arXiv subjects

C. S. Davies

Publications and source records attributed to C. S. Davies.

14 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

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↗

arXiv2019

Influence of nonmagnetic dielectric spacers on the spin wave response of one-dimensional planar magnonic crystals

The one-dimensional planar magnonic crystals are usually fabricated as a sequence of stripes intentionally or accidentally separated by non-magnetic spacers. The influence of spacers on shaping the spin wave spectra is complex and still not completely clarified. We performed the detailed numerical studies of the one-dimensional single- and bi-component magnonic crystals comprised of a periodic array of thin ferromagnetic stripes separated by non-magnetic spacers. We showed that the dynamic dipolar interactions between the stripes mediated by non-magnetic spacer, even ultra-narrow, significantly shift up the frequency of the ferromagnetic resonance and simultaneously reduce the spin wave group velocity, which is manifested by the flattening of the magnonic band. We attributed these changes in the spectra to the modifications of dipolar pinning and shape anisotropy both dependent on the width of the spacers and the thickness of the stripes, as well as to the dynamical magnetic volume charges formed due to inhomogeneous spin wave amplitude.

cond-mat.mes-hall↗

arXiv2019

Supervised learning of an opto-magnetic neural network with ultrashort laser pulses

The explosive growth of data and its related energy consumption is pushing the need to develop energy-efficient brain-inspired schemes and materials for data processing and storage. Here, we demonstrate experimentally that Co/Pt films can be used as artificial synapses by manipulating their magnetization state using circularly-polarized ultrashort optical pulses at room temperature. We also show an efficient implementation of supervised perceptron learning on an opto-magnetic neural network, built from such magnetic synapses. Importantly, we demonstrate that the optimization of synaptic weights can be achieved using a global feedback mechanism, such that the learning does not rely on external storage or additional optimization schemes. These results suggest there is high potential for realizing artificial neural networks using optically-controlled magnetization in technologically relevant materials, that can learn not only fast but also energy-efficient.

cs.ET↗

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↗

arXiv2017

Broadband conversion of microwaves into propagating spin waves in patterned magnetic structures

We have used time-resolved scanning Kerr microscopy (TRSKM) and micromagnetic simulations to demonstrate that, when driven by spatially uniform microwave field, the edges of patterned magnetic samples represent both efficient and highly tunable sources of propagating spin waves. The excitation is due to the local enhancement of the resonance frequency induced by the non-uniform dynamic demagnetizing field generated by precessing magnetization aligned with the edges. Our findings represent a crucial step forward in the design of nanoscale spin-wave sources for magnonic architectures, and are also highly relevant to the understanding and interpretation of magnetization dynamics driven by spatially-uniform magnetic fields in patterned magnetic samples.

cond-mat.mtrl-sci↗

arXiv2017

Mapping the magnonic landscape in patterned magnetic structures

We report the development of a hybrid numerical / analytical model capable of mapping the spatially-varying distributions of the local ferromagnetic resonance (FMR) frequency and dynamic magnetic susceptibility in a wide class of patterned and compositionally modulated magnetic structures. Starting from the numerically simulated static micromagnetic state, the magnetization is deliberately deflected orthogonally to its equilibrium orientation, and the magnetic fields generated in response to this deflection are evaluated using micromagnetic software. This allows us to calculate the elements of the effective demagnetizing tensor, which are then used within a linear analytical formalism to map the local FMR frequency and dynamic magnetic susceptibility. To illustrate the typical results that one can obtain using this model, we analyze three micromagnetic systems boasting non-uniformity in either one or two dimensions, and successfully explain the spin-wave emission observed in each case, demonstrating the ubiquitous nature of the Schlömann excitation mechanism underpinning the observations. Finally, the developed model of local FMR frequency could be used to explain how spin waves could be confined and steered using magnetic non-uniformities of various origins, rendering it a powerful tool for the mapping of the graded magnonic index in magnonics.

cond-mat.mes-hall↗