arXiv subjects

A. K. Zvezdin

Publications and source records attributed to A. K. Zvezdin.

At least 19 recordsLinked to original sources

arXiv2026

Degeneracy and trajectory control of spin eigenmodes excited by fs-optical pulses in a nearly compensated ferrimagnet

We investigate optically excited spin dynamics in a uniaxial ferrimagnet near the magnetization compensation point under a magnetic field applied along the magnetic anisotropy axis. Experiment and numerical modeling reveal an unusual regime where the frequencies of two spin eigenmodes approach each other and become highly field sensitive. The modes, corresponding to opposite rotations of the Neel vector, simultaneously reverse their handedness at a critical field where their frequencies become degenerate. At this point, the two-frequency precessional dynamics collapses into a linear oscillations directed along the inverse-Faraday-effect excitation induced by a single pump pulse. We further show that a double-pulse excitation scheme enables control of the spin trajectory. These results uncover an unconventional dynamical regime in ferrimagnets and establish new opportunities for manipulating spin motion in magnonic systems and devices.

cond-mat.mtrl-sci↗

arXiv2026

The prospects of nonthermal magnetization switching in near-compensated rare earth iron garnets

Ultrafast spin dynamics in a magnetically compensated rare earth iron garnet film driven by femtosecond optical pulses through the inverse Faraday effect is theoretically investigated. Numerical simulations based on the equations of motion for the Néel vector reveal the temporal evolution of the system and its trajectories in the effective potential landscape tuned by external field and temperature. The results demonstrate a clear threshold behavior: weak pulses induce only oscillations around the initial equilibrium state, while a stronger excitation results in a deterministic magnetization switching. The switching threshold is determined by the magnetic state of the sample on its phase diagramme as well as on the laser pulse helicity. This mechanism demonstrates a non-thermal and even non-absorptive pathway towards optomagnonic logic and memory devices.

cond-mat.mtrl-sci↗

arXiv2026

Terahertz spin-orbit torque as a drive of spin dynamics in the insulating antiferromagnet Cr$_{2}$O$_{3}$

Contrary to conventional wisdom that spin dynamics induced by current are exclusive to metallic magnets, we theoretically predict that such phenomena can also be realized in magnetic insulators, specifically in the magnetoelectric antiferromagnet $\mathrm{Cr}_{2}\mathrm{O}_{3}$. We reveal that the displacement current driven by the THz electric field is able to generate a N{é}el spin-orbit torque in this insulating system. By introducing an alternative electric dipole order parameter arising from the dipole moment at $\mathrm{Cr}^{3+}$ sites, we combine symmetry analysis with a Lagrangian approach and uncover that the displacement current couples to the antiferromagnetic spins and enables ultrafast control of antiferromagnetic order. The derived equations of motion show that this effect competes with the linear magnetoelectric response, offering a novel pathway for manipulating antiferromagnetic order in insulators. Our findings establish insulator antiferromagnets as a viable platform for electric field driven antiferromagnetic spintronics and provide general design principles for non-metallic spin-orbit torque materials.

cond-mat.mtrl-sci↗

arXiv2026

Josephson-like magnetic tunnel junction -- transition from classical to quantum regime

We theoretically propose and analyze a Josephson-like magnetic tunnel junction (MTJ) structure that exhibits quantum spin dynamics analogous to those in superconducting Josephson junctions. By exploiting the isomorphism between the equations of motion for low-dissipation MTJs with easy-plane anisotropy and the Josephson phase dynamics, we construct a theoretical framework for realizing spintronic qubits. Within this framework, we identify the physical parameters -- such as anisotropy constants, Gilbert damping, spin current amplitude, and geometric factors -- that govern the transition from classical to quantum behavior. We show that different types of spintronic qubits, including analogs of charge, flux, and transmon superconducting qubits, can be implemented depending on the hierarchy of energy scales. A Hamiltonian formalism is developed for each regime, enabling an analytical treatment of the two-level quantum dynamics and estimation of coherence times. In particular, we demonstrate that the spin current can be used not only to excite but also to stabilize the qubit states through dissipation control. These findings provide a route toward integrating spintronic qubits into CMOS-compatible architectures and lay the groundwork for a fully spintronic platform for quantum computation.

cond-mat.mes-hall↗

arXiv2025

Competition between terahertz magnetoelectric and Néel spin-orbit torque driven spin dynamics in metallic antiferromagnets

Although magnetoelectric effects in metals are usually neglected, assuming that applied electric fields are screened by free charge carriers, the skin depth, defining the penetration depth of the fields, is non-zero and for THz electric fields typically reaches 400 nm. Hence, if the thickness of an antiferromagnetic film is of the order of tens of nm, electric field induced effects cannot be neglected. Here, we theoretically study the THz electric field induced spin dynamics in the metallic antiferromagnet $\mathrm{Mn}_{2}\mathrm{Au}$, whose spin arrangements allow it to exhibit a linear magnetoelectric effect. We show that the THz magnetoelectric torque in metallic antiferromagnets is proportional to the time derivative of the polarization induced by the THz electric field. Our simulations reveal that the magnetoelectric driven spin dynamics is indeed not negligible, and for a fair explanation of previously published experimental results in $\mathrm{Mn}_{2}\mathrm{Au}$ competition between THz magnetoelectric and Néel spin-orbit torques must be taken into account. Thus, it is shown that even in metallic antiferromagnets the THz magnetoelectric effect on spins can be strong and thus cannot be neglected.

cond-mat.mtrl-sci↗

arXiv2025

THz electric field control of spins in collinear antiferromagnet Cr$_{2}$O$_{3}$

The idea to find a magnet that responds to an electric field as efficiently as to its magnetic counterpart has long intrigued people's minds and recently became a cornerstone for future energy efficient and nano-scalable technologies for magnetic writing and information processing. In contrast to electric currents, a control by electric fields promises much lower dissipations and in contrast to magnetic fields, electric fields are easier to apply to a nanoscale bit. Recently, the idea to find materials and mechanisms facilitating a strong and simultaneously fast response of spins to electric field has fueled an intense research interest to electromagnons in non-collinear antiferromagnets. Here we show that THz spin resonance at the frequency 0.165 THz in collinear antiferromagnet Cr$_{2}$O$_{3}$, which does not host any electromagnons, can be excited by both THz magnetic and electric fields. The mechanisms result in comparable effects on spin dynamics, when excited by freely propagating electromagnetic wave, but have different dependencies on the orientation of the applied THz electric field and the antiferromagnetic Néel vector. Hence this discovery opens up new chapters in the research areas targeting to reveal novel principles for the fastest and energy efficient information processing - ultrafast magnetism, antiferromagnetic spintronics, and THz magnonics.

cond-mat.mtrl-sci↗

arXiv2023

Micromagnetic textures in exchange coupled-ferromagnetic multiferroic films

The development of new computing technologies has given a new stimulus in the study of multiferroics. The use of multiferroics allows the realization of competitive energy efficient scalable logic and storage devices. The low-power consumption in Magneto Electric-Spin Orbital logics and Magnetic Random Access Memory components is provided by magnetoelectric switching in multiferroic based systems using a low-energy electric field. Our work concerns the modelling of the Magneto Electric-Spin Orbital elements with an emphasis on the magnetoelectric component and simulation of magnetization reversal processes in a model system. The use of the proposed approach makes it possible to analyze the influence of dimensional factors (film thicknesses, transverse dimensions, sample shape) affecting the magnetic states of multiferroic nanoelements; taking into interfacial interactions (magnetic anisotropy and interlayer exchange); energy-efficient external influences that allow switching magnetic states using magnetic and electric fields.

cond-mat.mtrl-sci↗

arXiv2023

Ultrafast Emergence of Ferromagnetism in Antiferromagnetic FeRh in High Magnetic Fields

Ultrafast heating of FeRh by a femtosecond laser pulse launches a magneto-structural phase transition from an antiferromagnetic to a ferromagnetic state. Aiming to reveal the ultrafast kinetics of this transition, we studied magnetization dynamics with the help of the magneto-optical Kerr effect in a broad range of temperatures (from 4 K to 400 K) and magnetic fields (up to 25 T). Three different types of ultrafast magnetization dynamics were observed and, using a numerically calculated H-T phase diagram, the differences were explained by different initial states of FeRh corresponding to a (i) collinear antiferromagnetic, (ii) canted antiferromagnetic and (iii) ferromagnetic alignment of spins. We argue that ultrafast heating of FeRh in the canted antiferromagnetic phase launches practically the fastest possible emergence of magnetization in this material. The magnetization emerges on a time scale of 2 ps, which corresponds to the earlier reported time-scale of the structural changes during the phase transition.

cond-mat.mtrl-sci↗

arXiv2023

Effective rectification of THz electromagnetic fields in a ferrimagnetic iron garnet

It is found that single-cycle THz electromagnetic fields efficiently excite a GHz spin resonance mode in ferrimagnetic Tm$_3$Fe$_5$O$_{12}$, despite the near absence of GHz spectral components in the exciting THz pulse. By analyzing how the efficiency of excitation depends on the orientation and strength of the THz electric field, we show that it can be explained in terms of the nonlinear THz inverse Cotton-Mouton effect. Here, the THz electric field gets effectively rectified and acts on the ferrimagnetic spins as a uni-polar effective magnetic field pulse. This interpretation is confirmed by a theoretical model based on the phenomenological analysis of the effective magnetic field, combined with the equations of motion derived from the effective Lagrangian for a ferrimagnet. Moreover, by using the outcome of two-dimensional THz spectroscopy, we conjecture a quantum-mechanical interpretation of the observed effect in terms of stimulated Raman scattering of THz photons by the crystal-field split f-f electronic transitions of Tm$^{3+}$.

cond-mat.mtrl-sci↗

arXiv2023

Unconventional spin dynamics in the non-collinear phase of a ferrimagnet

Ferrimagnets containing several partially compensated magnetic sublattices are considered the most promising materials for all-optical data storage and for ultrafast communications based on spin waves. There are two magnetic phases of the ferrimagnets: collinear and non-collinear ones. Up to now spin dynamics in ferrimagnets has been studied mostly in the collinear state without paying much attention to the kind of the magnetic phase. Here we investigate laser induced ultrafast spin dynamics in a rare-earth iron garnet film in the noncollinear phase as well. We identify a crucial influence of the magnetic phase on the excited spin modes which allowed us to discover several prominent effects previously overlooked. In particular, the non-collinearity makes the quasi-antiferromagnetic mode sensitive to the external magnetic field and brings its frequency close to the frequency of the quasiferromagnetic mode. The latter maximizes near the magnetization compensation point and vanishes towards the collinear phase. Spectacularly, at the phase transition the quasiferromagnetic mode becomes soft and its amplitude significantly increases reaching 7°. This opens new opportunities for the ultrafast control of spins in ferrimagnets for nonthermal data storage and data processing.

cond-mat.mtrl-sci↗

arXiv2022

Two-dimensional THz spectroscopy of nonlinear phononics in the topological insulator $\mathrm{MnBi}_2\mathrm{Te}_4$

The interaction of a single-cycle THz electric field with the topological insulator $\mathrm{MnBi}_2\mathrm{Te}_4$ triggers strongly anharmonic lattice dynamics, promoting fully coherent energy transfer between the otherwise non-interacting Raman-active $E_g$ and infrared (IR)-active $E_u$ phononic modes. Two-dimensional (2D) THz spectroscopy combined with modeling based on the classical equations of motion and symmetry analysis reveals the multi-stage process underlying the excitation of the Raman-active $E_g$ phonon. In this process, the THz electric field first prepares a coherent IR-active $E_u$ phononic state and subsequently interacts with this state to efficiently excite the $E_g$ phonon.

cond-mat.mtrl-sci↗

arXiv2022

Phase transitions in rare-earth ferrimagnets with surface anisotropy near the magnetization compensation point

We report of a theoretical model for calculating the H-T phase diagrams of a rare-earth ferrimagnet, taking into account anisotropies originated by both magnetization sublattices' and by the surface. The possibility of an exchange spring formation due to surface anisotropy is considered. This situation is realized in heterostructures containing a ferrimagnet and a heavy metal. We derive the stability lose lines of the collinear phase from the free energy of the two sublattice ferrimagnet. We numerical calculate the magnetic phase diagrams for the cases when the magnetic field applied along and perpendecular to the easy axis. We demonstrate that tricritical point down at the low field range due to surface anisotropy effect. Moreover, the line of the first order phase transition between angular and collinear phases reduces due to surface anisotropy. In the case when magnetic field is applied perpendicular to the easy axis we show the possibility of the first order phase transition between two collinear phases in contrast to the phase diagram without surface anisotropy.

physics.app-ph↗

arXiv2022

Accumulation and control of spin waves in magnonic dielectric microresonators by a comb of ultrashort laser pulses

Spin waves in magnetic microresonators are at the core of modern magnonics. Here we demonstrate a new method of tunable excitation of different spin wave modes in magnetic microdisks by using a train of laser pulses coming at a repetition rate higher than the decay rate of spin precession. The microdisks are etched in a transparent bismuth iron garnet film and the light pulses influence the spins nonthermally through the inverse Faraday effect. The high repetition rate of the laser stimulus of 10 GHz establishes an interplay between the spin wave resonances in the frequency and momentum domains. As a result, scanning of the focused laser spot near the disk boarder changes interference pattern of the magnons and leads to a resonant dependence of the spin wave amplitude on the external magnetic field. Apart from that, we achieved a switching between volume and surface spin waves by a small variation of the external magnetic field.

physics.optics↗

arXiv2021

Ultrafast kinetics of the antiferromagnetic-ferromagnetic phase transition in FeRh

Understanding how fast short-range interactions build up long-range order is one of the most intriguing topics in condensed matter physics. FeRh is a test specimen for studying this problem in magnetism, where the microscopic spin-spin exchange interaction is ultimately responsible for either ferro- or antiferromagnetic macroscopic order. Femtosecond laser excitation can induce ferromagnetism in antiferromagnetic FeRh, but the mechanism and dynamics of this transition are topics of intense debates. Employing double-pump THz emission spectroscopy has enabled us to dramatically increase the temporal detection window of THz emission probes of transient states without sacrificing any loss of resolution or sensitivity. It allows us to study the kinetics of emergent ferromagnetism from the femtosecond up to the nanosecond timescales in FeRh/Pt bilayers. Our results strongly suggest a latency period between the initial pump-excitation and the emission of THz radiation by ferromagnetic nuclei.

cond-mat.mtrl-sci↗

arXiv2021

Optical excitation and probing of the antiferromagnetic modes with non-uniform in-depth distribution in birefringent antiferromagnetic crystals

Optical pump-probe setups are commonly used for excitation and investigation of the spin dynamics in various types of magnetic materials. However, usually the spatially homogeneous excitation is considered. In the present study we describe an approach for optical excitation of the nonuniform THz spin dynamics and for probing its spatial distribution inside a magnetic crystal. We propose to illuminate a crystal with laser pulses of properly adjusted polarization to benefit from a strong optical birefringence inherent to the crystal. It results in an unusual behavior of the effective magnetic field generated by the pulses due to the inverse Faraday effect and the peculiar sign-changing dependence of the direct Faraday effect inside the crystal. The study is performed exemplary for yttrium orthoferrite crystal although the proposed approach is applicable for various magnetic materials with optical anisotropy.

physics.optics↗

arXiv2021

Sample-dependent Dirac point gap in MnBi$_2$Te$_4$ and its response to the applied surface charge: a combined photoemission and ab initio study

Recently discovered intrinsic antiferromagnetic topological insulator MnBi$_2$Te$_4$ presents an exciting platform for realization of the quantum anomalous Hall effect and a number of related phenomena at elevated temperatures. An important characteristic making this material attractive for applications is its predicted large magnetic gap at the Dirac point (DP). However, while the early experimental measurements reported on large DP gaps, a number of recent studies claimed to observe a gapless dispersion of the MnBi$_2$Te$_4$ Dirac cone. Here, using micro($μ$)-laser angle-resolved photoemission spectroscopy, we study the electronic structure of 15 different MnBi$_2$Te$_4$ samples, grown by two different chemists groups. Based on the careful energy distribution curves analysis, the DP gaps between 15 and 65 meV are observed, as measured below the Néel temperature at about 10-16 K. At that, roughly half of the studied samples show the DP gap of about 30 meV, while for a quarter of the samples the gaps are in the 50 to 60 meV range. Summarizing the results of both our and other groups, in the currently available MnBi$_2$Te$_4$ samples the DP gap can acquire an arbitrary value between a few and several tens of meV. Further, based on the density functional theory, we discuss a possible factor that might contribute to the reduction of the DP gap size, which is the excess surface charge that can appear due to various defects in surface region. We demonstrate that the DP gap is influenced by the applied surface charge and even can be closed, which can be taken advantage of to tune the MnBi$_2$Te$_4$ DP gap size.

cond-mat.mtrl-sci↗

arXiv2021

One-dimensional optomagnonic microcavities for selective excitation of perpendicular standing spin waves

Here we propose a method of the excitation of perpendicular standing spin waves (PSSWs) of different orders in an optomagnonic microcavity by ultrashort laser pulses. The microcavity is formed by a magnetic dielectric film surrounded by dielectric non-magnetic Bragg mirrors. Optical cavity modes in the magnetic layer provide concentration and strongly non-uniform distribution of the optical power over the layer thickness and therefore induce the effective field of the inverse Faraday effect also spatially non-uniform. It results in excitation of PSSWs. PSSWs whose wavevector is closest to the wavevector characterizing distribution of the inverse Faraday effect field are excited most efficiently. Consequently, a key advantage of this approach is a selectivity of the PSSW excitation which allows to launch PSSWs of required orders only. All-optical operation of the optomagnonic cavities opens new possibilities for their applications for quantum technologies.

physics.optics↗

arXiv2021

Domain wall dynamics of ferrimagnets induced by spin-current near the angular momentum compensation temperature

We report on a theoretical study of the spin-current excited dynamics of domain walls (DWs) in ferrimagnets in the vicinity of the angular momentum compensation point. Effective Lagrangian and nonlinear dynamic equations are derived for a two-sublattice ferrimagnet taking into account both spin-torques and external magnetic field. The dynamics of the DW before and after the Walker breakdown is calculated for any direction of the spin current polarization. It is shown that for the in-plane polarization of the spin current, the DW mobility reaches a maximum near the temperature of the angular momentum compensation. For the out-of-plane spin polarization, in contrast, a spin current with the densities below the Walker breakdown does not excite the dynamics of the DW. After overcoming the Walker breakdown, the domain wall velocity increases linearly with increasing the current density. In this spin-current polarization configuration the possibility of a gigahertz oscillation dynamics of the quasi-antiferromagnetic vector under the action of a damping-like torque in the angular momentum compensation point is demonstrated. Possible structures for experimental demonstration of the considered effects are discussed.

physics.app-ph↗