arXiv subjects

P. A. Rikvold

Publications and source records attributed to P. A. Rikvold.

85 records · Page 5Linked to original sources

arXiv1997

Magnetization Switching in Single-Domain Ferromagnets

A model for single-domain uniaxial ferromagnetic particles with high anisotropy, the Ising model, is studied. Recent experimental observations have been made of the probability that the magnetization has not switched. Here an approach is described in which it is emphasized that a ferromagnetic particle in an unfavorable field is in fact a metastable system, and the switching is accomplished through the nucleation and subsequent growth of localized droplets. Nucleation theory is applied to finite systems to determine the coercivity as a function of particle size and to calculate the probability of not switching. Both of these quantities are modified by different boundary conditions, magnetostatic interactions, and quenched disorder.

cond-mat.mtrl-sci↗

arXiv1997

Kinetic Ising System in an Oscillating External Field: Stochastic Resonance and Residence-Time Distributions

Experimental, analytical, and numerical results suggest that the mechanism by which a uniaxial single-domain ferromagnet switches after sudden field reversal depends on the field magnitude and the system size. Here we report new results on how these distinct decay mechanisms influence hysteresis in a two-dimensional nearest-neighbor kinetic Ising model. We present theoretical predictions supported by numerical simulations for the frequency dependence of the probability distributions for the hysteresis-loop area and the period-averaged magnetization, and for the residence-time distributions. The latter suggest evidence of stochastic resonance for small systems in moderately weak oscillating fields.

cond-mat↗

arXiv1997

Monte Carlo Simulation of Magnetization Reversal in Fe Sesquilayers on W(110)

Iron sesquilayers grown at room temperature on W(110) exhibit a pronounced coercivity maximum near a coverage of 1.5 atomic monolayers. On lattices which faithfully reproduce the morphology of the real films, a kinetic Ising model is utilized to simulate the domain-wall motion. Simulations reveal that the dynamics is dominated by the second-layer islands, which act as pinning centers. The simulated dependencies of the coercivity on the film coverage, as well as on the temperature and the frequency of the applied field, are very similar to those measured in experiments. Unlike previous micromagnetic models, the presented approach provides insight into the dynamics of the domain-wall motion and clearly reveals the role of thermal fluctuations.

cond-mat↗

arXiv1997

Nucleation Theory of Magnetization Switching in Nanoscale Ferromagnets

A nucleation picture of magnetization switching in single-domain ferromagnetic nanoparticles with high local anisotropy is discussed. Relevant aspects of nucleation theory are presented, stressing the effects of the particle size on the switching dynamics. The theory is illustrated by Monte Carlo simulations and compared with experiments on single particles.

cond-mat.mtrl-sci↗

arXiv1997

Projected dynamics for metastable decay in Ising models

The magnetization switching dynamics in the kinetic Ising model is projected onto a one-dimensional absorbing Markov chain. The resulting projected dynamics reproduces the direct simulation results with great accuracy. A scheme is proposed to utilize simulation data for small systems to obtain the metastable lifetime for large systems and/or for very weak magnetic fields, for which direct simulation is not feasible.

cond-mat↗

arXiv1995

Underpotential deposition of Cu on Au(111) in sulfate-containing electrolytes: a theoretical and experimental study

We study the underpotential deposition of Cu on single-crystal Au(111) electrodes in sulfate-containing electrolytes by a combination of computational statistical-mechanics based lattice-gas modeling and experiments. The experimental methods are in situ cyclic voltammetry and coulometry and ex situ Auger electron spectroscopy and low-energy electron diffraction. The experimentally obtained voltammetric current and charge densities and adsorbate coverages are compared with the predictions of a two-component lattice-gas model for the coadsorption of Cu and sulfate. This model includes effective, lateral interactions out to fourth-nearest neighbors. Using group-theoretical ground-state calculations and Monte Carlo simulations, we estimate effective electrovalences and lateral adsorbate--adsorbate interactions so as to obtain overall agreement with experiments, including both our own and those of other groups. In agreement with earlier work, we find a mixed R3xR3 phase consisting of 2/3 monolayer Cu and 1/3 monolayer sulfate at intermediate electrode potentials, delimited by phase transitions at both higher and lower potentials. Our approach provides estimates of the effective electrovalences and lateral interaction energies, which cannot yet be calculated by first-principles methods.

cond-mat↗

arXiv1995

Response of Kinetic Ising System to Oscillating External Fields: Amplitude and Frequency Dependence

The S=1/2, nearest-neighbor, kinetic Ising model has been used to model magnetization switching in nanoscale ferromagnets. For this model, earlier work based on the droplet theory of the decay of metastable phases and Monte Carlo simulations has shown the existence of a size dependent spinodal field which separates deterministic and stochastic decay regimes. We extend the above work to study the effects of an oscillating field on the magnetization response of the kinetic Ising model. We compute the power spectral density of the time-dependent magnetization for different values of the amplitude and frequency of the external field, using Monte Carlo simulation data. We also investigate the amplitude and frequency dependence of the probability distributions for the hysteresis loop area and the period-averaged magnetization. The time-dependent response of the system is classified by analyzing the behavior of these quantities within the framework of the distinct deterministic and stochastic decay modes mentioned above.

cond-mat↗

arXiv1995

Lattice-Gas Models of Adsorption in the Double Layer

The theory of statistical-mechanical lattice-gas modeling of adsorption is reviewed and shown to be applicable to a range of electrochemical problems dominated by effective, lateral adsorbate--adsorbate interactions. A general strategy for applying the method to specific systems is outlined, which includes microscopic model formulation, calculation of zero-temperature phase diagrams, numerical calculation of thermodynamical and structural quantities at nonzero temperatures, and estimation of effective, lateral interaction energies that cannot be obtained by first-principles methods. Phenomena that are discussed include poisoning and enhanced adsorption, and illustrative applications to specific systems are reviewed. Particular problems considered are: the poisoning by sulfur of hydrogen adsorption on platinum (111), the electrochemical adsorption of naphthalene on polycrystalline copper and of urea on single-crystal platinum (100), and the underpotential deposition of copper on single-crystal gold (111).

cond-mat↗

arXiv1995

A Method to Study Relaxation of Metastable Phases: Macroscopic Mean-Field Dynamics

We propose two different macroscopic dynamics to describe the decay of metastable phases in many-particle systems with local interactions. These dynamics depend on the macroscopic order parameter $m$ through the restricted free energy $F(m)$ and are designed to give the correct equilibrium distribution for $m$. The connection between macroscopic dynamics and the underlying microscopic dynamic are considered in the context of a projection- operator formalism. Application to the square-lattice nearest-neighbor Ising ferromagnet gives good agreement with droplet theory and Monte Carlo simulations of the underlying microscopic dynamic. This includes quantitative agreement for the exponential dependence of the lifetime on the inverse of the applied field $H$, and the observation of distinct field regions in which the derivative of the lifetime with respect to $1/H$ depends differently on $H$. In addition, at very low temperatures we observe oscillatory behavior of this derivative with respect to $H$, due to the discreteness of the lattice and in agreement with rigorous results. Similarities and differences between this work and earlier works on finite Ising models in the fixed-magnetization ensemble are discussed.

cond-mat↗

arXiv1995

Coadsorption of Copper and Sulfate on Au(111) Electrodes: Monte Carlo Simulation of a Lattice-Gas Model

We report ground-state calculations and Monte Carlo simulations for a lattice-gas model of the underpotential deposition of copper on Au(111) in sulfate-containing electrolytes. In a potential range of approximately 100$\sim$150 mV, this system exhibits a $(\sqrt3\!\times\!\sqrt3)$ mixed phase with 2/3 monolayer (ML) copper and 1/3 ML sulfate. Our simulation results agree well with experimental results and with other theoretical work.

cond-mat↗

arXiv1994

Computational Lattice-Gas Modeling of the Electrosorption of Small Molecules and Ions

We present two recent applications of lattice-gas modeling techniques to electrochemical adsorption on catalytically active metal substrates: urea on Pt(100) and (bi)sulfate on Rh(111). Both involve the specific adsorption of small molecules or ions on well-characterized single-crystal electrodes, and they provide a particularly good fit between the adsorbate geometry and the substrate structure. The close geometric fit facilitates the formation of ordered submonolayer adsorbate phases in a range of electrode potential positive of the range in which an adsorbed monolayer of hydrogen is stable. In both systems the ordered-phase region is separated from the adsorbed- hydrogen region by a phase transition, signified in cyclic voltammograms by a sharp current peak. Based on data from {\it in situ\/} radiochemical surface concentration measurements, cyclic voltammetry, and scanning tunneling micro- scopy, and {\it ex situ\/} Auger electron spectroscopy and low-energy electron diffraction, we have developed specific lattice-gas models for the two systems. These models were studied by group-theoretical ground-state calcu- lations and numerical Monte Carlo simulations, and effective lattice-gas inter- action parameters were determined so as to provide agreement with experiments.

cond-mat↗

arXiv1994

Recent Results on the Decay of Metastable Phases

We review some aspects of current knowledge regarding the decay of metastable phases in many-particle systems. In particular we emphasize recent theoretical and computational developments and numerical results regarding homogeneous nucleation and growth in kinetic Ising and lattice-gas models. An introductory discussion of the droplet theory of homogeneous nucleation is followed by a discussion of Monte Carlo and transfer-matrix methods commonly used for numerical study of metastable decay, including some new algorithms. Next we discuss specific classes of systems. These include a brief discussion of recent progress for fluids, and more exhaustive considerations of ferromagnetic Ising models ({\it i.e.}, attractive lattice-gas models) with weak long-range interactions and with short-range interactions. Whereas weak-long-range-force (WLRF) models have infinitely long-lived metastable phases in the infinite-range limit, metastable phases in short-range-force (SRF) models eventually decay, albeit extremely slowly. Recent results on the finite-size scaling of metastable lifetimes in SRF models are reviewed, and it is pointed out that such effects may be experimentally observable.

cond-mat↗

arXiv1994

Application of a constrained-transfer-matrix method to metastability in the $d$$=$$2$ Ising ferromagnet

Applying a numerical transfer-matrix formalism, we obtain complex-valued constrained free energies for the two-dimensional square-lattice nearest-neighbor Ising ferromagnet below its critical temperature and in an external magnetic field. In particular, we study the imaginary part of the constrained free-energy branch that corresponds to the metastable phase. Although droplets are not introduced explicitly, the metastable free energy is obtained in excellent agreement with field-theoretical droplet-model predictions. The finite-size scaling properties are different in the weak-field and intermediate-field regimes, and we identify the corresponding different critical-droplet shapes. For intermediate fields, we show that the surface free energy of the critical droplet is given by a Wulff construction with the equilibrium surface tension. We also find a prefactor exponent in complete agreement with the field-theoretical droplet model. Our results extend the region of validity for known results of this field-theoretical droplet model, and they indicate that this transfer-matrix approach provides a nonperturbative numerical continuation of the equilibrium free energy into the metastable phase.

cond-mat↗