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G. Brown

Publications and source records attributed to G. Brown.

36 records · Page 2Linked to original sources
arXiv2007

Two Modes of Magnetization Switching in a Simulated Iron Nanopillar in an Obliquely Oriented Field

Finite-temperature micromagnetics simulations are employed to study the magnetization-switching dynamics driven by a field applied at an angle to the long axis of an iron nanopillar. A bi-modal distribution in the switching times is observed, and evidence for two competing modes of magnetization-switching dynamics is presented. For the conditions studied here, temperature $T = 20$ K and the reversal field 3160 Oe at an angle of 75$^\circ$ to the long axis, approximately 70% of the switches involve unstable decay (no free-energy barrier) and 30% involve metastable decay (a free-energy barrier is crossed). The latter are indistinguishable from switches which are constrained to start at a metastable free-energy minimum. Competition between unstable and metastable decay could greatly complicate applications involving magnetization switches near the coercive field.

cond-mat.stat-mech↗
arXiv2006

Reply to ``Remarks on the simulation of Cl electrosorption on Ag(100) reported in Electrochimica Acta 50 (2005) 5518"

We reply to remarks by Lang and Horanyi on the meaning of the notion of "electrosorption valency" used in I. Abou Hamad et al., Electrochim. Acta 50 (2005) 5518. It is concluded that, contrary to the assertion of Lang and Horanyi, the magnitude of the current in the external circuit upon adsorption of an ion of charge ze with partial charge transfer is indeed given by an electrosorption valency gamma such that |gamma e| < |ze|. We believe the conclusion of Lang and Horanyi to the contrary is the result of an excessively severe charge-neutrality requirement.

physics.chem-ph↗
arXiv2006

Finite-temperature Simulations for Magnetic Nanostructures

We examine different models and methods for studying finite-temperature magnetic hysteresis in nanoparticles and ultrathin films. This includes micromagnetic results for the hysteresis of a single magnetic nanoparticle which is misaligned with respect to the magnetic field. We present results from both a representation of the particle as a one-dimensional array of magnetic rotors, and from full micromagnetic simulations. The results are compared with the Stoner-Wohlfarth model. Results of kinetic Monte Carlo simulations of ultrathin films are also presented. In addition, we discuss other topics of current interest in the modeling of magnetic hysteresis in nanostructures, including kinetic Monte Carlo simulations of dynamic phase transitions and First-Order Reversal Curves.

cond-mat.mtrl-sci↗
arXiv2005

Cl electrosorption on Ag(100): Lateral interactions and electrosorption valency from comparison of Monte Carlo simulations with chronocoulometry experiments

We present Monte Carlo Simulations using an equilibrium lattice-gas model for the electrosorption of Cl on Ag(100) single-crystal surfaces. Fitting the simulated isotherms to chronocoulometry experiments, we extract parameters such as the electrosorption valency gamma and the next-nearest-neighbor lateral interaction energy phi_nnn. Both coverage-dependent and coverage independent gamma were previously studied assuming a constant phi_nnn [I. Abou Hamad, Th. Wandlowski, G. Brown, P.A. Rikvold, J. Electroanal. Chem. 554-555 (2003) 211]. Here, a self-consistent, entirely electrostatic picture of the lateral interactions with a coverage-dependent phi_nnn is developed, and a relationship between phi_nnn and gamma is investigated for Cl on Ag(100).

cond-mat.mtrl-sci↗
arXiv2004

Determination of the basic timescale in kinetic Monte Carlo simulations by comparison with cyclic-voltammetry experiments

While kinetic Monte Carlo simulations can provide long-time simulations of the dynamics of physical and chemical systems, it is not yet possible in general to identify the inverse Monte Carlo attempt frequency with a physical timescale. Here we demonstrate such an identification by comparing simulations with experimental data. Using a dynamic lattice-gas model for the electrosorption of Br on Ag(100), we measure the scan-rate dependence of the separation between positive-and negative-going peaks in cyclic-voltammetry (CV) and compare simulated and experimental peak separations. By adjusting the Monte Carlo attempt frequency, good agreement between simulated and experimental peak separations is achieved. It is also found that the uniqueness of such a determination is dependent on the relative values of the adsorption/desorption and diffusion free-energy barriers.

cond-mat.mtrl-sci↗
arXiv2004

Reversal Modes of Simulated Iron Nanopillars in an Obliquely Oriented Field

Stochastic micromagnetic simulations are employed to study switching in three-dimensional magnetic nanopillars exposed to highly misaligned fields. The switching appears to proceed through two different decay modes, characterized by very different average lifetimes and different average values of the transverse magnetization components.

cond-mat.stat-mech↗
arXiv2004

Projective Dynamics in Realistic Models of Nanomagnets

The free-energy extrema governing the magnetization-reversal process for a model of an iron nanopillar are investigated using the projective dynamics method. Since the time evolution of the model is computationally intensive, one could question whether sufficient statistics can be generated, with current resources, to evaluate the position of the metastable configuration. Justification for the fine-grained discretization of the model that we use here is given, and it is shown that tractable results can be obtained for this system on realistic time scales.

cond-mat.stat-mech↗
arXiv2003

Angular Dependence of Switching Properties in Single Fe Nanopillars

The continued increase in areal densities in magnetic recording makes it crucial to understand magnetization reversal in nanoparticles. We present finite-temperature micromagnetic simulations of hysteresis in Fe nanopillars with the long axis tilted at angles from 0 degrees to 90 degrees to the applied sinusoidal field. The field period is 15 ns, and the particle size is 9 x 9 x 150 nm. The system is discretized into a rectangular pillar of 7 x 7 x 101 spins each with uniform magnetization. At low angles, reversal begins at the endcaps and proceeds toward the center of the particle. At ninety degrees, reversal proceeds along the entire length of the particle (save at the ends). The switching field was observed to increase over the entire range of angles, consistent with recent experimental observations. A second, lower-resolution micromagnetic simulation with 1 x 1 x 17 spins, does not agree with experiment, but shows behavior very similar to that of the Stoner-Wohlfarth model of coherent rotation.

cond-mat.mtrl-sci↗
arXiv2003

Projective Dynamics Analysis of Magnetization Reversal

The computational Projective Dynamics method is used to analyze simulations of magnetization reversal in nanoscale magnetic pillars. It is shown that this method can be used to determine the magnetizations corresponding to the metastable minimum and saddle point in the free energy, and the free-energy barrier associated with those points. For the nanopillars studied here, entropy is found to provide a significant contribution to the free-energy barrier which determines the reversal time scale.

cond-mat.stat-mech↗
arXiv2002

Electrosorption of Br and Cl on Ag(100): Experiments and Computer Simulations

We present chronocoulometry experiments and equilibrium Monte Carlo simulations for the electrosorption of Br and Cl on Ag(100) single-crystal electrode surfaces. Two different methods are used to calculate the long-range part of the adsorbate-adsorbate interactions. The first method is a truncated-sum approach, while the second is a mean-field-enhanced truncated-sum approach. To compare the two methods, the resulting isotherms are fit to experimental adsorption isotherms, assuming both a constant electrosorption valency &gamma and also a coverage-dependent &gamma. While a constant &gamma fits the Br/Ag(100) well, a coverage-dependent or potential-dependent &gamma is needed for Cl/Ag(100).

cond-mat.mtrl-sci↗
arXiv2002

The Transition State in Magnetization Reversal

We consider a magnet with uniaxial anisotropy in an external magnetic field along the anisotropy direction, but with a field magnitude smaller than the coercive field. There are three representative magnetization configurations corresponding to three extrema of the free energy. The equilibrium and metastable configurations, which are magnetized approximately parallel and antiparallel to the applied field, respectively, both correspond to local free-energy minima. The third extremum configuration is the saddle point separating these minima. It is also called the transition state for magnetization reversal. The free-energy difference between the metastable and transition-state configurations determines the thermal stability of the magnet. However, it is difficult to determine the location of the transition state in both experiments and numerical simulations. Here it is shown that the computational Projective Dynamics method, applied to the time dependence of the total magnetization, can be used to determine the transition state. From large-scale micromagnetic simulations of a simple model of magnetic nanowires with no crystalline anisotropy, the magnetization associated with the transition state is found to be linearly dependent on temperature, and the free-energy barrier is found to be dominated by the entropic contribution at reasonable temperatures and external fields. The effect of including crystalline anisotropy is also discussed. Finally, the influence of the spin precession on the transition state is determined by comparison of the micromagnetic simulations to kinetic Monte Carlo simulations of precession-free (overdamped) dynamics.

cond-mat.stat-mech↗
arXiv2000

Static and Dynamic Monte Carlo Simulations of Br Electrodeposition on Ag(100)

We study the static and dynamic properties of bromine electrosorption onto single-crystal silver (100) electrodes by Monte Carlo simulation. At room temperature the system displays a second-order phase transition between a low-coverage disordered phase at more negative electrode potentials and a c(2x2) ordered phase with bromine coverage 1/2 at more positive potentials. We explore the phase diagram and demonstrate that the broad shoulder observed in room-temperature cyclic voltammograms is due to local fluctuations resembling ordered phases with coverage 1/4 which exist in the model at much lower temperatures. We construct a dynamic Monte Carlo algorithm using a thermally activated stochastic barrier-hopping model for the microscopic dynamics. We use this algorithm to study the phase ordering and disordering processes following sudden potential steps between the disordered phase and the c(2x2) phase and to study the sweep-rate dependence in simulated cyclic-voltammetry experiments.

cond-mat.mtrl-sci↗
arXiv2000

Dynamics of Br Electrosorption on Single-Crystal Ag(100): A Computational Study

We present dynamic Monte Carlo simulations of a lattice-gas model for bromine electrodeposition on single-crystal silver (100). This system undergoes a continuous phase transition between a disordered phase at low electrode potentials and a commensurate c(2X2) phase at high potentials. The lattice-gas parameters are determined by fitting simulated equilibrium adsorption isotherms to chronocoulometric data, and free-energy barriers for adsorption/desorption and lateral diffusion are estimated from ab initio data in the literature. Cyclic voltammograms in the quasi-static limit are obtained by equilibrium Monte Carlo simulations, while for nonzero potential scan rates we use dynamic Monte Carlo simulation. The butterfly shapes of the simulated voltammograms are in good agreement with experiments. Simulated potential-step experiments give results for the time evolution of the Br coverage, as well as the c(2X2) order parameter and its correlation length. During phase ordering following a positive potential step, the system obeys dynamic scaling. The disordering following a negative potential step is well described by random desorption with diffusion. Both ordering and disordering processes are strongly influenced by the ratio of the time scales for desorption and diffusion. Our results should be testable by experiments, in particular cyclic voltammetry and surface X-ray scattering.

cond-mat.mtrl-sci↗
arXiv2000

Fine-grid Simulations of Thermally Activated Switching in Nanoscale Magets

Numerical integration of the Landau-Lifshitz-Gilbert equation with thermal fluctuations is used to study the dynamic response of single-domain nanomagnets to rapid changes in the applied magnetic field. The simulation can resolve magnetization patterns within nanomagnets and uses the Fast Multipole method to calculate dipole-dipole interactions efficiently. The thermal fluctuations play an essential part in the reversal process whenever the applied field is less than the zero-temperature coercive field. In this situation pillar-shaped nanomagnets are found to reverse through a local curling mode that involves the formation and propagation of a domain wall. Tapering the ends of the pillars to reduce pole-avoidance effects changes the energies involved but not the fundamental process. The statistical distribution of switching times is well described by the independent nucleation and subsequent growth of regions of reversed magnetization at both ends of the pillar.

cond-mat.mtrl-sci↗
arXiv2000

Hysteresis in an Electrochemical System: Br Electrodeposition on Ag(100)

We examine hysteresis in cyclic voltammetry of Br electrosorption onto single-crystal Ag(100) by dynamic Monte Carlo simulations. At room temperature, this system displays a second-order phase transition from a low-coverage disordered phase to a doubly degenerate $c (2 \times 2)$ phase. The electrochemical potential is ramped back and forth across the phase transition, linearly in time, and the phase lag between the response of the adlayer and the potential is observed to depend on the sweep rate. The hysteresis in this system is caused by slow ordering/disordering kinetics and critical slowing-down.

cond-mat.mtrl-sci↗
arXiv1998

Computer Simulations of Thermal Switching in Small-grain Ferromagnets

We present Monte Carlo and Langevin micromagnetic calculations to investigate thermal switching of single-domain ferromagnetic particles. For the Monte Carlo study we place particular emphasis on the probability that the magnetization does not switch by time t. We find that this quantity has different behaviors in different regimes of applied field, temperature, and system size, and we explain this in terms of different reversal mechanisms that dominate in the different regimes. In the micromagnetic study of an array of Ni pillars, we show that the reversal mode is an outside-in mode starting at the perimeter of the array of pillars.

cond-mat.mtrl-sci↗
arXiv1998

Hard Simulation Problems in the Modeling of Magnetic Materials: Parallelization and Langevin Micromagnetics

We present recent results on two attempts at understanding and utilizing large-scale simulations of magnetic materials. In the first study we consider massively parallel implementations on a Cray T3E of the n-fold way algorithm for magnetization switching in kinetic Ising models. We find an intricate relationship between the average time increment and the size of the spin blocks on each processor. This narrows the regime of efficient implementation. The second study concerns incorporating noise into micromagnetic calculations using Langevin methods. This allows measurement of quantities such as the probability that the system has not switched within a given time. Preliminary results are reported for arrays of single-domain nanoscale pillars.

cond-mat.mtrl-sci↗