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Brazilian Journal of Physics, Volume: 31, Número: 3, Publicado: 2001
  • Workshop on magnetic fluids

  • Structure of particle assembly in water-based magnetite magnetic fluid under zero applied field

    Jeyadevan, B.; Nakatani, I.; Oka, H.; Tohji, K.

    Resumo em Inglês:

    The magnetic particle dispersion in magnetic fluid has been found to depend very much on the type of dispersing media. The water-based magnetic fluid has been found to contain large number of inherent clusters even in zero applied magnetic field. In this paper, we discuss the possible origin of these primary clusters and propose that these clusters are of magnetic origin. The scanning probe microscope observation of primary clusters were carried out and found to be of ring shape.
  • Overview of the phase diagram of ionic magnetic colloidal dispersions

    Cousin, F.; Dubois, E.; Cabuil, V.; Boué, F.; Perzynski, R.

    Resumo em Inglês:

    We study ionic magnetic colloidal dispersions, which are constituted of g-Fe2O3 nanoparticles dispersed in water, and stabilized with electrostatic interparticle repulsion. The phase diagram PV versus F (P: osmotic pressure, V: particle volume, F: particle volume fraction) is explored, especially in the range of high P and high F. The osmotic pressure P of the colloidal dispersion is known either by a measurement either because it is imposed during the sample preparation by osmotic compression. The structure of the colloidal dispersion is determined from Small Angle Neutron Scattering (SANS). Two regimes can be distinguished. At high pressure, fluid and solid phases can exist. Their structure is governed by strong electrostatic repulsion, the range of which is here evaluated. At low pressure, gas, liquid and glassy solids can exist. Their structure results from a sticky hard sphere potential.
  • Diffusion limited field induced aggregation of magnetic liposomes

    Licinio, Pedro; Frézard, Frédéric

    Resumo em Inglês:

    Magnetic liposomes are spherical vesicles containing magnetic particles. Their practical interest relies in their potential use as "intelligent" drug delivery systems, with magnetic driving or targeting possibilities. In the presence of a field, the vesicles become magnetized and interact. A strong interaction leads to aggregation and could in extreme lead to undesirable clogging in physiological conditions. We investigated the formation of chain aggregates in liposome dispersions in the presence of magnetic fields by light scattering measurements. Data analysis showed that aggregation number increased linearly with elapsed time, as for ordinary isotropic colloid diffusion limited aggregation. The rate of aggregation increased with the field also linearly for low field strengths, and saturates at higher fields. These results are in contrast with measurements for the pure magnetic fluid, i.e., non-encapsulated magnetic particle dispersions.
  • Shapes and textures of ferromagnetic liquid droplets

    Banerjee, Shubho; Widom, M.

    Resumo em Inglês:

    Theoretical calculations, computer simulations and experiments indicate the possible existence of a ferromagnetic liquid state. Should such a state exist, demagnetization effects would force a nontrivial magnetization texture governed by the shape of the liquid droplet. Since liquid droplets are deformable, the droplet shape couples to the magnetization texture. This paper solves the joint shape/texture problem subject to the assumption of cylindrical droplet symmetry. The shape undergoes a change in topology from spherical to toroidal as exchange energy grows or surface tension decreases.
  • Viscoelastic ferrogel: dynamic magnetic susceptibilities

    Raikher, Yu. L.; Rusakov, V. V.

    Resumo em Inglês:

    Theoretical model to describe magnetodynamics of a ferrogel, i.e., an assembly of ferromagnetic nanoparticles embedded in a gel, is proposed. The reorientations of the particles are determined by the influence of the elastic matrix and the rotational Brownian motion. The set of essential parameters, on which the components of the dynamic magnetic susceptibility tensor depend, is discussed. In the framework of the model, absorption of the energy of an ac field is studied. With allowance for the interaction of elastic and Brownian forces, the effective relaxation times and eigenfrequencies for the longitudinal and transverse components of the ferrogel magnetization are evaluated.
  • Rotational dynamics of the magnetic moments in ferrofluids

    Scherer, Claudio; Ricci, Trieste F.

    Resumo em Inglês:

    We present a set of six non-linear stochastic diffferential equations for the six variables which are relevant for the dynamical behavior of the magnetic moments in ferrofluids, namely, the three Euler angles of the magnetic particle, the two polar angles of the magnetic moment relative to the particle and the modulus of the magnetic moment. The interaction between the magnetic particle and the solvent fluid is modeled by dissipative and random noise torques, and so is the interaction between the particle and its magnetic moment, treated as an independent physical entity. In the appropriate limits, the model system reduces to the cases of super-paramagnetic or of non-super-paramagnetic (blocked magnetic moments) particles. Numerical results show that for non-zero moment of inertia the precession of the magnetic moment around the magnetic field is accompanied by "nutation". It is also indicated how the dynamic complex susceptibility may be calculated from the equations of motion and the numerical results show that the nutation leads to a second resonance peak.
  • Static magneto optical birefringence of new Electric Double Layered Magnetic Fluids

    Depeyrot, J.; Silva, G. J. da; Alves, C. R.; Sousa, E. C.; Magalhães, M.; Figueiredo Neto, A. M.; Sousa, M. H.; Tourinho, F. A.

    Resumo em Inglês:

    Magnetic birefringence measurements are performed under a static field on Electric Double Layered Magnetic Fluids based on copper and zinc ferrite nanostructures. The optical birefringence is related to a single-particle effect and well described by a Langevin model which includes a log- normal distribution of particles. By the field-induced birefringence level, these new magnetic fluids are comparable to usual ones, a result which could offer a new way for biological applications.
  • Determination of particle size distribution by FMR measurements

    Folly, Walter S. D.; Biasi, Ronaldo S. de

    Resumo em Inglês:

    Knowledge of particle size distribution is very important for the study of magnetic fluids, magnetic powders and other magnetic systems. In this work, we describe a simple method for its determination from FMR measurements. The method was applied to the case of MgFe2O4 precipitates in (Mg,Fe)O.
  • Raman spectroscopy as a powerful technique in the characterization of ferrofluids

    Rubim, Joel C.; Sousa, Marcelo H.; Silva, Jean C. O.; Tourinho, Francisco A.

    Resumo em Inglês:

    Raman spectroscopy has been used to get some insight into the chemical composition and structure of magnetic fluids based on ferrites. The inner as well as the surface structure of Fe-maghemite and Zn-maghemite have characterized by Raman spectroscopy. It has been shown that different chemical species are present on the maghemite surface by changing the laser excitation energy. Maghemites modified by the adsorption of aspartic and glutamic acids as well as those modified by the adsorption of methylene blue(MB) have also been investigated by Raman spectroscopy. It has been shown that while FTIR (Fourier transform infrared) spectroscopy gives almost no information regarding to the surface species, Raman spectroscopy in and off resonance gives suitable information regarding to the adsorbate structure and bonding. The Raman spectroscopy study of the modified maghemites have shown that the organic acids adsorb on the maghemite surfarce as glutamate and aspartate chemically bounded to Fe(III) ions on the maghemite surface, and that MB, a cation, adsorbs on the maghemite surface as a monomer by ion pair formation with coadsorbed nitrate.
  • Mössbauer spectroscopy, superparamagnetism and ferrofluids

    Pfannes, H.-D.; Dias Filho, J. H.; Magalhães-Paniago, R.; López, J. L.; Paniago, R.

    Resumo em Inglês:

    Some fundamentals of Mössbauer spectroscopy and of fluctuating magnetic hyperfine interactions are reviewed. An expression for calculation of spin-phonon interaction transition probabilities on the ground of a Debye model is given. Basic ideas of superparamagnetism and classical results for low-temperature relaxation rates are presented. A theory of superparamagnetism based on spin-phonon interaction is introduced. It is shown how to calculate from this Mössbauer spectra taking all spin-levels into account. Experimental spectra of ferrofluids are compared with simulated spectra and found in good accordance.
  • The effect of the ferrofluid magnetic particle concentration on the thermal diffusivity

    Shibli, S.M.; Dantas, A.L.L.; Bee, A.

    Resumo em Inglês:

    Thermal diffusivity results obtained with the collinear mirage technique, are reported for different concentrations surfacted ferrofluid and for a set of acid ferrofluids with variable pH values. The collinear mirage technique was used because the materials are semitransparent and have low thermal diffusivities. The results show that, as the ferrofluid magnetic particles concentration increases, the thermal diffusivity also increases. Their behaviour differs from that reported for ferronematics [1]. The different geometry of thermal waves propagating through the material can account for the difference.
  • Interfacial instabilities in confined ferrofluids

    Miranda, José A.

    Resumo em Inglês:

    We investigate the flow of two immiscible, viscous fluids in the confined geometry of a Hele-Shaw cell. We consider that one of the fluids is a ferrofluid and that an external magnetic field is applied. The interfacial instabilities which arise between the fluids are studied for various situations: (a) different cell geometries (radial, rectangular); (b) frontal and parallel flows; (c) distinct applied field configurations (tangential, perpendicular, normal) and (d) motionless and rotating cells. The interplay between applied magnetic field and several other destabilizing/stabilizing factors in determining the interface behavior is analyzed. Stability analysis and numerical simulations are used to describe linear and nonlinear stages of the interface evolution.
  • Ferrofluid hydrodynamics: waves, jets and free drops

    Potts, H. E.; Diver, D. A.

    Resumo em Inglês:

    A novel approach to hydrodynamical studies uses the bulk force properties of magnetic liquids to probe the dynamics of (i) freely suspended drops; and (ii) unstable surface waves and jets. The fluid dynamics are imaged by a fast CCD camera, allowing thorough analysis of these time-dependent phenomena. Ferrofluid drops are freely suspended in air by using magnetic fields to create an attractive force opposing gravity. The suspended drop then undergoes forced oscillations by perturbing the supporting magnetic field, and exhibits high order nonlinear modes of oscillation which can be driven until the drop bifurcates. Fluid surface waves and jets are investigated in cylindrical geometry. Nonlinear waves are magnetically driven, resulting in dramatic jets when the critical amplitude is exceeded. Such jets are observed to have a maximum acceleration exceeding 70g.
  • Dynamics of the labyrinthine patterns at the diffuse phase boundaries

    Cebers, A.

    Resumo em Inglês:

    The phase diagram of a magnetic colloid in a Hele-Shaw cell is calculated. As a function of the magnetic field strength, of the concentration and of the layer thickness the magnetic colloid can find itself in a stripe phase, the hexagonal phase or in an unmodulated state. Those results allow to interpret experiments observing the transformation of a labyrinthine pattern into a hexagonal structure. This possibility is confirmed directly by the numerical simulation presented here and showing the transformation of the labyrinthine pattern into the hexagonal structure.
  • Surface waves and wave resistance in magnetic fluids

    Browaeys, J.; Perzynski, R.; Bacri, J.-C.; Shliomis, M. I.

    Resumo em Inglês:

    We present here the experimental determination of the dispersion relation of surface waves of a ferrofluid submitted to a steady vertical magnetic field. The results are compared to a linear theory where all the magnetic characteristics of the fluid have been taken into account. The knowledge of the dispersion equation is then used to analyse the capillary-wave resistance, that is the drag force associated to the emission of waves by a moving disturbance at a free fluid surface. It undergoes a transition from zero to a finite value as the speed of the disturbance reaches a certain critical value. The effect of viscosity is explored, and a magnetic fluid is shown to allow controlling the critical speeds. Contrary to the theoretical model, however, the measured wave resistance reveals a non monotonic speed dependence after the threshold.
  • Scalar field cosmology in three-dimensions Regular Articles

    Oliveira-Neto, G.

    Resumo em Inglês:

    We study an analytical solution to the Einstein's equations in 2 + 1-dimensions. The space-time is dynamical and has a line symmetry. The matter content is a minimally coupled, massless, scalar field. Depending on the value of certain parameters, this solution represents three distinct space-times. The first one is at space-time. Then, we have a big bang model with a negative curvature scalar and a real scalar field. The last case is a big bang model with event horizons where the curvature scalar vanishes and the scalar field changes from real to purely imaginary.
  • New method for obtaining complex roots in the semiclassical coherent-state propagator formula Regular Articles

    Xavier Jr., Ademir Luix

    Resumo em Inglês:

    A semiclassical formula for the coherent-state propagator requires the determination of specific classical paths inhabiting a complex phase-space and governed by a Hamiltonian flux. Such trajectories are constrained to special boundary conditions which render their determination difficult by common methods. In this paper we present a new method based on Runge-Kutta integrator for a quick, easy and accurate determination of these trajectories. Using nonlinear one dimensional systems we show that the semiclassical formula is highly accurate as compared to its exact counterpart. Further, we clarify how the phase of the semiclassical approximation is correctly retrieved during the time evolution.
  • The age problem and growing of structures for open system cosmology Regular Articles

    Campos, M. de; Tomimura, N.A.

    Resumo em Inglês:

    A new equation for the density contrast is derived in the framework of reexamined Newtonian cosmology if we take into account adiabatic matter creation in the universe. The age of the universe and the reach of non linear regime of the density contrast are usually treated separately in the literature and this may leadtocontroversial conclusions regarding the most adequate scenario to describe the universe. We relate the age of the universe and the growing mode of the density contrast by introducing a variable x that relates both of them, so that both aspects are treated simultaneously. We apply this procedure to the Friedmann type model where the source of particle production is Y = 3nbH.
  • Normal modes propagation in a helical free-electron laser with an axial guide field Regular Articles

    El-Bahi, Riadh; Rhimi, Mohamed Nazih; Cheikhrouhou, Abdel Wahab

    Resumo em Inglês:

    Following up the detailed three-dimensional analysis of the particle dynamics in a free electron laser with a helical wiggler and a uniform axial guide magnetic field, achieved by Rhimi et al. [1], we study the motion of the electrons in the neighborhood of the ideal trajectory. The normal modes <img src="http:/img/fbpe/bjp/v31n3/shace.gif">± of the quadratic Hamiltonian are analyzed numerically in both the normal and reversed-field configurations for different values of the ratio <img src="http:/img/fbpe/bjp/v31n3/20eq01.gif">w/<img src="http:/img/fbpe/bjp/v31n3/20eq01.gif">0. The electron trajectories are thus discussed. It is found that pseudo-circular and elliptic trajectories are characterizing both group-I and reversed-field group-II modes while the motion in the normal group-II mode is much more complicated.
  • On the biphoton wavelength Regular Articles

    Ribeiro, P. H. Souto

    Resumo em Inglês:

    We report on an experiment showing that the wavelength of a biphoton is clearly dependent onthe measurement scheme and on the way it is defined. It is shown that it can take any value, depending on the control of the interferometer phase differences. It is possible to identify the interference of the single and two-photon wavepackets as particular cases of the most general interference process. The variable wavelength has no implication on the energy of the individual photons neither on the total energy of the biphoton.
  • Molecular dynamics simulation of morphological instabilities in solid-fluid interfaces Regular Articles

    Costa, B.V.; Coura, P.Z.; Mesquita, O.N.

    Resumo em Inglês:

    We perform molecular dynamics simulations of directional growth of a binary alloy. We fix the temperature gradient, pulling speed, impurity concentration and only vary the impurity segregation coefficient. By changing the range of the Lennard Jones potential of impurity atoms as compared to the range of the potential of solvent atoms, the elastic energy cost causes a decreases of solubility of the impurity atoms into the matrix (solvent) solid phase and consequently a decrease in the impurity segregation coefficient. Within certain range of segregation coefficients, the growing interface is planar; below it, the interface becomes unstable and a cellular structure emerges.
  • Construction of exact solutions for the Stern-Gerlach effect Regular Articles

    Díaz Bulnes, J.; Oliveira, I.S.

    Resumo em Inglês:

    We obtain exact solutions for the Schrödinger-Pauli matrix equation for a neutral particle of spin 1/2 in a magnetic eld with a field gradient. The analytical wavefunctions are written on the symmetry plane Y = 0, which contains the incident and splitted beams, in terms of the Airy functions. The time-evolution of the probability densities, |<FONT FACE=Symbol>Y+|</FONT>² and |<FONT FACE=Symbol>Y-|² </FONT>, and the eigenenergies are calculated. These include a small contribution from the field gradient, alpha, proportional to (alpha<img src="http:/img/fbpe/bjp/v31n3/sh.gif">)2/3, which amounts to equal energy displacements on both magnetic levels. The results are generalized for spin S = 3/2, and in this case we found that the m = ±1/2 and m = ±3/2 magnetic sublevels are unequaly splitted by the field gradient, being the difference in energy of the order 0.4 MHz. Replacing real experimental parameters we obtained a spatial splitting of the spin up and spin down states of the order deltaz ~ 4 mm, in accordance to a real Stern-Gerlach experiment.
  • Vacuum ultraviolet and visible spectroscopy diagnostics on the NOVA-UNICAMP tokamak Regular Articles

    Daltrini, A.M.; Machida, M.; Monteiro, M.J.R.

    Resumo em Inglês:

    Three visible and one VUV spectrometers covering four toroidal positions have been set to study He and H plasma created by NOVA-UNICAMP tokamak. Ion temperatures have been measured at the beginning of the tokamak discharge by Doppler broadening of C, O and He lines. The time evolution of carbon line emissions, with different degrees of ionization, showed to have opposite behavior between high and low plasma ionization condition. The high density operation and 2 kHz oscillation present throughout discharge, caused by periodical touching of the plasma with NOVA-UNICAMP tokamak limiter, restrained the increase of ion temperature.
  • Comparison of two multipass configurations for scattered light amplification Regular Articles

    Monteiro, M.J.R.; Machida, M.; Daltrini, A.M.; Berni, L.A.

    Resumo em Inglês:

    Scattered light amplification by using multipass method is a very useful technique to increase Thomson and Rayleigh scattering signals for local plasma electron temperature and density measurements. Using Rayleigh scattering in nitrogen neutral gas chamber, the well-known technique of multipass system with two concentrical spherical mirrors is compared with new configuration by using two sets of lens and at mirrors. The focusing capability of both configurations makes the difference to stray light and scattered signal related to spectrometer slit aperture and number of multipass. For the same number of laser beam pass and best slit aperture, the lens and plan mirrors new configuration presented lower stray light and higher scattered signal, although the gain of the usual two spherical mirrors still remains higher.
  • Circular cosmic string loop in Brans-Dicke theory Regular Articles

    Barros, A.; Sen, A. A.; Romero, C.

    Resumo em Inglês:

    The gravitational field of a stationary circular cosmic string loop, externally supported against collapse, is investigated in the context of Brans-Dicke theory in the weak field approximation of the field equations. The solution is quasi-conformally related to the corresponding solution in Einstein's General Relativity(GR) and goes over to the corresponding solution in GR when the Brans-Dicke parameter w becomes infinitely large.
  • The temperature dependence of critical current in YBa2Cu3O7<FONT FACE=Symbol>-d thin films deposited on MgO by an eclipse PLD</FONT> Regular Articles

    Ohmukai, Masato; Fujita, Takehiro; Ohno, Takayuki

    Resumo em Inglês:

    YBa2Cu3O7-d thin films were deposited on MgO single crystals by means of an eclipse pulsed laser deposition (PLD) method. A deposited lm exhibited the critical temperature of 74 K. The dependence of critical current on temperature was well explained by the Ginzburg-Landau theory. Although the theory is described only near critical temperature, experimental results are well fitted in a wide range between 25 K and 74 K. It stems from the strongly Josephson-coupling regime.
  • Dissociation of fast ions analyzed by time-of-flight Regular Articles

    Ponciano, C.R.; Ladeia, R.C.C.; Collado, V.M.; Silveira, E.F. da

    Resumo em Inglês:

    The fragmentation of metastable ions, having keV of kinetic energy, is analyzed by time-of-flight technique. Assuming isotropic distribution of fragments in a free field region, it is deduced an analytical expression to describe the corresponding peak shapes in linear TOF spectrometers. Metastable ion mean-life and the kinetic energy release (Q-value) are the quantities extracted from data fitting. As an illustration, the dissociation of C8H10N+ metastable ions, desorbed by 252Cf ssion fragment impact on organic target, is studied.
  • How reliable is the mean-field nuclear matter description for supporting chiral effective Lagrangians? Regular Articles

    Delfino, A.; Malheiro, M.; Frederico, T.

    Resumo em Inglês:

    The link between non-linear chiral effective Lagrangians and the Walecka model description of bulk nuclear matter is questioned. This fact is by itself due to the Mean Field Approximation (MFA) which innuclear mater makes the picture of a nucleon-nucleon interaction based on scalar (vector) meson exchange equivalent to the description of a nuclear matter based on attractive and repulsive contact interactions. We present a linear chiral model where this link between the Walecka model and an underlying to chiral symmetry realization still holds, due to MFA.
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