arXiv subjects

E. M. Chudnovsky

Publications and source records attributed to E. M. Chudnovsky.

At least 19 recordsLinked to original sources

arXiv2026

Magnetization, excitations, and microwave power absorption in transition-metal/rare-earth ferrimagnets with disorder

Efficient numerical routines are developed for numerical studies of the dependence of the equilibrium magnetic states, excitations, and microwave power absorption on temperature and composition in transition-metal/rare-earth ferrites, including the reversal of the Néel vector occurring on both temperature and the concentration of the rare-earth atoms. It results in a drastic change in the behavior at the magnetization and angular-momentum compensation points. Dominant uniform oscillation modes are obtained by computing the magnetization correlation function. They are compared with the analytical solution, which is analyzed in detail. The fluctuation-dissipation theorem is used to compute the frequency dependence of the absorbed microwave power. A good agreement with analytical results is demonstrated. Disorder caused by random positions of rare-earth atoms in a diluted RE system leads to multiple localized modes that converge into broad absorption maxima as the size of the system increases. The power absorption integrated over frequency exhibits a minimum at the compensation point.

cond-mat.mtrl-sci↗

arXiv2025

Skyrmion Crystal in a Microwave Field

Temperature and field dependences of the frequencies of uniform modes of the skyrmion lattice in a 2D ferromagnetic film with Dzyaloshinskii-Moriya interaction, as well as their damping, are computed within the model of classical spins. We show that the magnetization of the film exhibits Rabi-like oscillations when subjected to the microwave field at resonance with the low-frequency mode. Melting of the skyrmion lattice by resonant microwaves is investigated in terms of the time dependence of the orientational and translational order parameters. A distinct single-stage melting transition has been observed.

cond-mat.mtrl-sci↗

arXiv2023

Integral Absorption of Microwave Power by Random-Anisotropy Magnets

We study analytically and numerically on lattices containing $10^5$ spins, the integral absorption of microwaves by a random-anisotropy magnet, $\int dωP(ω)$. It scales as $D^2_R/J$ on the random-anisotropy strength $D_R$ and the strength of the ferromagnetic exchange $J$ in low-anisotropy amorphous magnetic materials. At high anisotropy and in low-anisotropy materials sintered of sufficiently large ferromagnetic grains, the integral power scales linearly on $D_R$. The maximum bandwidth, combined with the maximum absorption power, is achieved when the amorphous structure factor, or grain size, is of an order of the domain wall thickness in a conventional ferromagnet that is of the order of $(J/D_R)^{1/2}$ lattice spacings.

cond-mat.mtrl-sci↗

arXiv2022

Localized Spin-Wave Modes and Microwave Absorption in Random-Anisotropy Ferromagnets

The theory of localized spin-wave excitations in random-anisotropy magnets has been developed. Starting with a pure Heisenberg ferromagnet, we study the evolution of standing spin waves in a finite-size sample towards localized modes on increasing the strength of random anisotropy. Profiles of the localized modes and their phases are analyzed and visualized in a 2D sample. Localization length is obtained by several methods and its dependence on random anisotropy is computed. The connection between the localization of spin excitations and the broadband nature of the absorption of microwave power by random-anisotropy magnets is elucidated.

cond-mat.dis-nn↗

arXiv2022

Dynamics of the collapse of a ferromagnetic skyrmion in a centrosymmetric lattice

Time dependence of the size and chirality of a ferromagnetic skyrmion in a Heisenberg model with the magnetic field on a square lattice has been studied analytically and numerically. The lattice and the magnetic field generate strong time dependence of the skyrmion chirality. Due to nonlinearity, the lattice alone also generates strong intrinsic damping that leads to the skyrmion collapse via the emission of spin waves. In the absence of the magnetic field the collapse is slow for a large skyrmion but it becomes exponentially fast in the presence of the Landau-Lifshitz damping when the field is turned on. Magnons emitted by a collapsing skyrmion must have a discrete spectrum due to the quantization of the skyrmion magnetic moment.

cond-mat.mes-hall↗

arXiv2021

Nonlinear and Thermal Effects in the Absorption of Microwaves by Random Magnets

Abstract We study the temperature dependence of the absorption of microwaves by random-anisotropy magnets. It is governed by strong metastability due to the broad distribution of energy barriers separating different spin configurations. At a low microwave power, when the heating is negligible, the spin dynamics is close to linear. It corresponds to the precession of ferromagnetically ordered regions that are in resonance with the microwave field. Previously we have shown (doi.org/10.1103/PhysRevB.103.214414) that in this regime a dielectric substance packed with random magnets would be a strong microwave absorber in a broad frequency range. Here we demonstrate that on increasing the power, heating and over barrier spin transitions come into play, resulting in the nonlinear behavior. At elevated temperatures the absorption of microwave power decreases dramatically, making the dielectric substance with random magnets transparent for the microwaves.

cond-mat.mtrl-sci↗

arXiv2021

Quantum States of a Skyrmion in a 2D Antiferromagnet

Quantum states of a skyrmion in a 2D antiferromagnetic lattice are obtained by quantizing the scaling parameter of Belavin-Polyakov model. Skyrmion classical collapse due to violation of the translational invariance of the continuous spin-field model by the lattice is replaced in quantum mechanics by transitions between discrete energy levels of the skyrmion. Rates of transitions due to the emission of magnons are computed. Ways of detecting quantization of skyrmion states are discussed.

cond-mat.mes-hall↗

arXiv2021

Absorption of Microwaves by Random-Anisotropy Magnets

Microscopic model of the interaction of spins with a microwave field in a random-anisotropy magnet has been developed. Numerical results show that microwave absorption occurs in a broad range of frequencies due to the distribution of Imry-Ma domains on sizes and effective anisotropy. That distribution is also responsible for the weak dependence of the absorption on the damping. At a fixed frequency of the ac-field the spatial pattern of the amplitude of spin oscillations exhibits maxima corresponding to the oscillations of resonant magnetic domains. The dependence of the peak absorption frequency on the ratio of the magnitude of per-spin random anisotropy to the strength of the ferromagnetic exchange agrees with the scaling derived from the Imry-Ma argument. The effect noticeably increases in low-dimensional systems, suggesting that materials comprised of microscopic amorphous leaves and wires can be promising candidates for enhanced microwave absorption.

cond-mat.mtrl-sci↗

arXiv2020

Skyrmion Mass from Spin-Phonon Interaction

Inertial mass of a skyrmion arising from spin-phonon interaction is computed exactly within a toy model of the magnetoelastic coupling in a ferromagnetic film. The mass scales as the square of the strength of the magnetoelastic coupling, as the square of the film thickness, and as the first power of the lateral size of the skyrmion. For nanometer skyrmions it is in the ballpark of a few electron masses but may be significantly greater in materials with large magnetostriction. These findings are expected to stand for any complex structure of spin-phonon interaction in real materials. They must be taken into account when addressing the speed of information processing based upon skyrmions.

cond-mat.mes-hall↗

arXiv2020

Skyrmion-Skyrmion Interaction in a Magnetic Film

Interaction of two skyrmions stabilized by the ferromagnetic exchange, Dzyaloshinskii-Moriya interaction (DMI), and external magnetic field has been studied numerically on a 2D lattice of size large compared to the separation, $d$, between the skyrmions. We show that two skyrmions of the same chirality (determined by the symmetry of the crystal) repel. In accordance with earlier analytical results, their long-range pair interaction falls out with the separation as $\exp(-d/δ_H)$, where $δ_H$ is the magnetic screening length, independent of the DMI. The prefactor in this expression depends on the DMI that drives the repulsion. The latter results in the spiral motion of the two skyrmions around each other, with the separation between them growing logarithmically with time. When two skyrmions of the total topological charge $Q = 2$ are pushed close to each other, the discreteness of the atomic lattice makes them collapse into one skyrmion of charge $Q = 1$ below a critical separation. Experiment is proposed that would allow one to measure the interaction between two skyrmions by holding them in positions with two magnetic tips. Our findings should be of value for designing topologically protected magnetic memory based upon skyrmions.

cond-mat.mes-hall↗

arXiv2018

Topological order generated by random field in a 2D exchange model

We study a 2D exchange model with a weak static random field on lattices containing over one hundred million spins. Ferromagnetic correlations persist on the Imry-Ma scale inversely proportional to the random-field strength and decay exponentially at greater distances. We find that the average energy of the correlated area is close to the ground-state energy of a skyrmion, while the topological charge of the area is close to $\pm 1$. Correlation function of the topological charge density exhibits oscillations with a period determined by the ferromagnetic correlation length, while its Fourier transform exhibits a maximum. These findings suggest that static randomness transforms a 2D ferromagnetic state into a skyrmion-antiskyrmion glass.

cond-mat.dis-nn↗

arXiv2017

Skyrmion Glass in a Disordered Magnetic Film

We show that high concentration of skyrmions can be achieved in magnetic films with quenched disorder. A 2D system of Heisenberg spins with feromagnetic exchange, $J$, and random magnetic anisotropy of strength $D_R \ll J$ has been studied. Analytical theory for the dependence of the average skyrmion size on the magnetic field $H$, and for the stability of pinned skyrmions, is complemented by numerical studies of 2D lattices containing up to 40 million spins. At low fields the average size of the skyrmion, $λ$, is determined by the average size of Imry-Ma domains. On increasing the field the skyrmions first shrink, with $λ\propto D_R/H$, and then collapse at fields distributed around $H_c \propto D_R^{4/3}$. Concentration of the skyrmions goes down with the field as $\exp[-(H/H_c)^{3/2}]$.

cond-mat.mes-hall↗

arXiv2017

Skyrmion Clusters From Bloch Lines in Ferromagnetic Films

Conditions under which various skyrmion objects emerge in experiments on thin magnetic films remain largely unexplained. We investigate numerically centrosymmetric spin lattices in films of finite thickness with ferromagnetic exchange, magnetic anisotropy, and dipole-dipole interaction. Evolution of labyrinth domains into compact topological structures on application of the magnetic field is found to be governed by the configuration of Bloch lines inside domain walls. Depending on the combination of Bloch lines, the magnetic domains evolve into individual skyrmions, biskyrmions, or more complex topological objects. While the geometry of such objects is sensitive to the parameters, their topological charge is uniquely determined by the topological charge of Bloch lines inside the magnetic domain from which the object emerges.

cond-mat.other↗

arXiv2017

Switching of Magnetic Moments of Nanoparticles by Surface Acoustic Waves

We report evidence of the magnetization reversal in nanoparticles by surface acoustic waves (SAWs). The experimental system consists of isolated magnetite nanoparticles dispersed on a piezoelectric substrate. Magnetic relaxation from a saturated state becomes significantly enhanced in the presence of the SAW at a constant temperature of the substrate. The dependence of the relaxation on SAW power and frequency has been investigated. The effect is explained by the effective ac magnetic field generated by the SAW in the nanoparticles.

cond-mat.mes-hall↗

arXiv2016

Ordered vs Disordered States of the Random-Field Model in Three Dimensions

We report numerical investigation of the glassy behavior of random-field exchange models in three dimensions. Correlation of energy with the magnetization for different numbers of spin components has been studied. There is a profound difference between the models with two and three spin components with respect to the stability of the magnetized state due to the different kinds of singularities: vortex loops and hedgehogs, respectively. Memory effects pertinent to such states have been investigated. Insight into the mechanism of the large-scale disordering is provided by numerically implementing the Imry-Ma argument in which the spins follow the random field averaged over correlated volumes. Thermal stability of the magnetized states is investigated by the Monte Carlo method.

cond-mat.stat-mech↗

arXiv2016

Thermal fluctuations of the Josephson current in a ring of superconducting grains

Thermal fluctuations of the Josephson current $I$ induced by the magnetic flux through a ring of $N$ superconducting grains are studied. When a half-fluxon is threading the ring, $I$ exhibits incoherent transitions between the two degenerate states due to thermal phase slips. We propose a new numerical method to deal with both equilibrium and dynamic properties of Josephson systems. Computed transition rate has the form $Γ= A(N)\exp[-B(N)/T]$, where $B(N)$ agrees with the analytical result derived for the energy barrier associated with phase slips. In the non-degenerate case (e.g., at a quarter-fluxon) the equilibrium value of $I$ decreases with $T$ due to harmonic excitations and then gets destroyed by phase slips.

cond-mat.supr-con↗

arXiv2016

Stability of Suspended Graphene under Casimir Force

We consider graphene sheet suspended above a conducting surface. Treating graphene as an elastic membrane subjected to Casimir force, we study its stability against attachment to the conductor. There exists a critical elevation at the edges below which the central part of suspended graphene nucleates a trunk that becomes attached to the conductor. The dependence of the critical elevation on temperature and dimensions of the graphene sheet is computed.

cond-mat.mes-hall↗

arXiv2016

Persistent current in a 2D Josephson junction array wrapped around a cylinder

We study persistent currents in a Josephson junction array wrapped around a cylinder. The $T=0$ quantum statistical mechanics of the array is equivalent to the statistical mechanics of a classical $xy$ spin system in 2+1 dimensions at the effective temperature $T^{*}=\sqrt{2JU}$, with $J$ being the Josephson energy of the junction and $U$ being the charging energy of the superconducting island. It is investigated analytically and numerically on lattices containing over one million sites. For weak disorder and $T^{*}\ll J$ the dependence of the persistent current on disorder and $T^{*}$ computed numerically agrees quantitatively with the analytical result derived within the spin-wave approximation. The high-$T^{*}$ and/or strong-disorder behavior is dominated by instantons corresponding to the vortex loops in 2+1 dimensions. The current becomes destroyed completely at the quantum phase transition into the Cooper-pair insulating phase.

cond-mat.supr-con↗