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Artigo Detecting quantum phase transitions in a frustrated spin chain via transfer learning of a quantum classifier algorithm(Physical Review A, 2024-05-20) Ferreira-Martins, André Juan; Silva, Leandro; Palhares Júnior, Alberto Bezerra de; Pereira, Rodrigo; Soares-Pinto, Diogo O.; Araújo, Rafael Chaves Souto; Canabarro, AskeryThe classification of phases and the detection of phase transitions are central and challenging tasks in diverse fields. Within physics, these rely on the identification of order parameters and the analysis of singularities in the free energy and its derivatives. Here, we propose an alternative framework to identify quantum phase transitions. Using the axial next-nearest-neighbor Ising (ANNNI) model as a benchmark, we show how machine learning can detect three phases (ferromagnetic, paramagnetic, and a cluster of the antiphase with the floating phase). Employing supervised learning, we demonstrate the feasibility of transfer learning. Specifically, a machine trained only with nearest-neighbor interactions can learn to identify a new type of phase occurring when next-nearest-neighbor interactions are introduced. We also compare the performance of common classical machine learning methods with a version of the quantum nearest neighbors (QNN) algorithmArtigo Synthesis and characterization of poly(ethyl methacrylate-co-methacrylic acid) magnetic particles via miniemulsion polymerization(Elsevier B. V. Ltda, 2006-10-18) Araújo, José Humberto de; Nunes, J. S.; Vasconcelos, C. L. de; Cabral, F. A. O.; Pereira, M. R.; Fonseca, J. L. C.Magnetic particles are very important systems with potential use in drug delivery systems, ferrofluids, and effluent treatment. In many situations, such as in biomedical applications, it is necessary to cover inorganic magnetic particles with an organic material, such as polymers. In this work, latices based on magnetite covered by poly(ethyl methacrylate-co-methacrylic acid) were obtained via miniemulsion polymerization. The resultant latices had particles in the nanometric range and presented a pronounced superparamagnetic behavior.Artigo Effect of High Energy Milling on the Magnetic Properties of the WC-10wt.%Co Cemented Carbides(Materials Science Forum, 2006-01) Araujo, Jose Humberto; Manuel, Joao Baptista; Costa, F. A.; Macedo, Harim Revoredo de; Gomes, Uilame Umbelino; Silva, Angelus Giuseppe Pereira daCemented tungsten carbides were produced by liquid-phase sintering. In these work high energy milling (HEM) was used to produce homogeneous and finely grained powder mixtures. The milling effect in the magnetic properties of sintered samples is studied. Differents mixtures in same composition (WC-10wt%Co) were prepared by conventional mixture technique and wet HEM up to 300 hs in planetary Mill. Magnetic hysteresis measurements on the sintered samples detected a significant increase in the coercitive field and a decrease in the saturation magnetization with milling time increasing. X-ray diffractogram show phase transformations with milling time. The Magnetic properties are correlated with phase relations and microstructural properties of the sintered samples.Artigo Mössbauer study of spin-glass Fe x Zn1−x F2 system(Hyperfine Interaction, 1991-11) Araújo, Jose Humberto de; Cunha, Joao Batista Marimon da; Vasquez, A.; Amaral, L.; Moro, J. T.; Montenegro, F. C.; Rezende, S. M.; Coutinho-Filho, Mauricio D.57Fe Mössbauer effect measurements in the diluted Ising antiferromagnet Fe x Zn1−x F2 withx=0.25 andx=0.57 at temperatures between 4.2 and 55 K, are reported. DC suceptibility measurements show a spin-glass (SG) phase at low temperatures forx≤0.31. Our Mössbauer spectra show a phase transition to a SG state with antiferromagnetic order (AFSG) forx=0.25 and only antiferromagnetic order forx=0.57.Artigo Structural and magnetic properties of nanoparticles of La2/3Sr1/3MnO3(Elsevier, 2006-10-01) Araújo, José Humberto de; Moreira, M. L.; Soares, J. M.; Azevedo, W. M. de; Rodrigues, A. R.; Machado, F. L. A.Nanoparticles of the manganite were synthesized using the Pechini process and their structural properties were characterized by X-ray diffraction (XRD) and by transmission electron microscopy (TEM). The magnetization of compacted powders was measured for temperatures in the range . The estimated average diameter (D) of the particles continuously increased from 20 to 95 nm when the calcination temperature was varied from 873 to 1273 K. From the magnetic data we found that the room temperature saturation magnetization scaled with while the magnetic transition temperature follows a power-law suggesting that the correlation length is mainly determined by the surface properties of the nanoparticles.Artigo Anisotropy field and transverse susceptibility in nanocrystalline hexaferrites(Elsevier, 2006-10-01) Araujo, Jose Humberto; Soares, J. M.; Machado, F. L. A.; Cabral, F. A. O.; Rodrigues, H. A. B.; Ginani, Marconi FloripeNanocrystalline strontium and barium hexaferrites were produced by an ionic coordination reaction method. The average particle size obtained using the Rietveld X-ray refinement technique and by scanning electron microscopy was quite uniform and close to 50 nm. Transverse susceptibility measurements yielded both the coercive and the anisotropy magnetic fields. The results were analysed using a theoretical model proposed by Aharoni et al. [Bull. Res. Counc. Isr. A 6 (1957) 215]. This overall procedure seems to be quite useful in determining the distribution of the anisotropy magnetic fields in granular materials.Artigo Evidence of magnetic polaronic states in La_{0.70}Sr_{0.30}Mn_{1-x}Fe_{x}O_{3} manganites(American Physical Society, 2003-02-19) Araújo, José Humberto de; Souza Filho, A. G.; Faria, J. L. B.; Guedes, I.; Sasaki, José Marcos; Freire, P. T. C.; Mendes Filho, J.; Xavier Jr, M. M.; Cabral, Francisco de Assis Olímpio; Costa, J. A. P. da; Freire, V. N.We have performed x-ray, electrical resistivity, and Raman scattering measurements in La0.70Sr0.30Mn12xFexO3 manganites, with x varying from 0 to 0.40. The appearance of new peaks in the Raman spectra as the Fe content increases is attributed to either otherwise forbidden Raman modes that become active due to symmetry breaking effect or to the phonon density of states features, which arise due to a disorder-induced phenomenon. The softening of some phonon modes suggests the evidence of magnetic polaronic states in the insulating paramagnetic phase of these manganites. Our observations are discussed in connection with the x-ray diffraction and electrical resistivity data.Artigo Giant magnetoimpedance in FeAg granular alloys(Applied Physics Letters, 2002-04-02) Araújo, José Humberto de; Soares, João Maria; Cabral, F. A. O.; Dumelow, T.We have observed giant magnetoimpedance (GMI) in an FeAg granular alloy. Impedance and transverse susceptibility (TS) measurements as a function of the external dc magnetic field were made at room temperature. The variation of the magnetoimpedance ratio with magnetic field has been related to a corresponding TS ratio change as a function of the magnetic field. The peaks in the GMI and TS curves are in the same position and have been associated in both cases to effective anisotropy fields.Artigo Effect of Fe doping in polycrystalline La0.7Sr0.3MnO3 on transport and magnetic properties(Elsevier, 2001-05) Araújo, José Humberto de; Xavier Júnior, M. M.; Cabral, F. A. O.; Feitosa, Carlos Chesman de Araújo; Dumelow, T.; Sasaki, José MarcosThe effect of Fe doping (⩽10%) on polycrystalline samples of La0.7Sr0.3MnO3 has been studied through X-ray diffraction, resistivity, AC susceptibility, magnetization, and magnetoresistance measurements. Fe doping was found to change substantially the magnetic and transport properties of the samples, without modifying the lattice structure. In a magnetic field of 5 T, the magnetoresistance of a sample doped with 10% Fe was 4 times greater than that of an undoped sample, near the resistivity peak. This result was explained in terms of the formation of antiferromagnetic clusters of Fe ions. Magnetoresistance did not depend on Fe doping at 5 K, but there was a large magnetoresistive effect in fields less than 0.5 T in all samples. This behavior was attributed to spin polarized tunneling at grain boundaries of polycrystalline samples.Artigo Transmission fingerprints in quasiperiodic magnetic structures(Elsevier B.V., 2003-11) Bezerra, Claudionor Gomes; Vasconcelos, Manoel Silva de; Albuquerque, Eudenilson Lins de; Mariz, Ananias MonteiroIn this paper we study the spin wave transmission spectra of quasiperiodic magnetic multilayers following Fibonacci, double period and Thue–Morse sequences. We consider multilayers that are composed of two simple cubic ferromagnetic materials surrounded by two semi-infinite slabs of a third ferromagnetic material. A transfer matrix treatment is employed with the calculations carried out for the exchange-dominated regime within the framework of the Heisenberg model and taking into account the random phase approximation. The numerical results illustrate the self-similar aspects of the spectra as well as their magnetic transmission fingerprints, which can be used to identify the sequence applied to construct the systemArtigo Magnetization in quasiperiodic magnetic multilayers with biquadratic exchange and uniaxial anisotropy(Elsevier Science B.V., 2002-02) Bezerra, Claudionor Gomes; Cottam, Michael G.A theoretical study is made of the magnetization versus applied field curves of ferromagnetic/nonmagnetic multilayers constructed according to a Fibonacci quasiperiodic sequence. The ferromagnetic films are assumed to have uniaxial anisotropy and are coupled by both bilinear and biquadratic effective exchange. The effects of quasiperiodicity in the magnetic phases are illustrated numerically for Fe/Cr systemsArtigo A Mössbauer study of plasma nitrided iron(AIP Publishing, 1998-10-13) Araujo, Jose Humberto; Cabral, F. A. O.; Araujo, R. C.; Gama, S.Iron powder has been nitrided in a glow discharge plasma of a mixture of nitrogen and hydrogen, in order to verify the existence of a metastable iron nitrogen solid solution, previously observed from the Sm2Fe17N3 phase decomposition process. Our results show that, for short nitridation times, we obtain a mixture of Fe4N, a small amount of α-Fe and an iron nitrogen solid solution the amount of which increases after heat-treatment at 610 °C. This iron nitrogen solid solution is characterized by Mössbauer spectroscopy which shows a typical α-Fe spectrum, but with lower hyperfine field and broader linewidth. TMA measurements show that the Curie temperature is similar to that of α-Fe.Artigo Atomic hopping in AlCuFe quasicrystal(Elsevier, 1996-03) Araujo, Jose Humberto; Gomes, A. A.; Cunha, J. B. M. daHigh temperature 57Fe MOsshaner effect measurements in annealed icosahedral A1CuFe alloy are reported. The anomalous decrease in Debye-Waller factor is qualitatively interpretated. It is suggested using consistent parameters that the isomer shift decreases linearly with temperature in agreement with a second-order Doppler shift in the Debye approximation.Artigo Mössbauer effect characterization of Brazilian iron ores for mining industrial purposes(Elsevier, 1993-04-04) Araujo, Jose Humberto; Costa Jr., M. I. da; Kunrath, J. I.; Cunha, J. B. M. da; Moro, J. T.Iron ore is one of the important export products of Brazil. As only ore bodies with over 50% iron contents are mined, continuous efforts in prospecting are made. The use of Mössbauer spectroscopy as an aid to these efforts is illustrated by an example given in this report.Artigo Spin waves in quasi-periodic Fe/Cr(100) thin films(IOP Publishing, 2002-02-15) Bezerra, Claudionor Gomes; Mauriz, Paulo Wilson; Albuquerque, Eudenilson Lins deIn this work we have calculated the ferromagnetic (FM) resonance curves of Fe/Cr(100) thin films, which follow the Fibonacci sequence. Our approach is based on the equations of motion for the small-signal magnetization deviation from the equilibrium directions. The equilibrium positions of the magnetizations are calculated from the total energy, which includes the following contributions: Zeeman, bilinear exchange, biquadratic exchange, dipolar, cubic anisotropy and surface anisotropy terms. We also consider the presence of an external magnetic field applied in the plane of the films and parallel to the easy axes. The experimental parameters used in our calculations were recently reported and lie in three regions of interest, namely (i) near to the first antiferromagnetic (AF) peak (strong bilinear exchange coupling), (ii) near to the first FM-AF transition (moderate bilinear exchange coupling) and (iii) near to the second AF peak (weak bilinear exchange coupling comparable to the biquadratic exchange coupling). Our results show the effect of the quasi-periodic arrangement in the spin wave dispersion relation of these artificial structuresArtigo Surface and bulk spin waves in magnetic superlattices with biquadratic exchange coupling(American Institute of Physics Publishing, 2002-05-13) Bezerra, Claudionor Gomes; Cottam, Michael G.A study is made of the bulk and surface spin wave modes propagating in magnetic superlattices that are composed of two simple cubic ferromagnetic materials which interact by bilinear and biquadratic exchange couplings at the interfaces. A transfer matrix treatment is employed with the calculations carried out for the exchange dominated regime within the framework of the Heisenberg model and taking into account the random phase approximation. The numerical results obtained show that from a certain value of the ratio R between the biquadratic and interfacial bilinear exchange couplings the surface acoustic modes are suppressed and only the optical modes are presentArtigo Multifractal spectra of spin waves in Fibonacci magnetic superlattices with biquadratic exchange coupling(Elsevier B.V., 2002-06-01) Bezerra, Claudionor Gomes; Cottam, Michael G.The multifractal properties are investigated for the bulk spin wave bands of quasiperiodic magnetic superlattices arranged according to the Fibonacci sequence. The superlattices are considered to be composed of layers of two simple cubic ferromagnetic materials. The layers are characterized by a bulk bilinear exchange coupling and they interact through an additional biquadratic exchange coupling at the interfaces. The function f(α), which describes the spectrum of singularities, is calculated for different values of the dimensionless in-plane wave vector kxa and ratio R=Jbq/JblI between the biquadratic and interfacial bilinear exchange couplings. Our results show the effects of the biquadratic exchange coupling on the bulk band structure, which becomes more disordered as the ratio R increasesArtigo Universality beyond power laws and the average avalanche shape(Springer Nature, 2011-01-23) Papanikolaou, Stefanos; Bohn, Felipe; Sommer, Rubens Luis; Durin, Gianfranco; Zapperi, Stefano; Sethna, James P.The study of critical phenomena and universal power laws has been one of the central advances in statistical mechanics during the second half of the past century, explaining traditional thermodynamic critical points1, avalanche behaviour near depinning transitions2,3 and a wide variety of other phenomena4. Scaling, universality and the renormalization group claim to predict all behaviour at long length and timescales asymptotically close to critical points. In most cases, the comparison between theory and experiments has been limited to the evaluation of the critical exponents of the power-law distributions predicted at criticality. An excellent area for investigating scaling phenomena is provided by systems exhibiting crackling noise, such as the Barkhausen effect in ferromagnetic materials5. Here we go beyond powerlaw scaling and focus on the average functional form of the noise emitted by avalanches—the average temporal avalanche shape4. By analysing thin permalloy films and improving the data analysis methods, our experiments become quantitatively consistent with our calculation for the multivariable scaling function in the presence of a demagnetizing field and finite field-ramp rateArtigo Quantitative Scaling of Magnetic Avalanches(American Physical Society, 2016-08-15) Durin, Gianfranco; Bohn, Felipe; Corrêa, Marcio Assolin; Sommer, Rubem Luis; Doussal, P. Le; Wiese, K. J.We provide the first quantitative comparison between Barkhausen noise experiments and recent predictions from the theory of avalanches for pinned interfaces, both in and beyond mean field. We study different classes of soft magnetic materials, including polycrystals and amorphous samples—which are characterized by long-range and short-range elasticity, respectively—both for thick and thin samples, i.e., with and without eddy currents. The temporal avalanche shape at fixed size as well as observables related to the joint distribution of sizes and durations are analyzed in detail. Both long-range and short-range samples with no eddy currents are fitted extremely well by the theoretical predictions. In particular, the short-range samples provide the first reliable test of the theory beyond mean field. The thick samples show systematic deviations from the scaling theory, providing unambiguous signatures for the presence of eddy currentsArtigo Annealing effects on the microwave linewidth broadening of FeCuNbSiB ferromagnetic films(AIP Publishing LLC, 2015-03-26) Alves, Marcos José Pereira; Gonzalez-Chavez, Diego Ernesto; Bohn, Felipe; Sommer, Rubem LuisWe systematically investigate the annealing effects on the microwave linewidth broadening of FeCuNbSiB ferromagnetic films with thickness of 100 nm. We correlate the non-uniform residual stress obtained from grazing incidence x-ray diffraction measurements with the ferromagnetic resonance (FMR) linewidth due to effective field inhomogeneities measured from broadband ferromagnetic resonance absorption measurements. We also estimate the annealing temperature effect on the Gilbert and two-magnon scattering contributions to the total ferromagnetic resonance FMR linewidth. We show that the effective field inhomogeneities constitute the main contribution to the microwave linewidth, while this contribution is related to the non-uniform residual stress in the films which is reduced by thermal annealing