arXiv subjects

A. Kirilyuk

Publications and source records attributed to A. Kirilyuk.

At least 19 recordsLinked to original sources

arXiv2026

Generalized Helicity-Dependent Magnetization Switching via Substrate Phonons

Coherent excitation of circularly-polarized substrate phonons can drive deterministic reversal of magnetization in an adjacent magnetic layer. However, the extent to which this substrate-mediated mechanism can be generalized across different materials remains unresolved. Here, we measure and compare the spectral dependence of helicity-dependent magnetization reversal in magnetic GdFeCo layers grown on a diverse set of diamagnetic substrates. Switching is observed for all investigated substrates, spanning six point groups and several infrared-active transverse-optical phonon symmetries. While the switching spectra broadly overlap with the Reststrahlen bands, their maxima are systematically shifted to wavelengths shorter than the corresponding transverse-optical phonon resonances. This spectral mismatch shows that substrate absorption alone does not determine the switching efficiency. Transfer-matrix calculations show that strong absorption concentrates energy deposition near the substrate interface, providing a possible source of local heating that could suppress magnetization reversal. Our results reveal the broad applicability of substrate-mediated phononic switching while showing that its efficiency is governed by the full optical response of the heterostructure rather than substrate absorption alone.

cond-mat.mtrl-sci↗

arXiv2026

A crystal-field route to THz-driven magnetization

Light carries angular momentum, but the microscopic pathways that transform it into magnetization remain elusive. Here we establish that crystal-field excitations, historically viewed primarily as equilibrium spectroscopic fingerprints of localized 4$f$ electrons, constitute an active microscopic route through which circularly-polarized terahertz (THz) light creates magnetic polarization. Using wavelength-selective ultrafast Faraday spectroscopy on the paramagnetic insulator CeF$_3$, we show that resonant excitation of localized 4$f$ crystal-field transitions generates a helicity-dependent magnetization that survives for up to about 100 ps. Most strikingly, while the optical helicity is held fixed, the THz-driven response reverses sign as the excitation wavelength is tuned across the crystal-field resonance. The resulting dispersive spectral response follows the crystal-field excitation spectrum rather than that of optical phonons, and is captured by resonant electronic theory of the inverse Faraday effect. Our results identify crystal-field excitations as a previously unrecognized dynamical reservoir for optical angular momentum and broaden the microscopic pathways through which THz light can create and manipulate magnetic states.

cond-mat.mtrl-sci↗

arXiv2025

Quality of Helicity-Dependent Magnetization Switching by Phonons

Optical control of magnetization has emerged as a promising approach to achieve ultrafast and energy-efficient magnetization reversal. Here, we investigate helicity-dependent switching of magnetization driven by the resonant excitation of circularly-polarized transverse-optical phonons, using a polarization-modulated transient grating. Our results show that the polarized phonons within the sample substrate induce robust, helicity-defined magnetization reversal in the magnetic overlayer. Moreover, the quality of switching remains largely unaffected when the degree of ellipticity of the infrared excitation is varied at frequencies resonant with the targeted phonon modes. Conversely, as the excitation is moved slightly off-resonance, switching quality becomes highly sensitive to the ellipticity of the incident light.

cond-mat.mes-hall↗

arXiv2023

Phonon-induced magnetization dynamics in Co-doped iron garnets

The developing field of strain-induced magnetization dynamics offers a promising path toward efficiently controlling spins and phase transitions. Understanding the underlying mechanisms is crucial in finding the optimal parameters supporting the phononic switching of magnetization. Here, we present an experimental and numerical study of time-resolved magnetization dynamics driven by the resonant excitation of an optical phonon mode in iron garnets. Upon pumping the latter with an infrared pulse obtained from a free-electron laser, we observe spatially-varying magnetization precession, with its phase depending on the direction of an external magnetic field. Our micromagnetic simulations effectively describe the magnetization precession and switching in terms of laser-induced changes in the crystal's magneto-elastic energy.

cond-mat.mtrl-sci↗

arXiv2023

Phononic Switching of Magnetization by the Ultrafast Barnett Effect

The Barnett effect, discovered more than a century ago, describes how an inertial body with otherwise zero net magnetic moment acquires spontaneous magnetization when mechanically spinning. Breakthrough experiments have recently shown that an ultrashort laser pulse destroys the magnetization of an ordered ferromagnet within hundreds of femtoseconds, with the spins losing angular momentum to circularly-polarized optical phonons as part of the ultrafast Einstein-de Haas effect. However, the prospect of using such high-frequency vibrations of the lattice to reciprocally switch magnetization in a nearby magnetic medium has not yet been experimentally explored. Here we show that the spontaneous magnetization temporarily gained via the ultrafast Barnett effect, through the resonant excitation of circularly-polarized optical phonons in paramagnetic substrates, can be used to permanently reverse the magnetic state of the substrate-mounted heterostructure. With the handedness of the phonons steering the direction of magnetic switching, the ultrafast Barnett effect offers a selective and potentially universal method for exercising ultrafast non-local control over magnetic order.

cond-mat.mes-hall↗

arXiv2022

Dynamic self-organisation and pattern formation by magnon-polarons

Nonlinear dynamics can give rise, via the processes of self-organisation and pattern formation, to the spontaneous manifestation of order in open and complex systems far from equilibrium. Self-organising systems, transforming the inflow of energy into information, are ubiquitously found in current topical areas of science ranging from brainwave entrainment and neuromorphic computing to energy-efficient data storage technologies. In the latter, magnetic materials play a pivotal role combining very fast switching with permanent retention of information. However, it has been shown that, at very short time scales, magnetisation dynamics become chaotic due to internal instabilities, resulting in incoherent spin-wave excitations that ultimately destroy magnetic ordering. Here, contrary to all expectations, we show that such chaos gives rise to a periodic pattern of reversed magnetic domains, with a feature size far smaller than the spatial extent of the excitation. We explain this pattern as a result of phase-synchronisation of magnon-polaron waves, driven by strong coupling of magnetic and elastic modes. Our results reveal not only the peculiar formation and evolution of magnon-polarons at short time-scales, but also present a novel mechanism of magnetization reversal driven by coherent packets of short-wavelength quasiparticles.

cond-mat.mtrl-sci↗

arXiv2022

Helicity-independent all-optical switching of magnetization in ferrimagnetic alloys

We review and discuss the process of single-shot helicity-independent all-optical switching of magnetization by which a single suitably-ultrafast excitation, under the right conditions, toggles magnetization from one stable state to another. For almost a decade, this phenomenon was only consistently observed in specific rare-earth-transition-metal ferrimagnetic alloys of GdFeCo, but breakthrough experiments in recent years have revealed that the same behavior can be achieved in a wide range of multi-sublattice magnets including TbCo alloys doped with minute amounts of Gd, Gd/Co and Tb/Co synthetic ferrimagnets, and the rare-earth-free Heusler alloy Mn$_2$Ru$_x$Ga. Aiming to resolve the conditions that allow switching, a series of experiments have shown that the process in the ferrimagnetic alloys GdFeCo and Mn$_2$Ru$_x$Ga is highly sensitive to the pulse duration, starting temperature and the alloy composition. We argue here that the switching displayed by these two very different ferrimagnetic alloys can be generally understood within a single phenomenological framework describing the flow of angular momentum between the constituent sublattices and from the sublattices to the environment. The conditions that facilitate switching stem from the properties of these channels of angular momentum flow in combination with the size of the angular momentum reservoirs. We conclude with providing an outlook in this vibrant research field, with emphasis on the outstanding open questions pertaining to the underlying physics along with noting the advances in exploiting this switching process in technological applications.

cond-mat.mtrl-sci↗

arXiv2022

Cavity-Dumping a Single Infrared Pulse from a Free-Electron Laser for Two-Color Pump-Probe Experiments

Electromagnetic radiation in the mid- to far-infrared spectral range represents an indispensable tool for the study of numerous types of collective excitations in solids and molecules. Short and intense pulses in this THz spectral range are, however, difficult to obtain. While wide wavelength-tunability is easily provided by free-electron lasers, the energies of individual pulses are relatively moderate, on the order of microjoules. Here we demonstrate a setup that uses cavity-dumping of a free-electron laser to provide single, picosecond-long pulses in the mid- to far-infrared frequency range. The duration of the Fourier-limited pulses can be varied by cavity detuning, and their energy was shown to exceed 100 μJ. Using the aforementioned infrared pulse as a pump, we have realized a two-color pump-probe setup facilitating single-shot time-resolved imaging of magnetization dynamics. We demonstrate the capabilities of the setup first on thermally-induced demagnetization and magnetic switching of a GdFeCo thin film and second by showing a single-shot time-resolved detection of resonant phononic switching of the magnetization in a magnetic garnet.

cond-mat.mes-hall↗

arXiv2020

Electrically tunable detector of THz-frequency signals based on an antiferromagnet

A concept of an electrically tunable resonance detector of THz-frequency signals based on antiferromagnetic/heavy metal (AFM/HM) hetero-structure is proposed. The conversion of a THz-frequency input signal into DC voltage is done using the inverse spin Hall effect in an (AFM/HM) bilayer. An additional bias DC current in the HM layer can be used to vary the effective anisotropy of the AFM, and, therefore, to tune the AFMR frequency. The proposed AFM/HM hetero-structure works as a resonance-type quadratic detector which can be tuned by the bias current in the range of at least 10 percent of the AFMR frequency and our estimations show that the sensitivity of this detector could be comparable to that of modern detectors based on the Schottky, Gunn or graphene-based diodes.

cond-mat.str-el↗

arXiv2020

Ultrafast phononic switching of magnetization

Identifying an efficient pathway to change the order parameter via a subtle excitation of the coupled high-frequency mode is the ultimate goal of the field of ultrafast phase transitions. This is an especially interesting research direction in magnetism, where the coupling between spin and lattice excitations is required for magnetization reversal. Despite several attempts however, the switching between magnetic states via resonant pumping of phonon modes has not yet been demonstrated. Here we show how an ultrafast resonant excitation of the longitudinal optical phonon modes in magnetic garnet films switches magnetization into a peculiar quadrupolar magnetic domain pattern, unambiguously revealing the magneto-elastic mechanism of the switching. In contrast, the excitation of strongly absorbing transverse phonon modes results in thermal demagnetization effect only.

cond-mat.mtrl-sci↗

arXiv2020

Sub-picosecond exchange-relaxation in the compensated ferrimagnet Mn$_2$Ru$_x$Ga

We study the demagnetization dynamics of the fully compensated half-metallic ferrimagnet Mn$_2$Ru$_x$Ga. While the two antiferromagnetically coupled sublattices are both composed of manganese, they exhibit different temperature dependencies due to their differing local environments. The sublattice magnetization dynamics triggered by femtosecond laser pulses are studied to reveal the roles played by the spin and intersublattice exchange. We find a two-step demagnetization process, similar to the well-established case of Gd(FeCo)$_3$, where the two Mn-sublattices have different demagnetization rates. The behaviour is analysed using a four-temperature model, assigning different temperatures to the two manganese spin baths. Even in this strongly exchange-coupled system, the two spin reservoirs have considerably different behaviour. The half-metallic nature and strong exchange coupling of Mn$_2$Ru$_x$Ga lead to spin angular momentum conservation at much shorter time scales than found for Gd(FeCo)$_3$ which suggests that low-power, sub-picosecond switching of the net moment of Mn$_2$Ru$_x$Ga is possible.

cond-mat.mtrl-sci↗

arXiv2019

Magnetization dynamics of the compensated ferrimagnet $Mn_{2}Ru_{x}Ga$

Here we study both static and time-resolved dynamic magnetic properties of the compensated ferrimagnet from room temperature down to 10K, thus crossing the magnetic compensation temperature $T_{M}$. The behaviour is analysed with a model of a simple collinear ferrimagnet with uniaxial anisotropy and site-specific gyromagnetic ratios. We find a maximum zero-applied-field resonance frequency of $\sim$160GHz and a low intrinsic Gilbert damping $α$$\sim$0.02, making it a very attractive candidate for various spintronic applications.

cond-mat.mtrl-sci↗

arXiv2019

Blueprint for deterministic all-optical switching of magnetization

We resolve a significant controversy about how to understand and engineer single-shot all-optical switching of magnetization in ferrimagnets using femto- or picosecond-long heat pulses. By realistically modelling a generic ferrimagnet as two coupled macrospins, we comprehensively show that the net magnetization can be reversed via different pathways, using a heat pulse with duration spanning all relevant timescales within the non-adiabatic limit. This conceptual understanding is fully validated by experiments studying the material and optical limits at which the switching process in GdFeCo alloys loses its reliability. Our interpretation and results constitute a blueprint for understanding how deterministic all-optical switching can be achieved in alternative ferrimagnets using short thermal pulses.

cond-mat.mes-hall↗

arXiv2019

High Field Anomalies of Equilibrium and Ultrafast Magnetism in Rare-Earth-Transition Metal Ferrimagnets

Magneto-optical spectroscopy in fields up to 30 Tesla reveals anomalies in the equilibrium and ultrafast magnetic properties of the ferrimagnetic rare-earth-transition metal alloy TbFeCo. In particular, in the vicinity of the magnetization compensation temperature, each of the magnetizations of the antiferromagnetically coupled Tb and FeCo sublattices show triple hysteresis loops. Contrary to state-of-the-art theory, which explains such loops by sample inhomogeneities, here we show that they are an intrinsic property of the rare-earth ferrimagnets. Assuming that the rare-earth ions are paramagnetic and have a non-zero orbital momentum in the ground state and, therefore, a large magnetic anisotropy, we are able to reproduce the experimentally observed behavior in equilibrium. The same theory is also able to describe the experimentally observed critical slowdown of the spin dynamics in the vicinity of the magnetization compensation temperature, emphasizing the role played by the orbital momentum in static and ultrafast magnetism of ferrimagnets.

cond-mat.mtrl-sci↗

arXiv2018

Spin-current-mediated rapid magnon localisation and coalescence after ultrafast optical pumping of ferrimagnetic alloys

Sub-picosecond magnetisation manipulation via femtosecond optical pumping has attracted wide attention ever since its original discovery in 1996. However, the spatial evolution of the magnetisation is not yet well understood, in part due to the difficulty in experimentally probing such rapid dynamics. Here, we find evidence of rapid magnetic order recovery in materials with perpendicular magnetic anisotropy via nonlinear magnon processes. We identify both localisation and coalescence regimes, whereby localised magnetic textures nucleate and subsequently evolve in accordance with a power law formalism. Coalescence is observed for optical excitations both above and below the switching threshold. Simulations indicate that the ultrafast generation of noncollinear magnetisation via optical pumping establishes exchange-mediated spin currents with an equivalent 100% spin polarised charge current density of $10^8$ A/cm$^2$. Such large spin currents precipitate rapid recovery of magnetic order after optical pumping. These processes suggest an ultrafast optical route for the stabilization of desired meta-stable states, e.g., isolated skyrmions.

cond-mat.mes-hall↗

arXiv2018

Selection Rules for All-Optical Magnetic Recording in Iron Garnet

Finding an electronic transition a subtle excitation of which can launch dramatic changes of electric, optical or magnetic properties of media is one of the long-standing dreams in the field of photo-induced phase transitions [1-5]. Therefore the discovery of the magnetization switching only by a femtosecond laser pulse [6-10] triggered intense discussions about mechanisms responsible for these laser-induced changes. Here we report the experimentally revealed selection rules on polarization and wavelengths of ultrafast photo-magnetic recording in Co-doped garnet film and identify the workspace of the parameters (magnetic damping, wavelength and polarization of light) allowing this effect. The all-optical magnetic switching under both single pulse and multiple-pulse sequences can be achieved at room temperature, in narrow spectral ranges with light polarized either along <110> or <100> crystallographic axes of the garnet. The revealed selection rules indicate that the excitations responsible for the coupling of light to spins are d-electron transitions in octahedral and tetrahedral Co-sublattices, respectively.

cond-mat.mtrl-sci↗

arXiv2018

Kramers degeneracy and relaxation in vanadium, niobium and tantalum clusters

In this work we use magnetic deflection of V, Nb, and Ta atomic clusters to measure their magnetic moments. While only a few of the clusters show weak magnetism, all odd-numbered clusters deflect due to the presence of a single unpaired electron. Surprisingly, for majority of V and Nb clusters an atomic-like behavior is found, which is a direct indication of the absence of spin-lattice interaction. This is in agreement with Kramers degeneracy theorem for systems with a half-integer spin. This purely quantum phenomenon is surprisingly observed for large systems of more than 20 atoms, and also indicates various quantum relaxation processes, via Raman two-phonon and Orbach high-spin mechanisms. In heavier, Ta clusters, the relaxation is always present, probably due to larger masses and thus lower phonon energies, as well as increased spin-orbit coupling.

physics.atm-clus↗

arXiv2017

Magnetic properties of Co doped Nb clusters

From magnetic deflection experiments on isolated Co doped Nb clusters we made the interesting observation of some clusters being magnetic, while others appear to be non-magnetic. There are in principle two explanations for this behavior. Either the local moment at the Co site is completely quenched or it is screened by the delocalized electrons of the cluster, i.e. the Kondo effect. In order to reveal the physical origin, we conducted a combined theoretical and experimental investigation. First, we established the ground state geometry of the clusters by comparing the experimental vibrational spectra with those obtained from a density functional theory study. Then, we performed an analyses based on the Anderson impurity model. It appears that the non-magnetic clusters are due to a complete quenching of the local Co moment and not due to the Kondo effect. In addition, the magnetic behavior of the clusters can be understood from an inspection of their electronic structure. Here magnetism is favored when the effective hybridization around the chemical potential is small, while the absence of magnetism is signalled by a large effective hybridization around the chemical potential.

cond-mat.str-el↗