KR Pirota

University of Campinas, Campinas, Estado de Sao Paulo, Brazil

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Publications (49)65.2 Total impact

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    ABSTRACT: Tailoring magnetic flux distribution is highly desirable in a wide range of applications such as magnetic sensors and biomedicine. In this paper we study the manipulation of induced currents in passive devices in order to engineer the distribution of magnetic flux intensity in a given region. We propose two different approaches, one based on especially designed wire loops (Lenz law) and the other based on solid conductive pieces (eddy currents). The gain of such devices is mainly determined by geometry giving perspective of high amplification. We consistently modeled, simulated, and executed the proposed devices. Doubled magnetic flux intensity is demonstrated experimentally for a moderate aspect ratio.
    The Review of scientific instruments 08/2013; 84(8):085120. · 1.52 Impact Factor
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    ABSTRACT: A very simple method of synthesis of goethite, α-FeOOH, nanowire is reported. To fabricate the nanowires, an anodiezed 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(NO 3) 3 satured solution) which occurs inside the nanopores. The geometry of AAO determines the morphology of the nanowires, and the confinement conditions in which occurs the heat treatment determines the composition of the nanostructure. The nanowires are characterized using scanning electron microscopy (SEM), high resolution transmission electron microscopy (HR-TEM) and magnetometry (magnetization vs. applied field (M vs. H)). TEM analysis indicates that nanowires are composed of several α-FeOOH single crystals. The nanowires have a clear magnetic oriented structure.
    X Latin American Workshop on Magnetism, Magnetic Materials and their Applications; 04/2013
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    ABSTRACT: a b s t r a c t Superparamagnetic ferrogels with high swelling ability and potential applications as sol-vent absorbers and stimuli-responsive drug delivery devices were obtained by a non-toxic and environmentally friendly route based on dispersion of poly(acrylic acid)-coated iron oxide nanoparticles (PAA-coated NPs) in poly(vinyl alcohol) (PVA) solutions followed by freezing–thawing. Presence of carboxylate groups arising from the PAA coating allowed hydrogen bonding formation between NPs and PVA and enabled the synthesis of optically homogenous, superparamagnetic materials formed by a homogenous distribution of NPs diffuse clusters in the PVA matrix. The addition of PAA-coated NPs produced a remarkable increase in crystallinity degree, thermal degradation and swelling percentage respect to the neat matrix, which demonstrates that ferrogels with improved properties can be obtained by this procedure. Thereafter, combination of a cryogenic technique with the use of non-toxic components and magnetic NPs coated by a pH sensitive polymer makes these ferrogels very promising for applications in the biomedical field.
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    L A S De Oliveira, K R Pirota
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    ABSTRACT: In this work, we report on the fabrication of highly ordered single crystalline BiFeO3 (BFO) nanotubes by a sol–gel technique using two-step anodic aluminum oxide (AAO) as template. We prepared BFO nanotubes with dimensions of 65 nm in diameter and 3 mm in length, as confirmed by scanning electron microscopy (SEM) measurements. The obtained single crystalline nanotubes present the expected pure phase (BiFeO3) as confirmed by energy-dispersive X-ray spectroscopy (EDX), selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM). In addition to the antiferromagnetic behavior, the magnetization curves of the BFO nanotubes also present a ferromagnetic response, which holds from 2 to 300 K. This desirable behavior is associated to the break of the antiferromagnetic helical spin ordering of the BFO nanotubes. Besides the magnetocrystalline anisotropy, the large length-to-diameter ratio induced an uniaxial shape anisotropy, attested by the applied magnetic field angle measurements.
    Materials Research Bulletin 01/2013; 48(4):1593–1597. · 1.97 Impact Factor
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    ABSTRACT: The size effects on magnetic properties of nanowires arrays were studied varying the nanowires diameter and maintaining the same periodicity among them, for two different nominal compositions of Co and Ni in the alloy form. The competition among magnetocrystalline and shape anisotropies changes drastically from smallest to biggest diameters altering the easy axis direction. In the case of 75% of Co in alloy, experimental values of the effective anisotropy constant (Keff) vary from positive to negative depending on the diameter, which means a reversal of the easy axis direction. For 50% of Co the shape anisotropy dominates over the magnetocrystalline for all studied diameters.
    Journal of Magnetism and Magnetic Materials 11/2012; 324(22):3679–3682. · 2.00 Impact Factor
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    ABSTRACT: Highly hexagonally ordered CoPd alloy nanowire arrays were synthesized through electrochemical deposition techniques into the nanopores of anodic alumina membranes used as templates. Two different electrolytes were used for this purpose, one with pH = 4.1 and the other with pH = 7. Under applying different electrodeposition parameters and by adjusting both, the current density and the electrolyte composition, it could be possible make to vary the composition of CoPd alloy nanowires in a wide range. Their composition and morphology were investigated by SEM and EDX. The magnetic properties of the nanowires array have been measured with a VSM as a function of the temperature, ranging from RT down to 50 K, for different CoPd alloy nanowires composition. Also, the temperature influence on the reversible-irreversible magnetization processes related with the magnetization reversal of the CoPd nanowires array has been analyzed by first order reversal curve (FORC) method.
    Journal of Nanoscience and Nanotechnology 06/2012; 12(6):4736-43. · 1.15 Impact Factor
  • Ceramics International 04/2012; · 2.09 Impact Factor
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    L.A.S. de Oliveira, K.R. Pirota
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    ABSTRACT: In this work, we report a sol–gel route to fab-ricate oxide nanowires of BiFeO 3 using two-step anodic aluminum oxide as template. We prepared oxide nanowires with uniform dimensions that can vary from 35 to 100 nm in diameter and with 4 lm in length, as confirmed by scanning electron microscopy and transmission electron microscopy measurements. Magnetization measurements, performed in a vibrating sample magnetometer, show that this nanostructures present ferromagnetism at room temperature.
    Journal of Sol-Gel Science and Technology 01/2012; 63(2):275-278. · 1.66 Impact Factor
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    ABSTRACT: Hysteretic giant magnetoimpedance (GMI) of amorphous ribbons with a well-defined transversal domain structure is investigated by means of first-order reversal curves (FORC) analysis. The FORCs are not confined to the hysteretic area, exceeding the major curve amplitude. Irreversible switches of the transverse permeability, caused by domain wall structure transitions, may be the origin of the observed FORC distribution. An interlinked hysteron/anti-hysteron model is proposed to interpret it, which allows analyzing the influence of frequency and magnetostriction upon the hysteretic GMI effect.
    Journal of Magnetism and Magnetic Materials 06/2011; 324(8). · 2.00 Impact Factor
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    F Béron, G Soares, K R Pirota
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    ABSTRACT: We present a setup allowing to characterize the local irreversible behavior of soft magnetic samples. It is achieved by modifying a conventional ac induction magnetometer in order to measure first-order reversal curves (FORCs), a magnetostatic characterization technique. The required modifications were performed on a home-made setup allowing high precision measurement, with sensibility less than 0.005 Oe for the applied field and 10(-6) emu for the magnetization. The main crucial point for the FORCs accuracy is the constancy of the applied field sweep rate, because of the magnetic viscosity. Therefore, instead of the common way to work at constant frequency, each FORC is acquired at a slightly different frequency, in order to keep the field variation constant in time. The obtained results exhibit the consequences of magnetic viscosity, thus opening up the path of studying this phenomenon for soft magnetic materials.
    The Review of scientific instruments 06/2011; 82(6):063904. · 1.52 Impact Factor
  • J. Schoenmaker, J.F.Q. Rey, K.R. Pirota
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    ABSTRACT: In the scope of renewable energy, we draw attention to a little known technique to harness solar and geothermal energy. The design here proposed and analyzed is a conceptual hybrid of several patents. By means of a modified organic Rankine cycle, energy is obtained utilizing buoyancy force of a working fluid. Based on thermodynamic properties we propose and compare the performance of Pentane and Dichloromethane as working fluids. Theoretical efficiencies up to 0.26 are estimated for a 51 m (Pentane) and 71.5 m (Dichloromethane) high column of water in a regime below 100 °C operation temperature. These findings are especially relevant in the scope of distributed energy systems, combined cycle plants, and low-temperature Rankine cycles.
    Renewable Energy 03/2011; 36(3):999–1002. · 3.36 Impact Factor
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    ABSTRACT: A simple method to quantitatively characterize the local magnetic behaviour of a patterned nanostructure, like a ferromagnetic thin film of antidot arrays, is proposed. The first-order reversal curve (FORC) analysis, coupled with simulations using physically meaningful hysterons, allows us to obtain a quantitative and physically related description of the interaction field and each magnetization reversal process. The hysterons system is built from previously known hypotheses on the magnetic behaviour of the sample. This method was successfully applied to a highly hexagonal ordered FePd antidot array with nanometric dimensions. We achieved a complete characterization of the two different magnetization reversal mechanisms in function of the in-plane applied field angle. For a narrow range of high fields, the magnetization initiates rotating reversibly around the pores, while at lower fields, domain walls are nucleated and propagated. This in-plane magnetization reversal mechanism, partly reversible and partly irreversible, is the only angularly dependent one. While going away from the easy axis, its reversible proportion increases, as well as its switching field distribution. Finally, the results indicate that the high surface roughness between adjacent holes of the antidot thin film induces a parallel interaction field. The proposed method demonstrates its ability also to be applied to characterizing patterned nanostructures with rather complex magnetization reversal processes.
    New Journal of Physics 01/2011; 13:013035. · 4.06 Impact Factor
  • Journal of Applied Physics 01/2011; 109:083919. · 2.21 Impact Factor
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    ABSTRACT: 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.
    Journal of Magnetism and Magnetic Materials 01/2011; 323(23):3022-3026. · 2.00 Impact Factor
  • ChemInform 12/2010; 41(51):no-no.
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    ABSTRACT: The goal of this review is to survey and identify accomplishments and advancements that have been made in the field of nanorobotics in point of experimental and theoretical views. It could determine which routes in the area of nanorobotics are scientifically plausible and technically useful. Also, we found recommendations to avoid possible problems and collateral effects in the future.
    Journal of Computational and Theoretical Nanoscience 09/2010; 7(10):1870-1877. · 1.03 Impact Factor
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    ABSTRACT: The magnetotransport behaviors of two types of permalloy nanostructures, thin films and antidots, are presented and discussed. Antidots samples were prepared by sputtering a Ni(80)Fe(20) layer on top of a nanoporous alumina membrane. A counterpart continuous thin film grown on a continuous Si substrate was also prepared. The magnetoresistance (MR) was measured both as a function of the external applied magnetic field and of the angular orientation, and thus compared with the magnetization curves. The introduction of antidots is found to reduce the anisotropic MR and the angular dependence of the MR, simultaneously increasing the coercive field of the samples. The influence of the sample geometry on the perpendicular MR behavior is reported and discussed. (C) 2010 American Institute of Physics. [doi:10.1063/1.3383039]
    Journal of Applied Physics 01/2010; 107(8). · 2.21 Impact Factor
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    ABSTRACT: Nanoporous anodic alumina films on Al substrates have been used as templates for the growth of electrodeposited Ni nanowires. The nanowire diameter ranges between 35 and 65 nm, and the hexagonal symmetry lattice constant is 105 nm. The magnetization curves for these nanowire arrays with and without the Al substrate have been measured in a temperature range from 5 to 300 K, from which the effective magnetic anisotropy, coercivity, and remanence have been determined. The effective easy magnetization axis changes from parallel to perpendicular to the Ni nanowire axis as temperature decreases, as a result of the magnetocrystalline and the magnetoelastic anisotropy terms, arising from the different thermal expansion coefficients of the ferromagnetic nanowires, the alumina and Al.
    Journal of Applied Physics 02/2008; 103(7):07D523-07D523-3. · 2.21 Impact Factor
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    ABSTRACT: The magnetocaloric response of an ensemble of oriented uniaxial magnetic objects, perpendicularly magnetized to their easy axes, for temperatures close to the blocking temperature is calculated with the aim of demonstrating that the control of the sample’s microstructure makes up an effective way to tailor its magnetocaloric response. Coexisting positive and negative magnetocaloric effect (MCE) is found for a model material with a single magnetic phase transition. Both MCE regimes are controlled by the magnitude of the applied magnetic field. As a proof of concept, experimental results for arrays of self-assembled ferromagnetic nanowires embedded into highly ordered nanoporous anodic alumina templates are shown, suggesting the validity of the numerical calculations.
    Physical Review B. 01/2008; 77(10):104434.
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    ABSTRACT: High-density magnetic antidot arrays have been fabricated by deposition of Fe(20)Ni(80) thin films on self-assembled nanoporous alumina membranes (NAM) with high-order hexagonal symmetry. The magnetic properties induced by the size and the geometry configuration of the holes introduced in a Fe(20)Ni(80) thin film are discussed based on hysteresis loops measured as a function of temperature. The precursor NAMs have pore diameters ranging between 35 and 95nm (55 and 75nm after the film deposition) and a lattice parameter of 105 nm. An enormous increase of coercitivity, as compared with the corresponding continuous films, was observed for temperatures between 2 and 300 K. This effect depends on the size and surface density of holes in the Fe(20)Ni(80) antidot arrays. Rutherford backscattering spectrometry (RBS) measurements were performed in order to better clarify the magnetic material that was eventually deposited within the NAM pores. (C) 2008 Elsevier B.V. All rights reserved.
    JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS. 01/2008; 320(14):E235-E238.

Publication Stats

275 Citations
65.20 Total Impact Points

Institutions

  • 2001–2013
    • University of Campinas
      • • Departamento de Física da Matéria Condensada (DFMC)
      • • Instituto de Física "Gleb Wataghin" (IFGW)
      Campinas, Estado de Sao Paulo, Brazil
  • 2006–2008
    • Instituto de Ciencia de Materiales de Madrid
      Madrid, Madrid, Spain
    • Spanish National Research Council
      • Instituto de Ciencia de Materiales de Madrid
      Madrid, Madrid, Spain
  • 2006–2007
    • Universidad Autónoma de Madrid
      • Departamento de Física Aplicada
      Madrid, Madrid, Spain
  • 2002
    • Universidad Pública de Navarra
      • Department of Physics
      Iruña, Navarre, Spain