites have been widely used in components for high-frequency range applications due to their high electrical resistivity, mechanical strength and chemical stability. Ni-Zn ferrite nanopowders doped with samarium with a nominal composition of Ni 0.5 Zn 0.5 Fe 2Àx Sm x O 4 (x ¼ 0.0, 0.05, and 0.1 mol) were obtained by combustion synthesis using nitrates and urea as fuel. The morphological aspects of Ni-Zn-Sm ferrite nanopowders were investigated by X-ray diffraction, nitrogen adsorption by BET, sedimentation, scanning electron microscopy and magnetic properties. The results indicated that the Ni-Zn-Sm ferrite nanopowders were composed of soft agglomerates of nanoparticles with a high surface area (55.8-64.8 m 2 /g), smaller particles (18-20 nm) and nanocrystallite size particles. The addition of samarium resulted in a reduction of all the magnetic parameters evaluated, namely saturation magnetization (24-40 emu/g), remanent magnetization (2.2-3.5 emu/g) and coercive force (99.3-83.3 Oe).

Journal of Magnetism and Magnetic Materials, 1996

Magnetization reversal on recoil curves of a sintered SmCo 5 magnet is compared with that for a n... more Magnetization reversal on recoil curves of a sintered SmCo 5 magnet is compared with that for a nanocrystalline SmFeCo alloy prepared by mechanical alloying. Recoil curves were found to be hysteretic and time-dependent in both cases, but magnetic viscosity was observed only for the nanocrystalline SmFeCo alloy. Magnetic interactions were studied via Henkel plots for different initial demagnetized states.

Journal of Superconductivity and Novel Magnetism, 2013

This work describes the study on the magnetization reversal for two different spin valves: the fi... more This work describes the study on the magnetization reversal for two different spin valves: the first one is characterized by in-plane magnetic anisotropy, and the second by a perpendicular anisotropy. Magnetoresistance-firstorder-reversal-curve analysis was used in order to map the magnetoresistance of each valve as a function of the coercive fields and the interaction fields of the particles in the layers. For the valve with in-plane anisotropy, it was possible to identify an exchange-spring behavior in the pinned layer induced by the contact with the AF layer, and meanfield effects produced by interparticle interactions. Also, it was determined that the rotation of the magnetization of the pinned layer cannot happen by coherent rotation. For the second valve, it was possible to identify the distribution of coercivities and interaction fields of each layer, as a function the different magnetic orientations that they may have.

Chemical Communications, 2013

We describe a fast and simple method to prepare composite films of magnetite nanoparticles and xa... more We describe a fast and simple method to prepare composite films of magnetite nanoparticles and xanthan networks. The particles are distributed close to hybrid film surface, generating a coercivity of 27 ± 2 Oe at 300 K. The proliferation of fibroblast cells on the hybrid composites was successful, particularly when an external magnetic field was applied.

Physica B: Condensed Matter, 2000

Recent studies of demagnetization processes in permanent magnets have used two di!erent methods f... more Recent studies of demagnetization processes in permanent magnets have used two di!erent methods for determining the reversible magnetization, often arriving at con#icting results. In the present work, enhanced-remanence SmCo "lms have been prepared by DC magnetron sputtering. The evolution of the reversible (M ) and irreversible (M ) magnetization along the initial magnetization curve and the demagnetization curve of the "lms was determined using the two methods. In one case, the DCD de"nition of the reversible magnetization was used to measure the quantity from a set of recoil curves. Alternatively, the moving Preisach model was used to separate the magnetization into its components M and M . We discuss the di!erent behaviors encountered for the reversible magnetization and the inter-relation function "(RM /RM ) & and interpret the di!erences.

Se realizó un estudio del proceso de inversión de la magnetización a 300 K en un material de ferr... more Se realizó un estudio del proceso de inversión de la magnetización a 300 K en un material de ferrita de cobalto co-precipitada. La evolución de las componentes reversible M rev e irreversible M irr de la magnetización se determinó por el Método de Magnetización Isotérmica Remanente (MIR) que consiste en medir un conjunto de curvas de retorno a partir de diferentes puntos de la curva de magnetización inicial y por el Método de Desmagnetización de Corriente Directa (DCD) que consiste en medir un conjunto de curvas de retorno a partir de diferentes puntos de la curva mayor de desmagnetización. Estudios recientes han demostrado que existe una interrelación entre M rev y M irr expresada fenomenológicamente por la ecuación constitutiva dM rev = χ i rev dH+ηdM irr , donde χ i rev es la susceptibilidad intrínseca reversible, H i es el campo interno y η(H i ,H irr )= (∂M rev /∂M irr ) H i es la denominada función de interrelación. En esta ecuación constitutiva el primer término se encuentra asociado con la rotación reversible del vector de magnetización y el segundo término se debe a cambios en la cantidad y el movimiento de las paredes de dominio. En este trabajo se realizó una determinación experimental completa del comportamiento de M rev (H i ,M irr ) y η(H i ,M irr ) Las curvas M rev (H i ,M irr ) para H i constante muestran un máximo bien definido (mínimo) para la curva de desmagnetización característico del movimiento de las paredes de dominio dentro de los granos del material. Por otro lado, el comportamiento experimental de R(H i ,M irr ) sugiere que el mecanismo dominante para la inversión de la magnetización en este material es el movimiento de las paredes de los dominios sujetas a anclaje.

... respectivamente. 2. Através da micrografia do pó foi possível observar a formação de aglomera... more ... respectivamente. 2. Através da micrografia do pó foi possível observar a formação de aglomerados moles (constituídos por forças fracas de Van der Waals) de nanopartículas. ... Mater. 164 (1996) 255-260. 15. K. Suresh, NRS Kumar, KC Patil, Advan. Mater. Comm. 3 [3] (1991) ...

SmCo films with a Cr underlayer have been investigated as potentially attractive candidates for h... more SmCo films with a Cr underlayer have been investigated as potentially attractive candidates for high density recording media. Magnetron sputtering was used here to produce Cr/SmCo x /Cr films on Si ͑100͒ substrates. The magnetic films were deposited at a substrate temperature of 350°C and an Ar pressure of 5 mTorr. Cr underlayers were deposited both at 25°C and at 350°C and exhibited different textures. Layer thicknesses were evaluated using Rutherford backscattering spectroscopy, while SmCo grain size and underlayer texture were determined from a Rietveld analysis of x-ray diffraction data. The magnetic properties were measured with a vibrating sample magnetometer. The resulting films had in-plane coercivities in the range 4-8 kOe. For a SmCo layer with a thickness of 200 nm, the Cr underlayer thickness was varied in the range 75-300 nm to study its effect on intergranular coupling in the films. For thermally demagnetized samples, both magnetizing and demagnetizing remanence curves were measured and used to evaluate switching field distributions and ␦m plots. The underlayer texture was found to affect magnetic properties more strongly than underlayer thickness.

Astrophysical Journal, 2011

The atmospheric parameters of the components of the 16 Cygni binary system, in which the secondar... more The atmospheric parameters of the components of the 16 Cygni binary system, in which the secondary has a gas giant planet detected, are measured accurately using high-quality observational data. Abundances relative to solar are obtained for 25 elements with a mean error of σ([X/H]) = 0.023 dex. The fact that 16 Cyg A has about four times more lithium than 16 Cyg B is normal considering the slightly different masses of the stars. The abundance patterns of 16 Cyg A and B, relative to iron, are typical of that observed in most of the so-called solar twin stars, with the exception of the heavy elements (Z > 30), which can, however, be explained by Galactic chemical evolution. Differential (A-B) abundances are measured with even higher precision (σ(Δ[X/H]) = 0.018 dex, on average). We find that 16 Cyg A is more metal-rich than 16 Cyg B by Δ[M/H] = +0.041 ± 0.007 dex. On an element-to-element basis, no correlation between the A-B abundance differences and dust condensation temperature (T C) is detected. Based on these results, we conclude that if the process of planet formation around 16 Cyg B is responsible for the observed abundance pattern, the formation of gas giants produces a constant downward shift in the photospheric abundance of metals, without a T C correlation. The latter would be produced by the formation of terrestrial planets instead, as suggested by other recent works on precise elemental abundances. Nevertheless, a scenario consistent with these observations requires the convective envelopes of ~= 1 M sun stars to reach their present-day sizes about three times quicker than predicted by standard stellar evolution models.

An evaluation was made of the microstructure and magnetic properties of Ni0.5-xCuxZn0,5Fe2O4 ferr... more An evaluation was made of the microstructure and magnetic properties of Ni0.5-xCuxZn0,5Fe2O4 ferrites with x = 0.1, 0.2, 0.3 and 0.4 prepared by combustion reaction with particle sizes 23 to 29 nm. The resulting powders were uniaxially compacted at 385 MPa into pellets and sintered at 1000 ºC/2 h. The samples were characterized by apparent density and porosity measurements, X-ray diffraction, scanning electron microscopy and M-H magnetic measurements. The X-ray diffraction revealed crystalline inverse spinel as the major phase in all the specimens and a small amount of hematite (a-Fe2O3) as the secondary phase in the x = 0.1 and 0.4 samples. An increase in the amount of copper caused a slight increase in grain size (0.65 to 0.68 µm), a decrease in the apparent porosity (33.7% to 6.6%) and a reduction in saturation magnetization from 69 to 54 emu/g.

Chromium-doped nickel nanoferrite powder was prepared by combustion reaction, using microwave ene... more Chromium-doped nickel nanoferrite powder was prepared by combustion reaction, using microwave energy as the external heating source, and the influence of the Cr3+concentration on the NiCrxFe2-xO4 system was investigated. The powder, which was prepared according to the concept of propellant chemistry, was heated in a microwave oven under a power of 980 W. The resulting material was characterized by XRD, BET, SEM, helium picnometry and magnetic measurements. The results indicate that it was possible to obtain chromium-doped nickel ferrite nanopowders and that raising the chromium concentration caused an increase in particle size (from 15 nm to 55 nm), and a reduction in the magnetic parameters.

Journal of Magnetism and Magnetic Materials, 2007

Fe polycrystalline nanowires were electrodeposited in nanoporous anodized alumina membranes. Thei... more Fe polycrystalline nanowires were electrodeposited in nanoporous anodized alumina membranes. Their magnetic properties were studied at 10 K. The behavior of the isothermal remanence MrMr and the demagnetization remanence MdMd was determined. The corresponding ΔMΔM plot revealed the character demagnetizing of the dominant interactions. A simple analysis suggests that curling is the reversal magnetization mode in these samples. The forms of the Mrev(Mirr)HiMrev(Mirr)Hi curves were different from those encountered until now, and it seems that these forms are associated with the curling reversal mode.

Materials Science Forum, 2012

Research in Mn-Zn ferrites have been growing in recent years, mainly because of the properties in... more Research in Mn-Zn ferrites have been growing in recent years, mainly because of the properties in high frequency application such as high saturation magnetization, low core losses, which are sensitive to the structure, processing conditions, such as sintering temperature, time and atmosphere. This study aims to evaluate the microstructure and magnetic characteristics Mn 0.65 Zn 0.35 Fe 2 O 4 ferrite sintered in N 2 atmosphere and to microwave energy at 1200 o C/2h. The samples were compacted by uniaxial pressing at 200 MPa and sintered in a microwave oven with a heating rate of 50 o C/min and sintering time of 30 min. For samples sintered in N 2 atmosphere was used heating rate of 5 o C/min with sintering time of 2 hours. The samples were characterized by apparent and bulk density, XRD, SEM and magnetic measurements. The results indicate the formation single phase of Mn-Zn ferrite for sample sintering in N 2 atmosphere, and to sintering by microwave oven observed the phase ferrite Mn-Zn with trace of secondary phase hematite. The density and saturation magnetization were 3.14 and 3.12 g/cm 3 , 5 and 83 emug -1 for the samples sintered by microwave energy and N 2 atmosphere respectively.

Materials Science Forum, 2014

This work involved a study of the reproducibility of the process of combustion synthesis to produ... more This work involved a study of the reproducibility of the process of combustion synthesis to produce Ni-Zn ferrites. The structural, morphological and magnetic characteristics of the samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and magnetometry using an alternating gradient magnetometer (AGM). The XRD diffractograms of the samples indicated that they are monophasic, crystalline, with crystallite sizes ranging from 21 to 38 nm, and have a homogeneous morphology consisting of agglomerates of spherical particles. The samples behaved as soft magnetic materials, with magnetization levels ranging from 37 to 47 emu·g -1 .

Materials Science Forum, 2014

This study proposes to evaluate the influence of the variation of sintering temperature on micros... more This study proposes to evaluate the influence of the variation of sintering temperature on microstructural characteristics and magnetic ferrite Ni 0,5 Zn 0,5 Fe 2 O 4 sintered by microwave energy. The samples were sintered at 900, 1000, 1100 and 1200 o C for exposure time of 10 minutes, with rate 50 o C/minutes and characterized by density and porosity, X-ray diffraction, scanning electron microscopy and magnetic measurements. The results indicate that the values of density and apparent porosity were 4.2, 4.5, 4.4 and 4.5 g/cm 3 and 3.4, 2.1, 2.2 and 2.4% for the sintering temperatures of 900, 1000, 1100 and 1200 o C respectively. The formation of the ferrite phase Ni 0,5 Zn 0,5 Fe 2 O 4 been identified for all conditions of sintering, with grain sizes of 52, 62, 71 and 58nm and saturation magnetization values of 63, 68, 69 and 27 emu/g to temperatures sintering 900,

Advanced Powder Technology Vii, 2010

This paper describes the synthesis of Ni-Zn nanoferrites by combustion reaction using microwave e... more This paper describes the synthesis of Ni-Zn nanoferrites by combustion reaction using microwave energy as a heating source, and evaluates the performance of these materials as absorbers of electromagnetic energy at frequencies between 4 -12 GHz. The influence of the synthesis conditions on the structure, morphology and absorption characteristics was investigated. The powders were characterized by DRX, BET, AGM and reflectivity measurements in the frequency bands of 8 to 12 GHz. The XRD results show the formation of Ni-Zn ferrite phase and Fe 2 O 3 and Ni as secondary phases. The crystallite sizes ranged from 32 to 42 nm. The parameters of exposure time and power of the microwave oven changed the final characteristics of the resulting powders. The morphology of all the powders consisted of soft nanoparticle agglomerates. The best saturation magnetization and attenuation results were 70 emu/g and -4.1 dB in the frequency of 10 GHZ.

Advanced Powder Technology Viii, Pts 1 and 2, 2012

This paper proposes to assess the sintering of Ni-Zn ferrites synthesized by combustion reaction ... more This paper proposes to assess the sintering of Ni-Zn ferrites synthesized by combustion reaction in advance to procure materials for applications as soft magnetic devices. The samples used for sintering have different morphological characteristics and were previously synthesized by microwave energy. The samples were uniaxially pressed and sintered in a microwave oven at 1200 o C/2h with a heating rate of 5 o C/min, and characterized by XRD, SEM and magnetic measurements. The results show that for all samples have the formation of Ni-Zn phase and traces hematite as secondary phase. The resulting microstructure after sintering was different and was influenced by previous morphological characteristics of the synthesized samples. As for the magnetic parameters, all samples were characteristic of soft magnetic material with saturation magnetization between 57 and 62 emu.g -1 , indicating are promising materials for the fabrication of soft magnetic devices.

Advanced Powder Technology Viii, Pts 1 and 2, 2012

This paper assesses the sintering in a conventional oven and microwave oven of Ni-Zn ferrite. The... more This paper assesses the sintering in a conventional oven and microwave oven of Ni-Zn ferrite. The samples were previously synthesized by combustion reaction in the microwave, and then sintered in a conventional oven and microwave oven 1200 o C/2h with a heating rate of 5 o C/min. The samples were characterized by: XRD, SEM and magnetic measurements by AGM. The results indicate the formation of inverse spinel crystalline phase of Ni-Zn ferrite, with inter-and intergranular porosity and average grain size of 0.15 µm and 0.30 µm, respectively for the samples sintered in a conventional oven and microwave oven. The sample sintered in conventional furnace showed a pattern characteristic of superparamagnetic materials with saturation magnetization of 7.6 emu.g -1 , while in the microwave sintered sample showed a typical behavior of soft magnetic materials with saturation magnetization of 70 emu.g -1 .

Magnetics International Conference, 1998

A series of densified exchange-enhanced SmFeCo magnets was produced by mechanical alloying, follo... more A series of densified exchange-enhanced SmFeCo magnets was produced by mechanical alloying, followed by a heat treatment. The nanocrystalline phases and the grain size distribution of the samples were characterized from x-ray diffraction patterns. Magnetic measurements in terms of different models were analyzed for the temperature behavior of the magnetization and coercivity. One analytical formulation was derived from the random anisotropy model predicting that with increasing M r /M s ratio a decreasing coercivity occurs. Another numerical approach, which is based on a micromagnetic calculation using finite element techniques was also applied. Attempts were made to improve the existing model derived from the finite element technique. The nucleation model was found to be unsatisfactory for describing the temperature-dependent coercivity.

Journal of Magnetism and Magnetic Materials, 2008

Ni-Zn ferrites have been widely used in components for high-frequency range applications due to t... more Ni-Zn ferrites have been widely used in components for high-frequency range applications due to their high electrical resistivity, mechanical strength and chemical stability. Ni-Zn ferrite nanopowders doped with samarium with a nominal composition of Ni 0.5 Zn 0.5 Fe 2Àx Sm x O 4 (x ¼ 0.0, 0.05, and 0.1 mol) were obtained by combustion synthesis using nitrates and urea as fuel. The morphological aspects of Ni-Zn-Sm ferrite nanopowders were investigated by X-ray diffraction, nitrogen adsorption by BET, sedimentation, scanning electron microscopy and magnetic properties. The results indicated that the Ni-Zn-Sm ferrite nanopowders were composed of soft agglomerates of nanoparticles with a high surface area (55.8-64.8 m 2 /g), smaller particles (18-20 nm) and nanocrystallite size particles. The addition of samarium resulted in a reduction of all the magnetic parameters evaluated, namely saturation magnetization (24-40 emu/g), remanent magnetization (2.2-3.5 emu/g) and coercive force (99.3-83.3 Oe).

Journal of Magnetism and Magnetic Materials, 1996

Magnetization reversal on recoil curves of a sintered SmCo 5 magnet is compared with that for a n... more Magnetization reversal on recoil curves of a sintered SmCo 5 magnet is compared with that for a nanocrystalline SmFeCo alloy prepared by mechanical alloying. Recoil curves were found to be hysteretic and time-dependent in both cases, but magnetic viscosity was observed only for the nanocrystalline SmFeCo alloy. Magnetic interactions were studied via Henkel plots for different initial demagnetized states.