Dmitry Berkov - Academia.edu (original) (raw)
Papers by Dmitry Berkov
arXiv (Cornell University), Sep 28, 2023
In the second part of this publication, we present simulation results for two three-dimensional m... more In the second part of this publication, we present simulation results for two three-dimensional models of Heusler-type alloys obtained by the mesoscopic micromagnetic approach. In the first model, we simulate the magnetization reversal of a single ferromagnetic (FM) inclusion within a monocrystalline antiferromagnetic (AFM) matrix, revealing the evolution of the complex magnetization distribution within this inclusion when the external field is changed. The main result of this "monocrystalline" model is the absence of any hysteretic behavior by the magnetization reversal of the FM inclusion. Hence, this model is unable to reproduce the basic experimental result for the corresponding nanocomposite-hysteresis in the magnetization reversal of FM inclusions with a vertical shift of the corresponding loops. To explain this latter feature, in the second model we introduce a polycrystalline AFM matrix, with exchange interactions between AFM crystallites and between the FM inclusion and these crystallites. We show that within this model we can not only reproduce the hysteretic character of the remagnetization process, but also achieve a semi-quantitative agreement with the experimentally observed hysteresis loop assuming that the concentration of FM inclusions strongly fluctuates. These findings demonstrate the reliability of our enhanced micromagnetic model and set the basis for its applications in future studies of Heusler alloys and FM/AFM nanocomposites.
Physical review, Dec 19, 2023
Optics and Spectroscopy - OPT SPECTROSC, 1986
New Journal of Physics
Motivated by recent experimental polarized neutron results, we present a numerical micromagnetic ... more Motivated by recent experimental polarized neutron results, we present a numerical micromagnetic study of the interfacial (intergrain) Dzyaloshinskii-Moriya interaction (DMI) in nanocrystalline terbium. We demonstrate that the DMI-induced spin misalignment between adjacent nanograins is the reason for the formation of the asymmetric positive-negative pattern seen in polarized neutron scattering experiments. Analysis of the remagnetization process suggests the generic impact of the DMI on the macroscopic magnetic parameters of polycrystalline defect-rich materials.
Journal of Applied Spectroscopy, 1987
Journal of Magnetism and Magnetic Materials, 1996
Various numerical methods used for simulation of quasistatic remagnetization processes in interac... more Various numerical methods used for simulation of quasistatic remagnetization processes in interacting systems of fine single-domain magnetic particles are analyzed. Among them methods based on the sequential alignment of magnetic moments along the effective field are found to be the fastest, especially in the most interesting case of a small single particle anisotropy when cooperative remagnetization modes play a dominant role. Further it is shown, that the random field approximation (RFA) which is often used to take the interparticle interaction effects into account is valid for large particle anisotropies only. The distribution density of the interaction field is found to transform from the Lorentzian to the Gaussian form with the increasing particle volume fraction. Finally hysteresis loops for a wide range of single-particle anisotropy values and particle concentration are calculated and the crossover from the single particle to the collective behavior with decreasing single particle anisotropy is investigated.
Journal of Applied Physics, 1998
Densely packed arrays of magnetic platelets with sizes in the sub-μm region are promising candida... more Densely packed arrays of magnetic platelets with sizes in the sub-μm region are promising candidates for magnetic metamaterials due to the possibility to control their dynamic properties both via their geometry (shape anisotropy) and magnetization configuration (which can be changed by the applied field). Simulations of these dynamical properties are extraordinary important for the prediction of the metamaterials features prior to its preparation. Here we present an example of the micromagnetic characterization of such a system for the case of a hexagonal array of thin nanodisks. Comparing our simulation results with experimental data obtained by various measurements techniques, we show that numerical simulations represent a reliable tool for the prediction of both static (hysteresis loops) and dynamic (excitation spectra) properties of magnetic metamaterials. Large-scale periodic arrays of thin magnetic nanoelem ents (disks, rectangles, triangles etc.) with lateral sizes ~ 10 – 10 ...
International Journal of Modern Physics C, 1994
The fast algorithm of the Maximum Entropy (MaxEnt) numerical solution of the linear inverse probl... more The fast algorithm of the Maximum Entropy (MaxEnt) numerical solution of the linear inverse problem is described. The minimization of a general functional intrinsic to the MaxEnt approach is reduced to an iteration procedure with each step being a constrained least-squares problem (minimization of a quadratic functional with linear inequality constraints). The algorithm is structurally simple and can be assembled from blocks available in standard program libraries. The algorithm is tested on “toy” tasks with exponential kernel, as well as on practical problems of the recovery of the spectral density of strongly correlated quantum systems from the imaginary time Green’s functions obtained by Monte Carlo.
physica status solidi (a), 2002
We report on recent progress achieved by the development of numerical methods based on the stocha... more We report on recent progress achieved by the development of numerical methods based on the stochastic (Langevin) dynamics applied to systems of interacting magnetic nanoparticles. The method enables direct simulations of the trajectories of magnetic moments taking into account (i) all relevant interactions, (ii) precession dynamics, and (iii) temperature fluctuations included via the random (thermal) field. We present several novel results obtained using new methods developed for the solution of the Langevin equations. In particular, we have investigated magnetic nanodots and disordered granular systems of single-domain magnetic particles. For the first case we have calculated the spectrum and the spatial distribution of spin excitations. For the second system the complex ac susceptibility c(w, T) for various particle concentrations and particle anisotropies were computed and compared with numerous experimental results.
IEEE Transactions on Magnetics, 1999
We present a new method to reconstruct the distribution of magnetic moments ρ(μ) of ferrofluid pa... more We present a new method to reconstruct the distribution of magnetic moments ρ(μ) of ferrofluid particles from the magnetization curve measured on a ferrofluid in the liquid state. The method employs the solution of the integral equation describing the magnetization of the fine particle system in an external field and requires no a priori assumptions concerning the shape of ρ(μ).
Physics and Technology of Thin Films - IWTF 2003 - Proceedings of the International Workshop, 2004
ABSTRACT The understanding of the equilibrium magnetisation structure and quasistatic remagnetiza... more ABSTRACT The understanding of the equilibrium magnetisation structure and quasistatic remagnetization processes in single- or polycrystalline layers is extremely important when developing technical applications such as magnetoresistive (MR) sensors and Magnetic Random-Access Memories (MRAM's). To optimise the design of single-and/or multilayer elements, we have developed a programme package, which allows to calculate quasistatic remagnetisation processes (e.g., the hysteresis loops) and corresponding magnetic domain structures for definite geometric arrangements, edge roughness, and material parameters of magnetic layers on the base of finite-difference approximations of the energy contributions. The equilibrium magnetisation state for the given external conditions is found by minimising the total magnetic free energy of the system, which includes four standard contributions: external field, anisotropy, exchange and demagnetising energies and - for multilayers - the interlayer exchange energy (ferro-or antiferromagnetic interlayer coupling). Various examples of simulations will be presented to illustrate some important topics of the MRAM design and the abilities of the program package.
Physical Review B, 2005
We demonstrate (using full-scale micromagnetic simulations) that the spin injection driven steady... more We demonstrate (using full-scale micromagnetic simulations) that the spin injection driven steadystate precession of a thin magnetic nanoelement exhibit a complicate transition from the quasimacrospin to the chaotic behaviour with the increasing element size. For nanoelement parameters typical for those used experimentally we have found that the macrospin approximation becomes invalid already for very small nanoelement sizes (∼ 30 nm), in contrast to the previously reported results (Li and Zhang, Phys. Rev., B68, 024404-1 (2003)).
Journal of Magnetism and Magnetic Materials, 1999
ABSTRACT
Physical Review B, 1996
ABSTRACT
Physical review, Dec 7, 2021
In this paper we present an algorithm which allows single-stage direct Langevin dynamics simulati... more In this paper we present an algorithm which allows single-stage direct Langevin dynamics simulations of transitions over arbitrary high energy barriers employing the concept of the energy-dependent temperature (EDT). In our algorithm, simulation time required for the computation of the corresponding switching rate does not increase with energy barrier. This is achieved by using in simulations an effective temperature which depends on the system energy: around the energy minima this temperature is high and tends towards the room temperature when the energy approaches the saddle point value. Switching times computed via our EDT algorithm show an excellent agreement with results obtained with the established forward flux sampling (FFS) method. As the simulation time required by our method does not increase with the energy barrier, we achieve a very large speedup when compared even to the highly optimized FFS version. In addition, our method does not suffer from stability problems occurring in multi-stage algorithms (like FFS and 'energy bounce' methods) due to the multiplication of a large number of transition probabilities between the interfaces.
7th Joint MMM-Intermag Conference. Abstracts (Cat. No.98CH36275)
arXiv (Cornell University), Sep 28, 2023
In the second part of this publication, we present simulation results for two three-dimensional m... more In the second part of this publication, we present simulation results for two three-dimensional models of Heusler-type alloys obtained by the mesoscopic micromagnetic approach. In the first model, we simulate the magnetization reversal of a single ferromagnetic (FM) inclusion within a monocrystalline antiferromagnetic (AFM) matrix, revealing the evolution of the complex magnetization distribution within this inclusion when the external field is changed. The main result of this "monocrystalline" model is the absence of any hysteretic behavior by the magnetization reversal of the FM inclusion. Hence, this model is unable to reproduce the basic experimental result for the corresponding nanocomposite-hysteresis in the magnetization reversal of FM inclusions with a vertical shift of the corresponding loops. To explain this latter feature, in the second model we introduce a polycrystalline AFM matrix, with exchange interactions between AFM crystallites and between the FM inclusion and these crystallites. We show that within this model we can not only reproduce the hysteretic character of the remagnetization process, but also achieve a semi-quantitative agreement with the experimentally observed hysteresis loop assuming that the concentration of FM inclusions strongly fluctuates. These findings demonstrate the reliability of our enhanced micromagnetic model and set the basis for its applications in future studies of Heusler alloys and FM/AFM nanocomposites.
Physical review, Dec 19, 2023
Optics and Spectroscopy - OPT SPECTROSC, 1986
New Journal of Physics
Motivated by recent experimental polarized neutron results, we present a numerical micromagnetic ... more Motivated by recent experimental polarized neutron results, we present a numerical micromagnetic study of the interfacial (intergrain) Dzyaloshinskii-Moriya interaction (DMI) in nanocrystalline terbium. We demonstrate that the DMI-induced spin misalignment between adjacent nanograins is the reason for the formation of the asymmetric positive-negative pattern seen in polarized neutron scattering experiments. Analysis of the remagnetization process suggests the generic impact of the DMI on the macroscopic magnetic parameters of polycrystalline defect-rich materials.
Journal of Applied Spectroscopy, 1987
Journal of Magnetism and Magnetic Materials, 1996
Various numerical methods used for simulation of quasistatic remagnetization processes in interac... more Various numerical methods used for simulation of quasistatic remagnetization processes in interacting systems of fine single-domain magnetic particles are analyzed. Among them methods based on the sequential alignment of magnetic moments along the effective field are found to be the fastest, especially in the most interesting case of a small single particle anisotropy when cooperative remagnetization modes play a dominant role. Further it is shown, that the random field approximation (RFA) which is often used to take the interparticle interaction effects into account is valid for large particle anisotropies only. The distribution density of the interaction field is found to transform from the Lorentzian to the Gaussian form with the increasing particle volume fraction. Finally hysteresis loops for a wide range of single-particle anisotropy values and particle concentration are calculated and the crossover from the single particle to the collective behavior with decreasing single particle anisotropy is investigated.
Journal of Applied Physics, 1998
Densely packed arrays of magnetic platelets with sizes in the sub-μm region are promising candida... more Densely packed arrays of magnetic platelets with sizes in the sub-μm region are promising candidates for magnetic metamaterials due to the possibility to control their dynamic properties both via their geometry (shape anisotropy) and magnetization configuration (which can be changed by the applied field). Simulations of these dynamical properties are extraordinary important for the prediction of the metamaterials features prior to its preparation. Here we present an example of the micromagnetic characterization of such a system for the case of a hexagonal array of thin nanodisks. Comparing our simulation results with experimental data obtained by various measurements techniques, we show that numerical simulations represent a reliable tool for the prediction of both static (hysteresis loops) and dynamic (excitation spectra) properties of magnetic metamaterials. Large-scale periodic arrays of thin magnetic nanoelem ents (disks, rectangles, triangles etc.) with lateral sizes ~ 10 – 10 ...
International Journal of Modern Physics C, 1994
The fast algorithm of the Maximum Entropy (MaxEnt) numerical solution of the linear inverse probl... more The fast algorithm of the Maximum Entropy (MaxEnt) numerical solution of the linear inverse problem is described. The minimization of a general functional intrinsic to the MaxEnt approach is reduced to an iteration procedure with each step being a constrained least-squares problem (minimization of a quadratic functional with linear inequality constraints). The algorithm is structurally simple and can be assembled from blocks available in standard program libraries. The algorithm is tested on “toy” tasks with exponential kernel, as well as on practical problems of the recovery of the spectral density of strongly correlated quantum systems from the imaginary time Green’s functions obtained by Monte Carlo.
physica status solidi (a), 2002
We report on recent progress achieved by the development of numerical methods based on the stocha... more We report on recent progress achieved by the development of numerical methods based on the stochastic (Langevin) dynamics applied to systems of interacting magnetic nanoparticles. The method enables direct simulations of the trajectories of magnetic moments taking into account (i) all relevant interactions, (ii) precession dynamics, and (iii) temperature fluctuations included via the random (thermal) field. We present several novel results obtained using new methods developed for the solution of the Langevin equations. In particular, we have investigated magnetic nanodots and disordered granular systems of single-domain magnetic particles. For the first case we have calculated the spectrum and the spatial distribution of spin excitations. For the second system the complex ac susceptibility c(w, T) for various particle concentrations and particle anisotropies were computed and compared with numerous experimental results.
IEEE Transactions on Magnetics, 1999
We present a new method to reconstruct the distribution of magnetic moments ρ(μ) of ferrofluid pa... more We present a new method to reconstruct the distribution of magnetic moments ρ(μ) of ferrofluid particles from the magnetization curve measured on a ferrofluid in the liquid state. The method employs the solution of the integral equation describing the magnetization of the fine particle system in an external field and requires no a priori assumptions concerning the shape of ρ(μ).
Physics and Technology of Thin Films - IWTF 2003 - Proceedings of the International Workshop, 2004
ABSTRACT The understanding of the equilibrium magnetisation structure and quasistatic remagnetiza... more ABSTRACT The understanding of the equilibrium magnetisation structure and quasistatic remagnetization processes in single- or polycrystalline layers is extremely important when developing technical applications such as magnetoresistive (MR) sensors and Magnetic Random-Access Memories (MRAM's). To optimise the design of single-and/or multilayer elements, we have developed a programme package, which allows to calculate quasistatic remagnetisation processes (e.g., the hysteresis loops) and corresponding magnetic domain structures for definite geometric arrangements, edge roughness, and material parameters of magnetic layers on the base of finite-difference approximations of the energy contributions. The equilibrium magnetisation state for the given external conditions is found by minimising the total magnetic free energy of the system, which includes four standard contributions: external field, anisotropy, exchange and demagnetising energies and - for multilayers - the interlayer exchange energy (ferro-or antiferromagnetic interlayer coupling). Various examples of simulations will be presented to illustrate some important topics of the MRAM design and the abilities of the program package.
Physical Review B, 2005
We demonstrate (using full-scale micromagnetic simulations) that the spin injection driven steady... more We demonstrate (using full-scale micromagnetic simulations) that the spin injection driven steadystate precession of a thin magnetic nanoelement exhibit a complicate transition from the quasimacrospin to the chaotic behaviour with the increasing element size. For nanoelement parameters typical for those used experimentally we have found that the macrospin approximation becomes invalid already for very small nanoelement sizes (∼ 30 nm), in contrast to the previously reported results (Li and Zhang, Phys. Rev., B68, 024404-1 (2003)).
Journal of Magnetism and Magnetic Materials, 1999
ABSTRACT
Physical Review B, 1996
ABSTRACT
Physical review, Dec 7, 2021
In this paper we present an algorithm which allows single-stage direct Langevin dynamics simulati... more In this paper we present an algorithm which allows single-stage direct Langevin dynamics simulations of transitions over arbitrary high energy barriers employing the concept of the energy-dependent temperature (EDT). In our algorithm, simulation time required for the computation of the corresponding switching rate does not increase with energy barrier. This is achieved by using in simulations an effective temperature which depends on the system energy: around the energy minima this temperature is high and tends towards the room temperature when the energy approaches the saddle point value. Switching times computed via our EDT algorithm show an excellent agreement with results obtained with the established forward flux sampling (FFS) method. As the simulation time required by our method does not increase with the energy barrier, we achieve a very large speedup when compared even to the highly optimized FFS version. In addition, our method does not suffer from stability problems occurring in multi-stage algorithms (like FFS and 'energy bounce' methods) due to the multiplication of a large number of transition probabilities between the interfaces.
7th Joint MMM-Intermag Conference. Abstracts (Cat. No.98CH36275)