R. Smirnov-Rueda

Brownian dynamics approach to interacting magnetic moments

The question of how to introduce thermal fluctuations in the equation of motion of a magnetic system is addressed. Using the approach of the fluctuation-dissipation theorem we calculate the properties of the noise for both, the fluctuating field and the additive fluctuating torque (force) representation. In contrast to earlier calculations we consider the general case of a system of interacting magnetic moments. We show that the interactions do not result in any correlations of thermal fluctuations in the field representation and that the same widely used formula can be used in the most general case. We further prove that close to the equilibrium where the fluctuation-dissipation theorem is valid, both, field and additive torque (force) representations coincide, being different far away from it. We also show that the uncorrelated character of the noise is due to the form of the Landau-Lifshitz (or Gilbert) damping and under different damping formalisms, the normal mode analysis is proper.

Experimental test on the applicability of the standard retardation condition to bound magnetic fields

Modern view on the fundamental structure of the whole EM field implies the existence of two components essentially different by nature. We found that it has some historical explanation. Experimental data do not support the validity of the standard retardation constraint (ν = c) generally accepted in respect to bound fields. Besides, non‐local characteristics of classical bound fields may shed a promising new light on a close relationship between quantum mechanics and classical EM theory.

Measurement of propagation velocity of bound electromagnetic fields in near zone

We start with the general approach based on conventional solutions to Maxwell’s equations and show that in the near zone of macroscopic electromagnetic sources the electromotive force produced in receiving loop antenna is intimately linked to the fundamental structure as well as to causal properties of the classical electromagnetic field as a superposition of bound and radiation components. As a consequence, we propose and implement a direct experimental procedure for the correct identification of retarded positions of bound field contributions on the oscilloscope time scale as a function of a distance from the emitting loop antenna. It provides unambiguous empirical information on causal characteristics of bound electromagnetic fields. According to the observation of no retardation inside the near zone of the emitting loop antenna, the experimental evidence for nonlocal properties of bound electromagnetic fields is reported.

Anomalously small retardation of bound (force) electromagnetic fields in antenna near zone

At fundamental level this work uncovers the actually incomplete knowledge of the energy transmission and propagation related to bound (force) electromagnetic (EM) fields. To deal with this problem, we present an experimental approach to a separate study of propagation characteristics of bound and radiation EM fields produced by antennas in a vacuum. A series of our recent experiments continues (J. Appl. Phys., 101 (2007) 023532; 102 (2007) 013529) with improved technical realizations extended for different ultra-high-frequency (UHF) radiation wavelengths. The experimental results show anomalously small retardation of bound EM fields within about the half of the near zone size.

Real time quantification of Monte Carlo steps for different time scales

Time quantification of Monte Carlo steps is studied by the implementation of a new technique which takes into account the realistic size of thermal fluctuations of magnetization along with Landau–Lifshitz–Gilbert dynamic correlations. The computational model has been specifically developed for an ensemble of isolated single-domain particles. The numerical results have been compared with Langevin dynamics calculations and theoretically predicted Brown’s asymptotes for relaxation time of single spin system. In addition we demonstrated that real time quantification of Monte Carlo steps is also possible for different time scales. Implementation of real time scales into Monte Carlo calculations for different sizes of time steps is shown to be convergent to the expected value if the Monte Carlo acceptance rate is taken into account.

ON THE RELAXATION OF SIMPLE MAGNETIC SYSTEMS

The relaxation at constant applied field and temperature of a simple magnetic system is evaluated in the framework of the micromagnetic approximation and using a Monte Carlo algorithm. Our results, and particularly, those corresponding to the time evolution of the probability of magnetic moment reversal, evidence remarkable differences with the classical Arrehnius–Neel predictions. These differences are linked to the fact that relaxation proceeds through the formation of structures involving a large number of degrees of freedom.

Simulation of magnetic relaxation by a Monte Carlo technique with correlations and quantified time steps

A new integrated numerical approach for simulation of fast and slow relaxation in magnets has been developed. It is based on Monte Carlo calculations which additionally take into account important dynamic information provided by the Langevin dynamics method. Real time quantification of Monte Carlo steps has been achieved by the new technique for a simple one-dimensional modelled system of interacting spins.

Modelling the time dependence of the magnetization in a system with many degrees of freedom

Abstract This paper presents the results of a micromagnetic simulation of thermally activated demagnetization in a 1D (3D) model of textured, highly anisotropic grains. Our results allow us to identify the role of the viscosity phenomenology of the domain walls separating reversed-unreversed grains.

Long-time calculation of the thermal magnetization reversal using Metropolis Monte Carlo

Abstract The standard Metropolis Monte Carlo is used to simulate thermal magnetization decay in ensemble of interacting and non-interacting particles. The fitting of demagnetization curves to simple Neel–Arrhenius model, using the volume distribution, suggests that in a non-interacting case one Monte Carlo step is proportional to a square root of time. The same dependence arises from the consideration of magnetic particle moving in the external potential according to Brownian dynamics. This constitutes the basis of the so-called Monte Carlo with quantified time step. The analytical calculations show that the method works reasonably in the case of small-to-intermediate size barriers and for a high anisotropy system. The application of the method to magnetic recording media reveals qualitatively the same dependence on the exchange parameter as obtained by kinetic Monte Carlo.

Influence of the system parameters on the non-Arrhenius magnetic relaxation of systems having distributed properties

We have investigated, in the framework of the micromagnetic approximation, the relaxation behavior of both simple systems and systems having distributed properties. In the case of single-particle-type systems, our study focused on the exchange constant dependence of the so-called “waiting time” for the onset of the relaxation to conclude that this parameter linearly increased with the increase of the exchange constant. Our results for the relaxation of polycrystalline-type systems having distributed anisotropy easy axes showed the occurrence in limited time ranges of a magnetization decrease which was adequately fitted by the M(t)−M(0)∝ ln(t+t0) law. The exchange constant dependence of the additive fitting parameter in the logarithmic law qualitatively reproduced that of the waiting time indicating that both parameters were linked to the same underlying characteristic of the demagnetization process: the coupling of the magnetic moments forming the domain-wall-like structures through which the system rever...