Navegando por Autor "Mello, V. D."

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Analysis of fractal groups of the type d-(m, r)-Cantor within the framework of Kaniadakis statistics
(Elsevier, 2014-05-02) Anselmo, Dory Hélio Aires de Lima; Souza, N. T. C. M.; Mello, V. D.; Silva Junior, Raimundo
We have used Kaniadakis statistics in the analysis of fractal structures of the type d-(m,r)-Cantor. The κ-entropy associated with these structures shows linearity with respect to the dimension L of the system and power-law type behavior with respect to the block size s used in the scanning of a determined sequence. The fractal dimension d f is related to the entropic parameter κ through an inverse-type law
Characterization of the magnetic phases of holmium nanofilms via magnetic neutron scattering
(Elsevier, 2019) Mello, V. D.; Santiago, F. A. L.; Anselmo, Dory Hélio Aires de Lima; Vasconcelos, Manoel Silva de; Almeida, N. S.
Magnetic phases of holmium films are associated with the intensity of the magnetic neutron scattering. It is shown that each magnetic phase of the system (fan, helifan, spin-slip, helix and ferromagnetic) exhibits a characteristic profile of scattering which can be used as a fingerprint to identify it. In this paper, we present theoretical results obtained for holmium films 24 monolayers thick at a fixed temperature and in the presence of a dc magnetic field applied along the basal plane. A self-consistent local field algorithm was used to obtain the equilibrium configurations of the magnetic moments of the film and, with these results, the spin-spin correlation functions which determine the shape of the neutron scattering intensity were calculated
Giant magnetocaloric effect of thin Ho films
(American Institute of Physics, 2011-03-24) Carriço, Artur da Silva; Medeiros Filho, F. C.; Mello, V. D.; Dantas, A. L.; Salles, F. H. S.
We report a theoretical study of the impact of finite size on the magnetocaloric effect (MCE) of thin Ho films. For strong external field strengths, the adiabatic temperature change ΔTad is comparable to the value found in bulk Ho, reaching about 12 K for an external field strength change ΔH = 50 kOe. For thicknesses below the helix period, there is a large enhancement. In this thickness range, the helical state does not form, leading to a giant MCE reaching ΔT/ΔH = 6.5 K/T for ΔH = 2 kOe.
Magnetic behavior of thin rare-earth films
(American Physical Society, 1999-03-01) Carriço, Artur da Silva; Almeida, N. S.; Mello, V. D.
We consider a thin rare-earth film with the c axis perpendicular to the surface, to study its nonuniform magnetic structure induced by the lower coordination near the surfaces. The influence of a dc magnetic field, applied perpendicular to the c axis, on the equilibrium configurations is also investigated. The spin-wave frequencies, in the q=0 limit, are calculated as a function of the strength of the external dc field. We use the bulk parameters of dysprosium and holmium to obtain the external field dependence of the equilibrium con- figuration, magnetization, and the lowest spin-wave modes of 30-monolayer-thick films.
Magnetic structures in ultra-thin Holmium films: Influence of external magnetic field
(Elsevier, 2015-03-01) Anselmo, Dory Hélio Aires de Lima; Rodrigues, L. J.; Mello, V. D.; Vasconcelos, Manoel Silva de
We address the magnetic phases in very thin Ho films at the temperature interval between 20 K and 132 K. We show that slab size, surface effects and magnetic field due to spin ordering impact significantly the magnetic phase diagram. Also we report that there is a relevant reduction of the external field strength required to saturate the magnetization and for ultra-thin films the helical state does not form. We explore the specific heat and the susceptibility as auxiliary tools to discuss the nature of the phase transitions, when in the presence of an external magnetic field and temperature effects. The presence of an external field gives rise to the magnetic phase Fan and the spin-slip structures
Magnetic surface phase of thin helimagnetic films
(American Physical Society, 2003) Mello, V. D.; Chianca, C. V.; Dantas, Ana L; Carriço, Artur da Silva
We study the magnetic phases of a thin helimagnetic film, with the c axis along the normal to the surface, in the presence of a dc external field H parallel to the c axis. For small values of H we have found a conical phase, as in the bulk, with small modifications near the surfaces. This surface modified conical phase collapses into a planar structure, the c-fan phase, when H is greater than a threshold value H*. The c-fan phase consists of a magnetic pattern in which the magnetic moments are in a plane perpendicular to the surface, containing one of the easy directions of the basal plane. We show that the c-fan phase does not form in the bulk and present results for a Dy thin film near the Ne`el temperature.
Magnetocaloric effect of thin Terbium films
(Elsevier, 2017-12) Anselmo, Dory Hélio Aires de Lima; Mello, V. D.; Vasconcelos, Manoel Silva de; Almeida, N. S.
We report a theoretical study of the magnetocaloric effect of Terbium (Tb) thin films due to finite size and surface effects in the helimagnetic phase, corresponding to a temperature range from TC=219 K to TN=231 K, for external fields of the order of kOe. For a Tb thin film of 6 monolayers submitted to an applied field (ΔH=30 kOe, ΔH=50 kOe and ΔH = 70 kOe) we report a significative change in adiabatic temperature, ΔT/ΔH, near the Néel temperature, of the order ten times higher than that observed for Tb bulk. On the other hand, for small values of the magnetic field, large thickness effects are found. For external field strength around few kOe, we have found that the thermal caloric efficiency increases remarkably for ultrathin films. For an ultrathin film with 6 monolayers, we have found ΔT/ΔH = 43 K/T while for thicker films, with 20 monolayers, ΔT/ΔH = 22 K/T. Our results suggest that thin films of Tb are a promising material for magnetocaloric effect devices for applications at intermediate temperatures
Non-additive model for specific heat of electrons
(Elsevier, 2016-10-14) Anselmo, Dory Hélio Aires de Lima; Vasconcelos, Manoel Silva de; Silva Junior, Raimundo; Mello, V. D.
By using non-additive Tsallis entropy we demonstrate numerically that one-dimensional quasicrystals, whose energy spectra are multifractal Cantor sets, are characterized by an entropic parameter, and calculate the electronic specific heat, where we consider a non-additive entropy Sq. In our method we consider an energy spectra calculated using the one-dimensional tight binding Schrödinger equation, and their bands (or levels) are scaled onto the [0, 1] interval. The Tsallis’ formalism is applied to the energy spectra of Fibonacci and double-period one-dimensional quasiperiodic lattices. We analytically obtain an expression for the specific heat that we consider to be more appropriate to calculate this quantity in those quasiperiodic structures
Octonacci photonic quasicrystals
(Elsevier, 2015-05-09) Anselmo, Dory Hélio Aires de Lima; Brandão, E. R.; Costa, C. H.; Vasconcelos, Manoel Silva de; Mello, V. D.
We study theoretically the transmission spectra in one-dimensional photonic quasicrystals, made up of iO2(A) and TiO2(B) materials, organized following the Octonacci sequence, where the nth-stage of the multilayer Sn is given by the rule Sn ¼ Sn1Sn2Sn1, for n P 3 and with S1 ¼ A and S2 ¼ B. The expression for transmittance was obtained by employing a theoretical calculation based on the transfer-matrix method. For normally incident waves, we observe that, for a same generation, the transmission spectra for transverse electric (TE) and transverse magnetic (TM) waves are equal, at least qualitatively, and they present a scaling property where a self-similar behavior is obtained, as an evidence that these spectra are fractals. The spectra show regions where the omnidirectional band gaps emerges for specific generations of Octonacci photonic structure, except to TM waves. For TE waves, we note that all of them have almost the same width, for different generations. We also report the localization of modes as a consequence of the quasiperiodicity of the heterostructure
Phononic topological states in 1D quasicrystals
(IOP Publishing, 2019) Silva, J.R.M.; Anselmo, Dory Hélio Aires de Lima; Vasconcelos, Manoel Silva de; Mello, V. D.
In this work, we address the study of phonons propagating on a one-dimensional quasiperiodic lattice, where the atoms are considered bounded by springs whose strength are modulated by equivalent Aubry-André hoppings. As an example, from the equations of motion, we obtained the equivalent phonon spectrum of the well known Hofstadter butterfly. We have also obtained extended, critical, and localized regimes in this spectrum. By introducing the equivalent Aubry-Andr´e model through the variation of the initial phase φ, we have shown that border states for phonons are allowed to exist. These states can be classified as topologically protected states (topological states). By Calculating the Inverse Participation Rate, we describe the localization of phonons and verify a phase transition, characterized by the critical value of λ = 1.0, where the states of the system change from extended to localized, precisely like in a metal-insulator phase transition
Propagation of magnetostatic modes on aperiodic rare-earth multilayers
(Elsevier, 2018) Anselmo, Dory Hélio Aires de Lima; Santiago, F. A. L.; Almeida, N. S.; Vasconcelos, Manoel Silva de; Cottam, M. G.; Mello, V. D.
The localization behavior and scaling properties of magnetostatic modes propagating on magnetic aperiodic superlattices made up by rare-earth materials is analyzed. Both surface and bulk modes are obtained, considering two different aperiodic rules of stacking: Thue-Morse and Fibonacci. The magnetization is considered to be perpendicular to the superlattice’s interfaces. Our model consider as magnetic materials both dysprosium and holmium, at different temperatures. A scaling behavior is observed for the allowed bulk bands, through specific ordering parameters, meaning that the allowed bands have a fractal (or multifractal) structure
Surface induced nucleation of the helifan phase
(ELSEVIER, 2001) Carriço, Artur da Silva; Mello, V. D.
We calculate the magnetic phases of thin films of helimagnetic materials. The surface of the film is perpendicular to the c-axis so that, for H = 0, the moments are in the basal plane. We study the effect of external fields, applied in the basal plane, and find that the field effects are controlled to a large extent by the c-axis anisotropy. If the c-axis an- isotropy is small, the effect of the external field is to produce small modifications in the basal plane projections, which remain nearly in a helical pattern, while the c-axis components oscillate throughout the film, with the surface regions displaying stronger oscillations. By increasing the external field strength, this phase evolves to a first order transition to a non-uniform fan phase, with major modifications near the surfaces. If the c-axis anisotropy is strong the out of plane oscillations are inhibited, and depending on the film thickness, a surface helifan phase is nucleated, starting from the gradual alignment of the magnetic moments near the surface region with the external field. The phase has twice the period of the helix and nucleates only in films where the thickness fits the double-period structure.
Thermal hysteresis of ferromagnetic/antiferromagnetic compensated bilayers
(Physical review B, 2009-10-12) Nascimento, F. I. F.; Dantas, Ana L.; Oliveira, L. L.; Mello, V. D.; Camley, R. E.; Carriço, Artur da Silva
We report a theoretical investigation of thermal hysteresis of fourfold anisotropy ferromagnetic FM film exchange coupled to a compensated antiferromagnetic substrate. Thermal hysteresis occurs if the temperature interval includes the reorientation transition temperature, below which the frustration of the interface exchange coupling leads to a 90° rotation of the magnetization of the ferromagnetic layer. The temperature width of the thermal hysteresis is tunable by external magnetic fields of modest magnitude, with values of 43 K for an external field of 110 Oe and of 14 K for a field of 210 Oe, for a Fe 12 nm/MnF2 110 bilayer. For a Fe 3 nm/FeF2 110 bilayer the width of the thermal hysteresis is 23 K at 110 Oe and 13 K at 300 Oe. We discuss how the thickness of the iron film affects the field tuning of the thermal hysteresis width, and also how the thermal loops may be used to identify the nature of the interface exchange energy.
A κ-statistical analysis of the Y-chromosome
(IOP Publishing, 2014-11-10) Anselmo, Dory Hélio Aires de Lima; Souza, N. T. C. M.; Silva Junior, Raimundo; Vasconcelos, Manoel Silva de; Mello, V. D.
An analysis of the coding sequence for the Y-chromosome (Homo sapiens) has been performed, embedded in the formalism of κ-statistics, which naturally encompasses long-range correlations. In this formalism, the entropy has been written as a function of κ (called the deformation —or entropic— parameter). The κ-entropy has been linked directly to dimensional parameters defined for the DNA chain associated with the chromosome Y. Our analysis indicates that there are certain regions of chromosome Y which exhibit linearity between Sκ entropy and sample sizes for some particular values of κ, implying that, on these regions, the information contained on the DNA increases monotonically linearly with the sample size, and also depicts an internal order

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Navegando por Autor "Mello, V. D."