Papers by Gonzalo dos Santos
Physical review, Oct 12, 2023
We use spin-lattice dynamics simulations to study the possibility of modeling the magnetic hyster... more We use spin-lattice dynamics simulations to study the possibility of modeling the magnetic hysteresis behavior of a ferromagnetic material. The temporal evolution of the magnetic and mechanical degrees of freedom is obtained through a set of two coupled Langevin equations. Hysteresis loops are calculated for different angles between the external field and the magnetocrystalline anisotropy axes. The influence of several relevant parameters is studied, including the field frequency, magnetic damping, magnetic anisotropy (magnitude and type), magnetic exchange, and system size. The role played by a moving lattice is also discussed. For a perfect bulk ferromagnetic system we find that, at low temperatures, the exchange and lattice dynamics barely affect the loops, while the field frequency and magnetic damping have a large effect on it. The influence of the anisotropy magnitude and symmetry are found to follow the expected behavior. We show that a careful choice of simulation parameters allows for an excellent agreement between the spin-lattice dynamics measurements and the paradigmatic Stoner-Wohlfarth model. Furthermore, we extend this analysis to intermediate and high temperatures for the perfect bulk system and for spherical nanoparticles, with and without defects, reaching values close to the Curie temperature. In this temperature range, we find that lattice dynamics has a greater role on the magnetic behavior, especially in the evolution of the defective samples. The present study opens the possibility for more accurate inclusion of lattice defects and thermal effects in hysteresis simulations.
Scientific Reports
Compression of a magnetic material leads to a change in its magnetic properties. We examine this ... more Compression of a magnetic material leads to a change in its magnetic properties. We examine this effect using spin-lattice dynamics for the special case of bcc-Fe, using both single- and poly-crystalline Fe and a bicontinuous nanofoam structure. We find that during the elastic phase of compression, the magnetization increases due to a higher population of the nearest-neighbor shell of atoms and the resulting higher exchange interaction of neighboring spins. In contrast, in the plastic phase of compression, the magnetization sinks, as defects are created, increasing the disorder and typically decreasing the average atom coordination number. The effects are more pronounced in single crystals than in polycrystals, since the presence of defects in the form of grain boundaries counteracts the increase in magnetization during the elastic phase of compression. Also, the effects are more pronounced at temperatures close to the Curie temperature than at room temperature. In nanofoams, the ef...
Physical review, Jan 25, 2023
Monte Carlo simulations and finite-size scaling theory have been carried out to study the critica... more Monte Carlo simulations and finite-size scaling theory have been carried out to study the critical behavior and universality for the isotropic-nematic (IN) phase transition in a system of straight rigid pentamers adsorbed on a triangular lattice with polarized nonhomogeneous intermolecular interactions. The model was inspired by the deposition of 2-thiophene molecules over the Au(111) surface, which was previously characterized by experimental techniques and density functional theory. A nematic phase, observed experimentally by the formation of a self-assembled monolayer of parallel molecules, is separated from the isotropic state by a continuous transition occurring at a finite density. The precise determination of the critical exponents indicates that the transition belongs to the three-state Potts universality class. The finite-size scaling analysis includes the study of mutability and diversity. These two quantities are derived from information theory and they have not previously been considered as part of the conventional treatment of critical phenomena for the determination of critical exponents. The results obtained here contribute to the understanding of formation processes of self-assembled monolayers, phase transitions, and critical phenomena from novel compression algorithms for studying mutual information in sequences of data.
arXiv (Cornell University), Apr 21, 2022
Spin-lattice dynamics is used to study the magnetic properties of Fe foams. The temperature depen... more Spin-lattice dynamics is used to study the magnetic properties of Fe foams. The temperature dependence of the magnetization in foams is determined as a function of the fraction of surface atoms in foams, n surf. The Curie temperature of foams decreases approximately linearly with n surf , while the critical exponent of the magnetization increases considerably more strongly. If the data are plotted as a function of the fraction of surface atoms, reasonable agreement with recent data on vacancy-filled Fe crystals and novel data on void-filled crystals is observed for the critical temperature. Critical temperature and critical exponent also depend on the coordination of surface atoms. Although the decrease we find is relatively small, it hints to the possibility of improved usage of topology to taylor magnetic properties.
Physical Review B, 2020
The magnetic behavior of bcc iron nanoclusters, with diameters between 2 and 8 nm, is investigate... more The magnetic behavior of bcc iron nanoclusters, with diameters between 2 and 8 nm, is investigated via spin dynamics (SD) simulations coupled to molecular dynamics (MD), using a distance-dependent exchange interaction. Finite-size effects in the total magnetization as well as the influence of the free surface and the surface/core proportion of the nanoclusters are analyzed in detail for a wide temperature range, reaching the Curie temperature. Comparisons with experimental data and theoretical models based on the mean-field Ising model are also presented, including one adapted to small clusters, and another developed to take into account the influence of low coordinated spins at free surfaces. Magnetization results show excellent agreement with experimental measurements for small Fe nanoclusters. Large differences are found with frozen-atom simulations. Finite-size effects on the thermal behavior of the magnetization increase as the size of the clusters is reduced, especially near the Curie temperature,. Analytical approximations to the magnetization as a function of temperature and size are proposed.
Computational Materials Science
Spin-lattice dynamics is used to study the magnetic properties of Fe foams. The temperature depen... more Spin-lattice dynamics is used to study the magnetic properties of Fe foams. The temperature dependence of the magnetization in foams is determined as a function of the fraction of surface atoms in foams, n surf. The Curie temperature of foams decreases approximately linearly with n surf , while the critical exponent of the magnetization increases considerably more strongly. If the data are plotted as a function of the fraction of surface atoms, reasonable agreement with recent data on vacancy-filled Fe crystals and novel data on void-filled crystals is observed for the critical temperature. Critical temperature and critical exponent also depend on the coordination of surface atoms. Although the decrease we find is relatively small, it hints to the possibility of improved usage of topology to taylor magnetic properties.
Journal of Physics D: Applied Physics, 2021
We study the Li diffusion in Li x V2O5 ( 0 <x⩽1 )—a potential cathode material for Lithium ion... more We study the Li diffusion in Li x V2O5 ( 0 <x⩽1 )—a potential cathode material for Lithium ion batteries. Different diffusion pathways in this material with dependence on the Li ion concentration are investigated by applying first-principles calculations. The results are used to obtain the corresponding diffusion coefficients by employing two complementary methodologies: Kinetic Monte Carlo (KMC) simulations and a statistical thermodynamics approach. The KMC simulations for two different crystal planes give new evidence that the diffusion occurs mainly along the [010] direction, while the corresponding diffusion coefficients show a temperature dependence obeying Arrhenius’ Law. The necessity of the consideration of concentration-dependent barrier heights in the KMC simulations are demonstrated by looking at the significant changes of the concentration-dependence of the diffusion coefficients. The simulated diffusion coefficients of the combined approach show a good quantitative a...
Computational Condensed Matter, 2022
Bulletin of the American Physical Society, 2020
Bulletin of the American Physical Society, 2020
Applied Physics Letters, 2021
Magnetization of clusters is often simulated using atomistic spin dynamics for a fixed lattice. C... more Magnetization of clusters is often simulated using atomistic spin dynamics for a fixed lattice. Coupled spin-lattice dynamics simulations of the magnetization of nanoparticles have, to date, neglected the change in the size of the atomic magnetic moments near surfaces. We show that the introduction of variable magnetic moments leads to a better description of experimental data for the magnetization of small Fe nanoparticles. To this end, we divide atoms into a surface-near shell and a core with bulk properties. It is demonstrated that both the magnitude of the shell magnetic moment and the exchange interactions need to be modified to obtain a fair representation of the experimental data. This allows for a reasonable description of the average magnetic moment vs cluster size, and also the cluster magnetization vs temperature.
Nano Express, 2020
The adsorption of rigid straight electrically polarized pentamers over a FCC(111) surface is stud... more The adsorption of rigid straight electrically polarized pentamers over a FCC(111) surface is studied. The model was inspired by the deposition of 2-thiophene molecules over the Au(111) surface, which was previously characterized by experimental techniques and simulated under the frame of the density functional theory. We now obtain and report the charge distribution of the molecule which allows to propose a deposition model followed by Monte Carlo simulations over an ad-hoc lattice gas model. We show that for a certain value of the chemical potential there exists an isotropic-nematic phase transition which can explain the formation of a self-assembled monolayer like the one observed in the transmission electron microscopy images. An order parameter is defined to characterize the transition which presents a step-like behavior at a critical chemical potential value. The possible nature of the nematic transition in conjunction with an ergodicity breakdown is discussed as future work by...
Langmuir, 2020
Natural gas (NG) is an interesting primary fuel; its larger-scale use hindered by the difficultie... more Natural gas (NG) is an interesting primary fuel; its larger-scale use hindered by the difficulties of storing it under high pressures or low temperatures; a viable alternative is its storage via physisorption in porous materials. Most NG adsorption studies have focused on adsorption of pure methane, its primary component. Here we investigate the influence of heavier alkanes commonly found in NG (propane, ethane) on the adsorption process. We present the results of extensive molecular dynamics simulations of mixtures of methane-propane and methane-ethane at T = 300 K and 400 K and P = 0-1,500 bar in slit-shaped pores with interlayer spacings H = 8-20 Å. We observed that heavier hydrocarbons adsorb preferentially, but remain mobile, promising for the intended application. We also solved a common problem with simulations of molecules with high adsorption affinity: the difficulty to determine their partial pressure. We developed an Arrhenius-type relationship allowing the calculation of these partial pressures from relationships between energy distributions of the different molecules in the simulations in conditions where a direct determination of these is impractical or impossible.
Modelling and Simulation in Materials Science and Engineering, 2020
The conditions for the formation of <100> dislocation loops in BCC iron were investigated via Mol... more The conditions for the formation of <100> dislocation loops in BCC iron were investigated via Molecular Dynamics simulations using a simplified model intended to mimic conditions in high energy collision cascades, focusing on the possible coherent displacement of atoms at the boundary of a subcascade. We report on the formation of <100> dislocation loops due to the fast displacement of a few hundred atoms with a coherent acceleration, in agreement with previous results for much larger cascade simulations. We analyze in detail the resulting atomic velocities and pressures, and find that they cannot be described within the usual formalism for a shock regime, since the pressure pulse only lasts less than 1 ps and does not match expected values from a Hugoniot shock. Our simulations include two interatomic potentials: Mendelev, which is extensively used for radiation damage simulations, and Ackland, which has been used for shock simulations because it can reproduce the experimentally observed transition from BCC to HCP structure at around 25 GPa, at high deformation rates. They both show similar evolution of defects, also indicating departure from a shock regime which is extremely different for these potentials.
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Papers by Gonzalo dos Santos