arXiv subjects

O. Chubykalo-Fesenko

Publications and source records attributed to O. Chubykalo-Fesenko.

At least 19 recordsLinked to original sources

arXiv2025

Ultrafast Laser-Induced Magnetic Relaxation in Artificial Spin Ice Driven by Dipolar Interactions

It is of great interest to develop methods to rapidly and effectively control the magnetic configurations in artificial spin ices, which are arrangements of dipolar coupled nanomagnets that have a variety of fascinating collective magnetic phenomena associated with them. This is not only valuable in terms of acquiring fundamental understanding but is also important for future high-performance applications. Here, we demonstrate ultrafast control of magnetic relaxation in square artificial spin ice through femtosecond laser pulsed excitation, enabling rapid access to low-energy states via dipolar interactions. Time-resolved magneto-optical Kerr effect measurements reveal that, after laser-induced demagnetization, the magnetization recovers within picoseconds. During this brief transient window, dipolar coupling drives a collective magnetic ordering. Ex-situ magnetic force microscopy confirms the emergence of extended Type I vertex domains, characteristic of ground-state ordering, thus establishing ultrafast laser-driven relaxation as a route to attain the low-energy states. Through complementary energy barrier calculations and micromagnetic simulations incorporating Landau-Lifshitz-Bloch dynamics, we elucidate the underlying mechanism: transient ultrafast demagnetization followed by rapid remagnetization that enables a dipolar-driven collective rearrangement. Moreover, a tailored decreasing-fluence laser annealing protocol is shown to enhance ground-state ordering, consistently achieving over 92% ground-state vertex populations. This work opens the way to ultrafast and spatially selective control of magnetic states in artificial spin ice for spin-based computation and memory technologies, and highlights the critical interplay of thermal fluctuations, magnetostatic coupling, and transient magnetization dynamics.

cond-mat.mes-hall↗

arXiv2023

Antiferromagnetic Switching in Mn$_2$Au Using a Novel Laser Induced Optical Torque on Ultrafast Timescales

Efficient manipulation of the Néel vector in antiferromagnets can be induced by generation of spin orbit (SOT) or spin-transfer (STT) torques. Here we predict another possibility for antiferromagnetic domain switching by using a non-zero staggered field induced from optical laser excitation. We present results on the atomistic scale dynamic simulations from the application of a novel laser induced torque using optical frequencies for all-optical switching (AOS) of the Néel vector in the antiferromagnet Mn$_2$Au. The driving mechanism takes advantage of the sizeable 'exchange enhancement' characteristic of antiferromagnets, allowing for small picosecond 90 and 180 degree precessional switching with laser fluences on the order of mJ/cm$^2$. The symmetry of these novel torques are highly dependent on the time-varying magnetisation direction, creating a sign change in the torque which greatly minimises the "over-shooting problem" common to SOT and STT. Lastly, we demonstrate the opportunity for this laser optical torque to deterministically switch single magnetic domains.

cond-mat.mtrl-sci↗

arXiv2023

Tuning domain wall oscillation frequency in bent nanowires through a mechanical analogy

In this work, we present a theoretical model for domain wall (DW) oscillations in a curved magnetic nanowire with a constant curvature under the action of a uniaxial magnetic field. Our results show that the DW dynamics can be described as that of the mechanical pendulum, and both the NW curvature and the external magnetic field influence its oscillatory frequency. A comparison between our theoretical approach and experimental data in the literature shows an excellent agreement. The results presented here can be used to design devices demanding the proper control of the DW oscillatory motion in NWs.

cond-mat.mes-hall↗

arXiv2023

Giant supermagnonic Bloch point velocities by jet propulsion effect in cylindrical ferromagnetic nanowires

Achieving high velocities of magnetic domain walls is a crucial factor for their use as information carriers in modern nanoelectronic applications. In nanomagnetism and spintronics, these velocities are often limited either by internal domain wall instabilities, known as the Walker breakdown phenomenon, or by spin wave emission, known as the magnonic regime. In the rigid domain wall model, the maximum magnon velocity acts as an effective "speed of light", providing a relativistic analogy for the domain wall speed limitation. Cylindrical magnetic nanowires are an example of systems with the absence of the Walker breakdown phenomenon. Here we demonstrate that in cylindrical nanowires with high magnetization such as Iron, also the magnonic limit could be outstandingly surpassed. Our numerical modelling shows the Bloch point domain wall velocities as high as 14 km/s, well above the magnonic limit estimated in the interval 1.7-2.0 km/s. The key ingredient is the conical shape of the domain wall which elongates and breaks during the dynamics, leading to domain wall acceleration due to the jet propulsion effect. This effect will be very important for three-dimensional spintronic networks based on cylindrical magnetic nanowires.

cond-mat.mes-hall↗

arXiv2022

Tailoring Elastic Scattering of Relativistic Antiferromagnetic Domain Walls for Collision-based Computing

Soliton-based computing is relied on their unique properties for transporting energy and emerging intact from head-on collisions. Magnetic domain walls are often referred to as solitons disregarding the strict mathematical definition requiring the above scattering property. Here we demonstrate the conditions of elastic and inelastic scattering for spin-orbit torque-induced dynamics of antiferromagnetic domain walls on the example of a technologically relevant Mn$_2$Au material. We show that even domain walls with opposite winding numbers can experience elastic scattering and we present a corresponding phase diagram as a function of the spin-orbit field strength and duration. The elastic collision requires minimum domain walls speed which we explain assuming an attractive potential created by domain wall pair. On the contrary, when the domain walls move at lower speeds, their collision is inelastic and results in a dispersing breather. Our findings will be important for the development soliton-based computing using antiferromagnetic spintronics and we discuss these perspective on our suggestions of how to create NOT and XOR gates.

cond-mat.mes-hall↗

arXiv2022

Domain Wall Propagation and Pinning Induced by Current Pulses in Cylindrical Modulated Nanowires

The future developments of three-dimensional magnetic nanotechnology require the control of domain wall dynamics by means of current pulses. While this has been extensively studied in planar magnetic strips (planar nanowires), few reports exist in cylindrical geometry, where Bloch point domain walls are expected to have intriguing properties. Here we report this investigation in cylindrical magnetic Ni nanowires with geometrical notches. Experimental work based on synchrotron X-ray magnetic circular dichroism (XMCD) combined with photoemission electron microscopy (PEEM) indicates that large current densities induce domain wall nucleation while smaller currents move domain walls preferably against the current direction. In the region where no pinning centers are present we found domain wall velocity of about 1 km/s. The domain wall motion along current was also detected in the vicinity of the notch region. Pinning of domain walls has been observed not only at geometrical constrictions but also outside of them. Thermal modelling indicates that large current densities temporarily raise the temperature in the nanowire above the Curie temperature leading to nucleation of domain walls during the system cooling. Micromagnetic modelling with spin-torque effect shows that for intermediate current densities Bloch point domain walls with chirality parallel to the Oersted field propagate antiparallel to the current direction. In other cases, domain walls can be bounced from the notches and/or get pinned outside their positions. We thus find that current is not only responsible for the domain wall propagation but is also a source of pinning due to the Oersted field action.

cond-mat.mtrl-sci↗

arXiv2022

Domain wall damped harmonic oscillations induced by curvature gradients in elliptical magnetic nanowires

Understanding the domain wall (DW) dynamics in magnetic nanowires (NW) is crucial for spintronic-based applications demanding the use of DWs as information carriers. This work focuses on the dynamics of a DW displacing along a bent NW with an elliptical shape under the action of spin-polarized electric currents and external magnetic fields. Our results evidence that a curvature gradient induces an exchange-driven effective tangential field responsible for pinning a DW near the maximum curvature point in a NW. The DW equilibrium position depends on the competition between the torques produced by the external stimuli and the curvature-induced effective fields. When the external stimuli are below a certain threshold, the DW follows a damped harmonic oscillation around the equilibrium position. Above this threshold, DW displaces along the NW under an oscillatory translational motion.

cond-mat.mes-hall↗

arXiv2021

Topological energy release from collision of relativistic antiferromagnetic solitons

Magnetic solitons offer functionalities as information carriers in multiple spintronic and magnonic applications. However, their potential for nanoscale energy transport has not been revealed. Here we demonstrate that antiferromagnetic solitons, e.g. domain walls, can uptake, transport and release energy. The key for this functionality resides in their relativistic kinematics; their self-energy increases with velocity due to Lorentz contraction of the soliton and their dynamics can be accelerated up to the effective speed of light of the magnetic medium. Furthermore, their classification in robust topological classes allows to selectively release this energy back into the medium by colliding solitons with opposite topology. Our work uncovers important energy-related aspects of the physics of antiferromagnetic solitons and opens up the attractive possibility for spin-based nanoscale and ultra-fast energy transport devices.

cond-mat.mtrl-sci↗

arXiv2019

Controlling magnetisation's reversal mechanism and hyperthermia efficiency in core/shell magnetic nanoparticles by tuning the interphase coupling

Magnetic particle hyperthermia, in which colloidal nanostructures are exposed to an alternating magnetic field, is a promising approach to cancer therapy. Unfortunately, the clinical efficacy of hyperthermia has not yet been optimized. Consequently, routes to improve magnetic particle hyperthermia such as designing hybrid structures comprised from different phase materials are actively pursued. Here we demonstrate enhanced hyperthermia efficiency in relative large spherical Fe/Fe-oxide core/shell nanoparticles through the manipulation of interactions between the core and shell phases. Experimental results on exemplary samples with diameters in the range 30-80 nm indicated a direct correlation of hysteresis losses to the observed temperature elevation rate with a maximum efficiency of around 0.9 kW/g. The absolute particle size, the core/shell ratio, and the interposition of a thin wüstite interlayer, are shown to have powerful effects on the specific absorption rate. By comparing our measurements to micromagnetic calculations we have unveiled topologically non-trivial magnetisation reversal modes under which interparticle interactions become negligible, aggregates formation is minimized, and the energy that is converted into heat is increased. This information has been overlooked till date and is in stark contrast to the existing knowledge on homogeneous particles.

cond-mat.mtrl-sci↗

arXiv2019

Domain wall pinning in a circular cross-section wire with modulated diameter

Domain wall propagation in cylindrical nanowires with modulations of diameter is a key phenomenon to design physics-oriented devices, or a disruptive three-dimensional magnetic memory. This chapter presents a combination of analytical modelling and micromagnetic simulations, with the aim to present a comprehensive panorama of the physics of pinning of domain walls at modulations, when moved under the stimulus of a magnetic field or a spin-polarized current. For the sake of considering simple physics, we consider diameters of a few tens of nanometers at most, and accordingly domain walls of transverse type. Modeling with suitable approximations provides simple scaling laws, while simulations are more accurate, refining the results and defining the range of validity of the models. While pinning increases with the relative change of diameter, a key feature is the much larger efficiency of pinning at an increase of diameter upon considering current rather than field, due to the drastic decrease of current density related to the increase of diameter.

cond-mat.mes-hall↗

arXiv2019

Configurational entropy of magnetic skyrmions as an ideal gas

The study of thermodynamics of topological defects is an important challenge to understand their underlying physics. Among them, magnetic skyrmions have a leading role for their physical properties and potential applications in storage and neuromorphic computing. In this paper, the thermodynamic statistics of magnetic skyrmions is derived. It is shown that the skyrmion free energy can be modelled via a parabolic function and the diameters statistics obeys the Maxwell-Boltzmann distribution. This allows for making an analogy between the behavior of the distribution of skyrmion diameters statistics and the diluted gas Maxwell-Boltzmann molecules distribution at thermodynamical equilibrium. The calculation of the skyrmion configurational entropy, due to thermally-induced changes of size and shape of the skyrmion, is essential for the determination of thermal fluctuations of the skyrmion energy around its average value. These results can be employed to advance the field of skyrmionics.

cond-mat.mes-hall↗

arXiv2018

Micromagnetic evaluation of the dissipated heat in cylindrical magnetic nanowires

Magnetic nanowires (NW) are promising candidates for heat generation under AC-field application due to their large shape anisotropy. They may be used for catalysis, hyperthermia or water purification treatments. In the present work we theoretically evaluate the heat dissipated by a single magnetic nanowire, originated from the domain wall dynamics under the action of an AC-field. We compare the Permalloy NWs (which demagnetize via the transverse wall propagation) with the Co fcc NWs whose reversal mode is via a vortex domain wall. The average hysteresis loop areas -which are proportional to the Specific Absorption Rate (SAR)- as a function of the field frequency have a pronounced maximum in the range 200MHz-1GHz. This maximum frequency is smaller in Permalloy than in Co and depends on the nanowire length. A simple model related to the nucleation and propagation time and domain wall velocity (higher for the vortex than for the transverse domain wall) is proposed to explain the non-monotonic SAR dependence on the frequency.

cond-mat.mes-hall↗

arXiv2017

Distinct magnetic field dependence of Néel skyrmion sizes in ultrathin nanodots

We investigate the dependence of the Néel skyrmion size and stability on perpendicular magnetic field in ultrathin circular magnetic dots with out-of-plane anisotropy and interfacial Dzyaloshinskii-Moriya exchange interaction. Our results show the existence of two distinct dependencies of the skyrmion radius on the applied field and dot size. In the case of skyrmions stable at zero field, their radius strongly increases with the field applied parallel to the skyrmion core until skyrmion reaches the metastability region and this dependence slows down. More common metastable skyrmions demonstrate a weaker increase of their size as a function of the field until some critical field value at which these skyrmions drastically increase in size showing a hysteretic behavior with coexistence of small and large radius skyrmions and small energy barriers between them. The first case is also characterized by a strong dependence of the skyrmion radius on the dot diameter, while in the second case this dependence is very weak.

cond-mat.mes-hall↗

arXiv2017

Oscillatory behavior of the domain wall dynamics in a curved cylindrical magnetic nanowire

Understanding the domain wall dynamics is an important issue in modern magnetism. Here we present results of domain wall displacement in curved cylindrical nanowires at a constant magnetic field. We show that the average velocity of a transverse domain wall increases with curvature. Contrary to what it is observed in stripes, in a curved wire the transverse domain wall oscillates along and rotates around the nanowire with the same frequency. These results open the possibility of new oscillation-based applications.

cond-mat.mes-hall↗

arXiv2017

Origin of temperature and field dependence of magnetic skyrmion size in ultrathin nanodots

Understanding the physical properties of magnetic skyrmions is important for fundamental research with the aim to develop new spintronic device paradigms where both logic and memory can be integrated at the same level. Here, we show a universal model based on the micromagnetic formalism that can be used to study skyrmion stability as a function of magnetic field and temperature. We consider ultrathin, circular ferromagnetic magnetic dots. Our results show that magnetic skyrmions with a small radius compared to the dot radius are always metastable, while large radius skyrmions form a stable ground state. The change of energy profile determines the weak (strong) size dependence of the metastable (stable) skyrmion as a function of temperature and/or field. These results can open a path toward the design of optimal materials for skyrmion based devices.

cond-mat.mes-hall↗

arXiv2017

Direct Observation of Transverse and Vortex Metastable Magnetic Domains observed in Cylindrical Nanowires

We present experimental evidence of metastable magnetic domains in cylindrical CoNi nanowires. Transverse (perpendicular) domains have been previously observed only in flat nanostrips. Here they are found together with more conventional vortex domains along the same cylindrical nanowire, denoting a bistable system with similar free energies. The surface and the inner magnetization distribution in both types of domains are analyzed by Photo-Emission Electron Microscopy (PEEM) with X-ray magnetic circular dichroism (XMCD) contrast, and hysteresis loop in individual nanowires are measured by Magneto-optical Kerr effect. These experimental data are understood and compared with complementary micromagnetic simulations. The transverse domain states are potentially more important for sensing and recording applications than commonly observed vortex domains since they create larger surface stray fields.

cond-mat.mtrl-sci↗

arXiv2016

Magnetization switching of FePt nanoparticle recording medium by femtosecond laser pulses

Manipulation of magnetization with ultrashort laser pulses is promising for information storage device applications. The dynamic of the magnetization response depends on the energy transfer from the photons to the spins during the initial laser excitation. A material of special interest for magnetic storage is FePt nanoparticles , on which optical writing with optical angular momentum was demonstrated recently by Lambert et al., although the mechanism remained unclear. Here we investigate experimentally and theoretically the all-optical switching of FePt nanoparticles. We show that the magnetization switching is a stochastic process. We develop a complete multiscale model which allows us to optimize the number of laser shots needed to write the magnetization of high anisotropy FePt nanoparticles in our experiments. We conclude that only angular momentum induced optically by the inverse Faraday effect will provide switching with one single femtosecond laser pulse.

cond-mat.mes-hall↗

arXiv2015

Shape-dependent exchange bias effect in magnetic nanoparticles with core-shell morphology

We study the low-temperature isothermal magnetic hysteresis of cubical and spherical nanoparticles with ferromagnetic (FM) core - antiferromagnetic (AF) shell morphology, in order to elucidate the sensitivity of the exchange bias effect to the shape of the particles and the structural imperfections at the core-shell interface. We model the magnetic structure using a classical Heisenberg Hamiltonian with uniaxial anisotropy and simulate the hysteresis loop using the Metropolis Monte Carlo algorithm. For nanoparticles with geometrically sharp interfaces, we find that cubes exhibit higher coercivity and lower exchange bias field than spheres of the same size. With increasing interface roughness, the shape-dependence of the characteristic fields gradually decays and eventually, the distinction between cubical and spherical particles is lost for moderately rough interfaces. The sensitivity of the exchange bias field to the microstructural details of the interface is quantified by a scaling factor (b) relating the bias field to the net moment of the AF shell (Heb = bMAF + Ho). Cubical particles exhibit lower sensitivity to the dispersed values of the net interfacial moment.

cond-mat.mes-hall↗