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Digitale Medien

Magnetic properties of disordered Fe0.9−xMn0.1Alx alloys (2000)

Restrepo, J. ; Pérez Alcázar, G. A.

[S.l.] : American Institute of Physics (AIP)

Journal of Applied Physics 87 (2000), S. 7425-7429

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ISSN: 1089-7550

Quelle: AIP Digital Archive

Thema: Physik

Notizen: We have examined the characteristics of the magnetic order present at room and lower temperatures in disordered samples corresponding to the Fe0.9−xMn0.1Alx (0.1≤x≤0.5) series prepared by arc melting, high temperature homogenization, and quenching. To that purpose we have measured the compositional variation of the lattice parameters, the Mössbauer spectra, the high field magnetization, and the saturation coercivity and also the compositional and temperature dependences of the magnetic susceptibility and the low field magnetization. Our results are summarized in a magnetic phase diagram including ferromagnetic, paramagnetic, pure spin glass, and reentrant spin glass regions. © 2000 American Institute of Physics.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.373004

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2

Digitale Medien

Magnetism of Fe–Al disordered alloys: An Ising–Monte Carlo approach (2001)

Restrepo, J.

[S.l.] : American Institute of Physics (AIP)

Journal of Applied Physics 89 (2001), S. 7341-7343

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ISSN: 1089-7550

Quelle: AIP Digital Archive

Thema: Physik

Notizen: Using a model of atoms randomly distributed on a cubic lattice, we have simulated the atomic disorder induced in quenched binary alloys. Our study has been developed within the framework of a random site-diluted Ising model with nearest-neighbor interactions and used a Monte Carlo algorithm implemented with Metropolis kinetics for sampling states. After equilibration, ensemble and configurational averages for magnetization, magnetic susceptibility, and heat capacity were computed. We conclude that, in agreement with previous experimental Mössbauer data for which a comparison is carried out, Fe–Al disordered alloys exhibit room-temperature ferromagnetic behavior up to around 43 at. % Al, beyond which the system becomes paramagnetic. This result contrasts with that for alloys with atomic order, which exhibit ferromagnetism only up to 30 at. % Al. © 2001 American Institute of Physics.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.1356043

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3

Digitale Medien

Room temperature magnetic properties of quenched-disordered FeAl alloys: A simulational study (2000)

Restrepo, J.

[S.l.] : American Institute of Physics (AIP)

Journal of Applied Physics 87 (2000), S. 6528-6530

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ISSN: 1089-7550

Quelle: AIP Digital Archive

Thema: Physik

Notizen: We present a Monte Carlo study of the room temperature magnetic properties of quenched-disordered Fe1−qAlq alloys with 0≤q≤0.6. Simulations were performed using a random site-diluted classical Heisenberg model with nearest neighbor interactions, and using Metropolis dynamics. Results for the Al concentration of the magnetization and susceptibility predict the occurrence of a critical concentration qc at around 40 at. % Al, at which a transition from a ferromagnetic state (q〈qc) to a paramagnetic one (q≥qc) takes place. The simulated dependence of the average magnetization on the Al content is compared with experimental results of the average hyperfine field obtained by Mössbauer spectroscopy. Finally, an analysis of the finite size scaling properties of this system is presented. © 2000 American Institute of Physics.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.372759

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4

Digitale Medien

Hardware architecture for real-time laser range sensing by triangulation (1994)

Dalglish, R. L. ; McGarrity, C. ; Restrepo, J.

[S.l.] : American Institute of Physics (AIP)

Review of Scientific Instruments 65 (1994), S. 485-491

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ISSN: 1089-7623

Quelle: AIP Digital Archive

Thema: Physik , Elektrotechnik, Elektronik, Nachrichtentechnik

Notizen: A hardware-based system is described which generates a 3D mapping directly into a memory array of all surfaces visible in its field of view. The system uses a conventional scanning strip-light illumination technique and a CCD camera. However, instead of processing the 2D camera data in software, dedicated hardware detects and locates all visible surface elements and automatically loads the 3D coordinate and brightness data direct from the detection hardware into dual-ported memory. This allows the hardware to operate in real time at camera speeds and allows a very compact representation of the scene information in the natural coordinate frame of a triangulation-based system.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.1145162

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5

Digitale Medien

Monte Carlo study of the magnetic properties of Fe-rich Al–Fe disordered alloys : The 41st annual conference on magnetism and magnetic materials (1997)

Restrepo, J. ; González, J. M.

[S.l.] : American Institute of Physics (AIP)

Journal of Applied Physics 81 (1997), S. 5270-5272

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ISSN: 1089-7550

Quelle: AIP Digital Archive

Thema: Physik

Notizen: We present a simulational study of the magnetic properties of disordered Fe-rich Al–Fe alloys. Our study has been developed in the framework of a simple site-diluted Ising model with nearest-neighbor interactions and by using a Monte Carlo algorithm. From our results, it is concluded that, in agreement with previous experimental data, disordered Al–Fe alloys exhibit ferromagnetic behavior up to higher Al concentrations than the ordered ones. © 1997 American Institute of Physics.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.364455

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6

Digitale Medien

Phase diagram of a highly diluted, disordered Ising system: The Al-rich, Al–Fe system : The 7th joint MMM-intermag conference on magnetism and magnetic materials (1998)

Restrepo, J. ; Pérez Alcázar, G. A.

[S.l.] : American Institute of Physics (AIP)

Journal of Applied Physics 83 (1998), S. 7249-7251

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ISSN: 1089-7550

Quelle: AIP Digital Archive

Thema: Physik

Notizen: Disordered solid solutions containing a highly diluted magnetic element in which competitive exchange interactions are present can allow the observation of magnetic ordering (ferromagnetism, antiferromagnetism, and spin-glass behavior) at dilutor element contents different from those corresponding to ordered alloys. In this work we present a simulational study of the magnetic properties of the Al-rich, Al–Fe disordered alloy system, which meets the above mentioned requirements and is therefore a good candidate to show differences between the magnetic phase diagrams corresponding to the ordered and disordered states. Our simulations were carried out on the basis of a randomly site-diluted Ising model, using the Monte Carlo method as energy minimization tool. In our model, the spins of the Fe atoms are assumed to be coupled by a direct nearest-neighbor ferromagnetic exchange J (depending on the Al concentration), and by an Al-mediated antiferromagnetic superexchange interaction −αJ. Our results suggest the presence of ferromagnetic order for Al concentrations higher than those for which that order disappears in ordered alloys and the occurrence, for Fe atomic percentages between 10% and 25%, of a re-entrant spin glass phase whose freezing temperature decreases with the degree of dilution. © 1998 American Institute of Physics.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.367698

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7

Digitale Medien

Description in a local model of the magnetic field distributions of Fe1−xNix disordered alloys : The 41st annual conference on magnetism and magnetic materials (1997)

Restrepo, J. ; Pérez Alcázar, G. A. ; Bohórquez, A.

[S.l.] : American Institute of Physics (AIP)

Journal of Applied Physics 81 (1997), S. 4101-4103

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ISSN: 1089-7550

Quelle: AIP Digital Archive

Thema: Physik

Notizen: A local model based on the idea of atoms distributed within a crystalline lattice and following a binomial probability method for describing the hyperfine field distributions obtained from the room temperature Mössbauer spectra of the disordered Fe–Ni system is presented. This model allows one to calculate the mean hyperfine field for each concentration from 5 to 95 at. % Ni, to determine what are the most probable sites and to interpret why the mean hyperfine field initially increases and then decreases with the Ni concentration in the BCC phase. The results are in good agreement with the experimental results except for Invar composition and for 85 at. % Ni which demand a different treatment. © 1997 American Institute of Physics.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.365564

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8

Digitale Medien

Stable and Unstable Solitary-Wave Solutions of the Generalized Regularized Long-Wave Equation (2000)

Bona, J. L. ; McKinney, W. R. ; Restrepo, J. M.

Springer

Journal of nonlinear science 10 (2000), S. 603-638

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ISSN: 1432-1467

Schlagwort(e): Key words. BBM equation, generalized BBM equation, RLW equation, generalized RLW equation, stable solitary-waves, unstable solitary-waves

Quelle: Springer Online Journal Archives 1860-2000

Thema: Mathematik , Physik

Notizen: Summary. Investigated here are interesting aspects of the solitary-wave solutions of the generalized Regularized Long-Wave equation u_t + u_x + \alpha (u^p)_x - \beta u_{xxt} = 0. For p〉5 , the equation has both stable and unstable solitary-wave solutions, according to the theory of Souganidis and Strauss. Using a high-order accurate numerical scheme for the approximation of solutions of the equation, the dynamics of suitably perturbed solitary waves are examined. Among other conclusions, we find that unstable solitary waves may evolve into several, stable solitary waves and that positive initial data need not feature solitary waves at all in its long-time asymptotics.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1007/s003320010003

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9

Digitale Medien

Most Probable Histories for Nonlinear Dynamics: Tracking Climate Transitions (2000)

Eyink, G. L. ; Restrepo, J. M.

Springer

Journal of statistical physics 101 (2000), S. 459-472

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ISSN: 1572-9613

Schlagwort(e): optimal ; estimation ; Onsager–Machlup ; effective action

Quelle: Springer Online Journal Archives 1860-2000

Thema: Physik

Notizen: Abstract The effective action provides an appropriate cost function to determine most probable (or optimal) histories for nonlinear dynamics with strong noise. In such strong-coupling problems, a nonperturbative technique is required to calculate the effective action. We have proposed a Rayleigh–Ritz variational approximation, which employs simple moment-closures or intuitive guesses of the statistics to calculate the effective action. We consider here an application to climate dynamics, within a simple “bimodal” Langevin model similar to that proposed by C. Nicolis and G. Nicolis [Tellus 33:225 (1981)]. Capturing climate state transitions even in this simple model is known to present a serious problem for standard methods of data assimilation. In contrast, it is shown that the effective action for the climate history is already well-approximated by a one-moment closure and that the optimal, minimizing history robustly tracks climate change, even with large observation errors. Furthermore, the Hessian of the effective action provides the ensemble variance as a realistic measure of confidence level in the predicted optimal history.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1023/A:1026437432570

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10

Digitale Medien

Magnetic properties of the mechanically alloyed Fe0.9-xMn0.1Alx system (1999)

Restrepo, J. ; Pérez Alcázar, G.A. ; González, J.M.

Springer

Hyperfine interactions 122 (1999), S. 189-199

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ISSN: 1572-9540

Schlagwort(e): mechanical alloying ; disordered alloys ; magnetic properties ; Mössbauer ; spin-glass

Quelle: Springer Online Journal Archives 1860-2000

Thema: Physik

Notizen: Abstract Samples of nominal composition Fe0.9-xMn0.1Alx (0.1 ⩽ x ⩽ 0.5) were prepared by mechanical alloying starting from pure elements. Milling times of 24, 72 and 144 h were considered. The magnetic properties of the samples were studied by using 57Fe Mössbauer spectroscopy, vibrating sample magnetometry and magnetic susceptibility measurements. The phase distribution was determined from X-ray diffractometry. The so obtained results evidence a strong dependence on the milling time and Al concentration of the room-temperature hyperfine field distributions and coercive forces. The susceptibility measurements in the range of temperature between 10 K and 180 K suggest the occurrence of different types of transitions as the temperature is increased: (a) from a ferromagnetic to a paramagnetic phase, (b) from a reentrant spin-glass phase to a ferromagnetic one and (c) from spin-glass to a paramagnetic phase. These transitions are also strongly influenced by the milling time and the Al concentration.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1023/A:1012614226942

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