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Dmitry Berkov

Publications and source records attributed to Dmitry Berkov.

16 recordsLinked to original sources
arXiv2023

Micromagnetics of ferromagnetic/antiferromagnetic nanocomposite materials. Part II: Mesoscopic modeling

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.

cond-mat.mtrl-sci↗
arXiv2023

Micromagnetics of ferromagnetic/antiferromagnetic nanocomposite materials. Part I: Towards the mesoscopic approach

In the first of two articles, we present here a novel mesoscopic micromagnetic approach for simulating materials composed of ferromagnetic and antiferromagnetic phases. Starting with the atomistic modeling of quasi one-dimensional systems, we explicitly show how the material parameters for the mesoscopic model of an antiferromagnet can be derived. The comparison between magnetization profiles obtained in atomistic and mesoscopic calculations (using a Heusler alloy as an example) proves the validity of our method. This approach opens up the possibility to recover the details of the magnetization distribution in ferromagnetic/antiferromagnetic materials with the resolution of a few nanometers covering length scales up to several hundreds of nanometers.

cond-mat.mtrl-sci↗
arXiv2023

Remanence Increase in SrFe$_{12}$O$_{19}$/Fe Exchange-Decoupled Hard-Soft Composite Magnets Owing to Dipolar Interactions

In the search for improved permanent magnets, fueled by the geostrategic and environmental issues associated with rare-earth-based magnets, magnetically hard (high anisotropy)-soft (high magnetization) composite magnets hold promise as alternative magnets that could replace modern permanent magnets, such as rare-earth-based and ceramic magnets, in certain applications. However, so far, the magnetic properties reported for hard-soft composites have been underwhelming. Here, an attempt to further understand the correlation between magnetic and microstructural properties in strontium ferrite-based composites, hard SrFe$_{12}$O$_{19}$ (SFO) ceramics with different contents of Fe particles as soft phase, both in powder and in dense injection molded magnets, is presented. In addition, the influence of soft phase particle dimension, in the nano- and micron-sized regimes, on these properties is studied. While Fe and SFO are not exchange-coupled in our magnets, a remanence that is higher than expected is measured. In fact, in composite injection molded anisotropic (magnetically oriented) magnets, remanence is improved by 2.4% with respect to a pure ferrite identical magnet. The analysis of the experimental results in combination with micromagnetic simulations allows us to establish that the type of interaction between hard and soft phases is of a dipolar nature, and is responsible for the alignment of a fraction of the soft spins with the magnetization of the hard. The mechanism unraveled in this work has implications for the development of novel hard-soft permanent magnets.

cond-mat.mtrl-sci↗
arXiv2022

Using small-angle scattering to guide functional magnetic nanoparticle design

Magnetic nanoparticles offer unique potential for various technological, biomedical, or environmental applications thanks to the size-, shape- and material-dependent tunability of their magnetic properties. To optimize particles for a specific application, it is crucial to interrelate their performance with their structural and magnetic properties. This review presents the advantages of small-angle X-ray and neutron scattering techniques for achieving a detailed multiscale characterization of magnetic nanoparticles and their ensembles in a mesoscopic size range from 1 to a few hundred nanometers with nanometer resolution. Both X-rays and neutrons allow the ensemble-averaged determination of structural properties, such as particle morphology or particle arrangement in multilayers and 3D assemblies. Additionally, the magnetic scattering contributions enable retrieving the internal magnetization profile of the nanoparticles as well as the inter-particle moment correlations caused by interactions within dense assemblies. Most measurements are used to determine the time-averaged ensemble properties, in addition advanced small-angle scattering techniques exist that allow accessing particle and spin dynamics on various timescales. In this review, we focus on conventional small-angle X-ray and neutron scattering (SAXS and SANS), X-ray and neutron reflectometry, gracing-incidence SAXS and SANS, X-ray resonant magnetic scattering, and neutron spin-echo spectroscopy techniques. For each technique, we provide a general overview, present the latest scientific results, and discuss its strengths as well as sample requirements. Finally, we give our perspectives on how future small-angle scattering experiments, especially in combination with micromagnetic simulations, could help to optimize the performance of magnetic nanoparticles for specific applications.

cond-mat.mtrl-sci↗
arXiv2021

Single-stage direct Langevin dynamic simulations of transitions over arbitrary high energy barriers: Concept of the energy-dependent temperature

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.

cond-mat.stat-mech↗
arXiv2020

Mechanical orientation of fine magnetic particles in powders by an external magnetic field: simulation-based optimization

We present a numerical algorithm for predicting the optimal conditions for the effective alignment of magnetic particles in dense powders during the compactization process using an externally applied field. This task is especially important for the permanent magnets development due to the fact that alignment of anisotropy axes of nanocomposite grains increases both remanence and coercivity of magnetic materials. In contrast to previously known methods where magnetic moment of each particle was assumed to be 'fixed' with respect to the particle itself, our approach takes into account the (field-dependent) deviation of this moment from the particle anisotropy axis that occurs even for magnetically 'hard' particles possessing a strong mechanical contact. We show, that this deviation leads to the existence of the optimal value of the applied field for which the particle orientation (or alignment) time is minimal. The influence of the external pressure and internal mechanical friction on the details of the compactization/orientation process is also studied.

cond-mat.mtrl-sci↗
arXiv2020

Resolving complex spin textures in nanoparticles by magnetic neutron scattering

In the quest to image the three-dimensional magnetization structure we show that the technique of magnetic small-angle neutron scattering (SANS) is highly sensitive to the details of the internal spin structure of nanoparticles. By combining SANS with numerical micromagnetic computations we study the transition from single-domain to multi-domain behavior in nanoparticles and its implications for the ensuing magnetic SANS cross section. Above the critical single-domain size we find that the cross section and the related correlation function cannot be described anymore with the uniform particle model, resulting e.g. in deviations from the well-known Guinier law. We identify a clear signature for the occurrence of a vortex-like spin structure at remanence. The micromagnetic approach to magnetic SANS bears great potential for future investigations, since it provides fundamental insights into the mesoscale magnetization profile of nanoparticles.

cond-mat.mes-hall↗
arXiv2019

Injection locking at fractional frequencies of magnetic tunnel junction (MTJ)-based read sensors' ferromagnetic resonance modes

Being nonlinear dynamic systems, magnetic read sensors should respond to an excitation signal of a frequency considerably different from their natural ferromagnetic resonance (FMR) frequencies. Because of the magnetization dynamics' inherent nonlinear nature, the sensors' response should be measured at the DC, excitation frequency, and its multiples (harmonics). In this paper, we present results of such measurements, accomplished using a one-port nonlinear vector network analyzer (NVNA), which show distinct resonances at fractional frequencies of the free layer (FL) FMR mode. Identification of these resonances, resulting from the nonlinear nature of the spin-torque (ST)-induced magnetization dynamics, was performed using micromagnetic modeling. In particular, we show that the measured DC response at the above-mentioned fractional frequencies can be explained by a low-order nonlinearity and strong magnetodipolar feedback between magnetic layers adjacent to an MgO barrier. Additionally, we determined that the simulated harmonic response is strongly enhanced by the mutual ST effect between these layers. Finally, we demonstrate that the read sensors' nonlinear magnetization dynamics and, by extension, their harmonic response are highly sensitive to various magnetic and ST parameters. Thus, this study shows that using NVNA measurements in conjunction with micromagnetic modeling can clarify the uncertainty in the definition of these parameters.

physics.app-ph↗
arXiv2019

Size-dependent spatial magnetization profile of manganese-zinc ferrite Mn0.2Zn0.2Fe2.6O4 nanoparticles

We report the results of an unpolarized small-angle neutron scattering (SANS) study on Mn-Zn ferrite (MZFO) magnetic nanoparticles with the aim to elucidate the interplay between their particle size and the magnetization configuration. We study different samples of single-crystalline MZFO nanoparticles with average diameters ranging between 8 to 80 nm, and demonstrate that the smallest particles are homogeneously magnetized. However, with increasing nanoparticle size, we observe the transition from a uniform to a nonuniform magnetization state. Field-dependent results for the correlation function confirm that the internal spin disorder is suppressed with increasing field strength. The experimental SANS data are supported by the results of micromagnetic simulations, which confirm an increasing inhomogeneity of the magnetization profile of the nanoparticle with increasing size. The results presented demonstrate the unique ability of SANS to detect even very small deviations of the magnetization state from the homogeneous one.

cond-mat.mes-hall↗
arXiv2017

Towards understanding of magnetization reversal in Nd$-$Fe$-$B nanocomposites: Analysis by high-throughput micromagnetic simulations

We demonstrate how micromagnetic simulations can be employed in order to characterize and analyze the magnetic microstructure of nanocomposites. For the example of nanocrystalline Nd$-$Fe$-$B, which is a potential material for future permanent-magnet applications, we have compared three different models for the micromagnetic analysis of this material class: (i) a description of the nanocomposite microstructure in terms of Stoner-Wohlfarth particles with and without the magnetodipolar interaction; (ii) a model based on the core-shell representation of the nanograins; (iii) the latter model including a contribution of superparamagnetic clusters. The relevant parameter spaces have been systematically scanned with the aim to establish which micromagnetic approach can most adequately describe experimental data for this material. According to our results, only the last, most sophisticated model is able to provide an excellent agreement with the measured hysteresis loop. The presented methodology is generally applicable to multiphase magnetic nanocomposites and it highligths the complex interrelationship between the microstructure, magnetic interactions, and the macroscopic magnetic properties.

cond-mat.mtrl-sci↗
arXiv2016

Optimization of nanocomposite materials for permanent magnets by micromagnetic simulations: effect of the intergrain exchange and the hard grains shape

In this paper we perform the detailed numerical analysis of remagnetization processes in nanocomposite magnetic materials consisting of magnetically hard grains (i.e. grains made of a material with a high magnetocrystalline anisotropy) embedded into a magnetically soft phase. Such materials are widely used for the production of permanent magnets, because they combine the high remanence with the large coercivity. We perform simulations of nanocomposites with Sr-ferrite as the hard phase and Fe or Ni as the soft phase, concentrating our efforts on analyzing the effects of ({\it i}) the imperfect intergrain exchange and ({\it ii}) the non-spherical shape of hard grains. We demonstrate that - in contrast to the common belief - the maximal energy product is achieved not for systems with the perfect intergrain exchange, but for materials where this exchange is substantially weakened. We also show that the main parameters of the hysteresis loop - remanence, coercivity and the energy product - exhibit non-trivial dependencies on the shape of hard grains, and provide detailed explanations for our results. Simulation predictions obtained in this work open new ways for the optimization of materials for permanent magnets.

cond-mat.mtrl-sci↗
arXiv2015

Dipolar spin-misalignment correlations in inhomogeneous magnets: comparison between neutron scattering and micromagnetic approaches

In inhomogeneous bulk ferromagnets, the dominating sources of spin disorder are related to spatial variations of (i) the magnitude of the local saturation magnetization and of (ii) the magnitude and/or direction of the magnetic anisotropy field. For the particular example of a porous ferromagnet, where the magnetization inhomogeneity is at maximum, we demonstrate, by means of experimental neutron scattering data and micromagnetic simulations, the anisotropic character of magnetization fluctuations induced by the dipolar interaction.

cond-mat.mes-hall↗
arXiv2014

Robust synchronization of an arbitrary number of spin-torque driven vortex nanooscillators

Non-linear magnetization dynamics in ferromagnetic nanoelements excited by the spin-polarized dc-current is one of the most intensively studied phenomena in solid state magnetism. Despite immense efforts, synchronization of oscillations induced in several such nanoelements (spin-torque driven nanooscillators, or STNO) still represents a major challenge both from the fundamental and technological points of view. In this paper we propose a system where synchronization of any number of STNOs, represented by magnetization vortices inside squared nanoelements, can be easily achieved. Using full-scale micromagnetic simulations we show that synchronization of these STNOs is extremely dynamically stable due to their very large coupling energy provided by the magnetodipolar interaction. Finally, we demonstrate that our concept allows robust synchronization of an arbitrary number of STNOs (arranged either as a 1D chain or as a 2D array), even when current supplying nanocontacts have a broad size distribution.

cond-mat.mes-hall↗
arXiv2012

Micromagnetic simulation of magnetic small-angle neutron scattering from two-phase nanocomposites

The recent development of a micromagnetic simulation methodology - suitable for multiphase magnetic nanocomposites - permits the computation of the magnetic microstructure and of the associated magnetic small-angle neutron scattering (SANS) cross section of these materials. In this review article we summarize results on the micromagnetic simulation of magnetic SANS from two-phase nanocomposites. The decisive advantage of this approach resides in the possibility to srutinize the individual magnetization Fourier contributions to the total magnetic SANS cross section, rather than their sum, which is generally obtained from experiment. The procedure furnishes unique and fundamental information regarding magnetic neutron scattering from nanomagnets.

cond-mat.mes-hall↗
arXiv2011

Magnetic nanocomposites: new methodology for micromagnetic modeling and SANS experiments

A new methodology for micromagnetic simulations of magnetic nanocomposites is presented. The methodology is especially suitable for simulations of two-phase composites consisting of magnetically hard inclusions in a soft magnetic matrix phase. The proposed technique allows to avoid the unnecessary discretization of the 'hard' inclusions (these are normally in a single-domain state), but enables an arbitrary fine discretization of the 'soft' phase. The method is applied to the determination of the equilibrium magnetization state of an iron-based nanocomposite from the Nanoperm (FeZrBCu) family of alloys and to the calculation of the corresponding small-angle neutron scattering (SANS) cross-section. For this highly interesting material, the results of our simulations exhibit a remarkable agreement with the nontrivial 'clover-leaf' SANS cross-sections observed experimentally (A. Michels et al., Phys. Rev. B, v. 74, 134407 (2006)).

cond-mat.mes-hall↗