György Kádár

Magnetic hysteresis in an Ising-like dipole-dipole model

Using zero temperature Monte Carlo simulations we have studied the magnetic hysteresis in a three-dimensional Ising model with nearest neighbor exchange and dipolar interaction. The average magnetization of spins located inside a sphere on a cubic lattice is determined as a function of magnetic field varied periodically. The simulations have justified the appearance of hysteresis and allowed us to have a deeper insight into the series of metastable states developed during this process.

On the product Preisach model of hysteresis

Abstract The product model is an output-dependent modification of the traditional Preisach model in order to remove its non-real congruency property. The differential susceptibility is proposed to be a product of a magnetization-dependent factor and an expression containing two terms: one for the reversible process and the other an integral of a Preisach type distribution for the irreversible part. Thus the magnetization is an indirect function, in which the saturation is a natural property of the hysteresis model, and the reversible and irreversible parts of the magnetization are added up indirectly. The envelope function is related to the paramagnetic process and in the specific case of uni-axial anisotropy it is a hyperbolic tangent function. The measurement of the anhysteretic curve may provide the direct link to the evaluation methods of experimental data applied for traditional Preisach modeling.

Band gaps in photonic crystals with dispersion

Purpose – The paper presents the band gap computation in one‐ and two‐dimensional photonic crystals built up from porous silicon. The frequency dispersion of the dielectric materials is taken into account.Design/methodology/approach – The behavior of the light in a photonic crystal can be well described by the Maxwell equations. The finite difference time domain (FDTD) method is applied to determine the band structure. The frequency dependence of the dielectric constant is taken into account by a sum of second‐order Lorenz poles. The material parameters are determined applying a conjugate gradient‐based minimization procedure. Passing a light pulse of Gaussian distribution through the photonic crystal and analyzing the transmitted wave can explore the photonic bands.Findings – The realized simulations and visualizations can lead to a much better understanding of the behavior of electromagnetic waves in dispersive photonic crystals, and can make possible to set up experimental conditions properly. The obta...

Hysteresis in a dipolar Ising model

Abstract Zero-temperature Monte Carlo simulations are performed to study the magnetic hysteresis in a three-dimensional model with only dipolar interactions among the Ising spins located inside a sphere on a cubic lattice. Relevant differences are found in the site energy distributions between the up and down spins during the magnetization process. Variations of the dipolar energy and the density of incomplete columns are also discussed.

Picosecond reflectance recovery dynamics of porous silicon multilayer

Light-induced reflectance dynamics of a porous silicon multilayered structure via the picosecond pump-probe technique is investigated. Fast recovery dynamics is inspected for a range of excitation energy densities (2.4-24 mJ/cm2), incident angles (0°-25°), and polarizations. Modeling performed using the transfer matrix method revealed the photonic origin of the phenomena observed. The spectral shift of the reflectance spectrum, i.e., photonic bandgap, responsible for the positive signal originates from the change of photoinduced carrier concentration with linear and nonlinear character for different porosity layers. The influence of temperature on the refractive index in this thermally nonconductive material is taken into account.