Papers by Stanislav Denisov
Acta Physica Polonica B, 2015
Using the continuous-time random walk (CTRW) approach, we study the phenomenon of relaxation of t... more Using the continuous-time random walk (CTRW) approach, we study the phenomenon of relaxation of two-state systems whose elements evolve according to a dichotomous process. Two characteristics of relaxation, the probability density function of the waiting times difference and the relaxation law, are of our particular interest. For systems characterized by the Erlang distributions of waiting times, we consider different regimes of relaxation and show that, under certain conditions, the relaxation process can be non-monotonic. By studying the asymptotic behavior of the relaxation process, we demonstrate that heavy and superheavy tails of waiting time distributions correspond to slow and superslow relaxation, respectively.
Physical Review E, 2000
We derive a rigorous expression for the mean first-passage time of an overdamped particle subject... more We derive a rigorous expression for the mean first-passage time of an overdamped particle subject to a constant bias in a force field with quenched disorder. Depending on the statistics of the disorder, the disorderaveraged mean first-passage time can undergo a transition from an infinite value for small bias to a finite value for large bias. This corresponds to a depinning transition of the particle. We obtain exact values for the depinning threshold for Gaussian disorder and also for a class of piecewise constant random forces, which we call generalized kangaroo disorder. For Gaussian disorder, we investigate how the correlations of the random force field affect the average motion of the particle. For kangaroo disorder, we apply the general results for the depinning transition to two specific examples, viz., dichotomous disorder and random fractal disorder.
Physical Review E, 2002
We study the temporal evolution of a system that has an absorbing state and that is driven by col... more We study the temporal evolution of a system that has an absorbing state and that is driven by colored Gaussian noise, whose amplitude depends on the system state x as ͉x͉ ␣. Exact, analytical expressions for the probability density functions of the system and of the absorption time are derived. We also calculate numerical characteristics of the probability density functions, namely, the fractional moments of the system and the mean absorption time, and analyze the role of the functional form of the noise correlation function.
Journal of Applied Physics, 2014
We develop an analytical model for describing the magnetization dynamics in ferromagnetic metal n... more We develop an analytical model for describing the magnetization dynamics in ferromagnetic metal nanoparticles, which is based on the coupled system of the Landau-Lifshitz-Gilbert (LLG) and Maxwell equations. By solving Maxwell's equations in the quasi-static approximation and finding the magnetic field of eddy currents, we derive the closed LLG equation for the magnetization that fully accounts for the effects of conductivity. We analyze the difference between the LLG equations in metallic and dielectric nanoparticles and show that these effects can strongly influence the magnetization dynamics. As an example illustrating the importance of eddy currents, the phenomenon of precessional switching of magnetization is considered.
Using the Landau-Lifshitz equation, the dependencies of the power loss of the nanoparticle magnet... more Using the Landau-Lifshitz equation, the dependencies of the power loss of the nanoparticle magnetic moment on the amplitude and frequency of alternating magnetic fields are calculated numerically. A special attention is paid to the different precessional modes of the magnetic moment and their influence on power loss value. The results for the circularly and linearly polarized fields are compared in order to choose the optimal one with respect to the power loss.
We study the role of conductivity in the magnetization dynamics of single-domain ferromagnetic pa... more We study the role of conductivity in the magnetization dynamics of single-domain ferromagnetic particles. Our approach is based on the coupled system of Maxwell's and Landau-Lifshitz-Gilbert (LLG) equations that describes both the induced electromagnetic field and the magnetization dynamics. We show that the effective LLG equation for a conducting particle contains two additional terms compared to the ordinary LLG equation. One of these terms accounts for the magnetic field of eddy currents induced by an external magnetic field, and the other is magnetization dependent and is responsible for the conductivity contribution to the damping parameter. By analytically solving Maxwell's equations, we determine this contribution and demonstrate the importance of conduction effects for large nanoparticles.
We derive the generalized Fokker-Planck equation for the probability density function of the nano... more We derive the generalized Fokker-Planck equation for the probability density function of the nanoparticle magnetic moment driven by Poisson white noise. Our approach is based on the reduced stochastic Landau-Lifshitz equation in which this noise is included into the effective magnetic field. We take into account that the magnetic moment under the noise action can change its direction instantaneously and show that the generalized equation has an integro-differential form.
Using the modified stochastic Landau-Lifshitz equation driven by Poisson white noise, we derive t... more Using the modified stochastic Landau-Lifshitz equation driven by Poisson white noise, we derive the generalized Fokker-Planck equation for the probability density function of the nanoparticle magnetic moment. In our calculations we employ the Ito interpretation of stochastic equations and take into account the fact that the magnetic moment direction can be changed by this noise instantaneously. The analysis of the stationary solution of the generalized Fokker-Planck equation shows that the distribution of the free magnetic moment in Poisson white noise is not uniform. This feature of the stationary distribution arises from using the Ito interpretation of the stochastic Landau-Lifshitz equation.
We develop a numerical method to study the long-time behavior of continuous-time random walks cha... more We develop a numerical method to study the long-time behavior of continuous-time random walks characterized by superheavy-tailed distributions of waiting time. To test the method, we consider symmetric jump-length distributions with both finite second moments and heavy tails for which the asymptotic behavior of the walking particle is known exactly. Our numerical results for the distributions of the particle position are in excellent agreement with the analytical ones.
We present a highly-parallel implementation of the Langevin simulation method for modeling ferrof... more We present a highly-parallel implementation of the Langevin simulation method for modeling ferrofluids on Graphical Processor Units (GPU). Our method is based on the Barnes-Hut algorithm. As a benchmark we use the straightforward 'all-to-all interaction' algorithm. The obtained results are in good agreement with known theoretical model. With the proposed method we were able to follow the evolution of a system of one million interacting particles over long timescales , the task hitherto is out of reach with the standard, CPU-based numerical schemes.
The mean velocity of domain walls in randomly inhomogeneous magnets is determined. The dynamic co... more The mean velocity of domain walls in randomly inhomogeneous magnets is determined. The dynamic coercivity field of domain walls and the dependence of their velocity on the applied magnetic field are calculated for two classes of models of inhomogeneous magnets. The experimentally observed behavior of mean velocities in magnetic fields close to the dynamic coercivity field is interpreted.
1997 IEEE International Magnetics Conference (INTERMAG'97)
Physics of the Solid State, 1997
Physical Review E, 2011
We study the long-time behavior of decoupled continuous-time random walks characterized by superh... more We study the long-time behavior of decoupled continuous-time random walks characterized by superheavy-tailed distributions of waiting times and symmetric heavy-tailed distributions of jump lengths. Our main quantity of interest is the limiting probability density of the position of the walker multiplied by a scaling function of time. We show that the probability density of the scaled walker position converges in the long-time limit to a non-degenerate one only if the scaling function behaves in a certain way. This function as well as the limiting probability density are determined in explicit form. Also, we express the limiting probability density which has heavy tails in terms of the Fox H-function and find its behavior for small and large distances.
Physical Review E, 2010
We present analytical results for the biased diffusion of particles moving under a constant force... more We present analytical results for the biased diffusion of particles moving under a constant force in a randomly layered medium. The influence of this medium on the particle dynamics is modeled by a piecewise constant random force. The long-time behavior of the particle position is studied in the frame of a continuous-time random walk on a semi-infinite one-dimensional lattice. We formulate the conditions for anomalous diffusion, derive the diffusion laws and analyze their dependence on the particle mass and the distribution of the random force.
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Papers by Stanislav Denisov