Ultrafast Demagnetization Processes Research Articles

Ultrafast Demagnetization Through Femtosecond Generation of Non‐Thermal Magnons

AbstractUltrafast laser excitation of ferromagnetic metals gives rise to correlated, highly non‐equilibrium dynamics of electrons, spins and lattice, which are, however, poorly described by the widely‐used three‐temperature model. Here, a fully ab initio parameterized out‐of‐equilibrium theory based on a quantum kinetic approach – termed (N+2) temperature model – is developed to describe magnon occupation dynamics due to electron‐magnon scattering. This model is applied to perform quantitative simulations on the ultrafast, laser‐induced generation of magnons in iron and demonstrates that on these timescales the magnon distribution is non‐thermal: predominantly high‐energy magnons are created, while the magnon occupation close to the center of the Brillouin zone even decreases, due to a repopulation toward higher energy states via a so‐far‐overlooked scattering term. It is demonstrated that the simple relation between magnetization and temperature computed at equilibrium does not hold in the ultrafast regime. The ensuing Gilbert damping, furthermore, becomes strongly magnon wavevector dependent and requires a description beyond the conventional Landau‐Lifshitz‐Gilbert spin dynamics. The ab initio parameterized calculations show that ultrafast generation of non‐thermal magnons provides a sizable demagnetization within 200 fs in excellent comparison with experimentally observed laser‐induced demagnetizations. This investigation thus emphasizes the importance of non‐thermal magnon excitations for the ultrafast demagnetization process.

Element-resolved evidence of superdiffusive spin current arising from ultrafast demagnetization process

Element-resolved evidence of superdiffusive spin current arising from ultrafast demagnetization process

Fluence and Temperature Dependences of Laser-Induced Ultrafast Demagnetization and Recovery Dynamics in L10-FePt Thin Film.

The fundamental mechanisms of ultrafast demagnetization and magnetization recovery processes in ferromagnetic materials remain incompletely understood. The investigation of different dynamic features which depend on various physical quantities requires a more systematic approach. Here, the femtosecond laser-induced demagnetization and recovery dynamics in L10-Fe0.5Pt0.5 alloy film are studied by utilizing time-resolved magneto-optical Kerr measurements, focusing on their dependences of excitation fluence and ambient temperature over broad ranges. Ultrafast demagnetization dominated by Elliott-Yafet spin-flip scattering, and two-step magnetization recovery processes are found to be involved in all observations. The fast recovery time corresponding to spin-lattice relaxation is much shorter than that of many ferromagnets and increase with excitation fluence. These can be ascribed to the strong spin-orbit coupling (SOC) demonstrated in FePt and the reduction of transient magnetic anisotropy, respectively. Surprisingly, the demagnetization time exhibits no discernible correlation with ambient temperature. Two competitive factors are proposed to account for this phenomenon. On the other hand, the spin-lattice relaxation accelerates as temperature decreases due to enhanced SOC at lower ambient temperature. A semiquantitative analysis is given to get a visualized understanding. These results offer a comprehensive understanding of the dynamic characteristics of ultrafast demagnetization and recovery processes in iron-based materials with strong SOC, highlighting the potential for regulating the magnetization recovery process through temperature and laser fluence adjustments.

Ultrafast demagnetization dynamics in the epitaxial FeGe(111) film chiral magnet

The exploring of ultrafast demagnetization is a significant subject for understanding the versatility of spin dynamics. However, the laser-induced ultrafast demagnetization process in chiral magnet FeGe has remained elusive. Here, we report the ultrafast demagnetization processes of FeGe film with various magnetic structures using time-resolved magneto-optical Kerr effect (TRMOKE) measurements. Our work reveals a transition from one-step ultrafast demagnetization (type I) to two-step demagnetization (type II) process by changing ambient temperature, laser fluence, and magnetic field. We confirm that the transition from one-step to two-step ultrafast demagnetization mainly results from the weak electron-phonon (e-p) coupling. In the skyrmion phase region, TRMOKE curves demonstrate a two-step ultrafast demagnetization process. The two-step ultrafast demagnetization will further enhance the magnetization loss, and consequently facilitate the magnetization switching and pave a pathway to engineer chiral magnetic devices on picosecond timescales.

Electric field tuning of ultrafast demagnetization in a magnetoelectric heterostructure

Voltage/electric-field control of ultrafast magnetization dynamics in magnetoelectrics provides a novel avenue for electronic tuning with orders of magnitude less power consumption, improved tuning response time, and more compact form factor, compared with conventional magnetic field control of magnetization dynamics. Magnetoelectrically tuned laser-driven magnetization dynamics has the potential to enable the next generation of optomagnetic devices from THz communication to optical magnetic recording. In this study, we fabricated a magnetoelectric heterostructure, specifically ferromagnetic ${({\mathrm{Fe}}_{81}{\mathrm{Ga}}_{19})}_{88}{\mathrm{B}}_{12}$ (FeGaB) thin film deposited on a $\mathrm{Pb}({\mathrm{Mg}}_{1/3}\phantom{\rule{0.16em}{0ex}}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_{3}\text{\ensuremath{-}}\mathrm{PbTi}{\mathrm{O}}_{3}$ ferroelectric substrate, to explore the electric field tuning of ultrafast demagnetization and to understand how magnetic anisotropy changes postdemagnetization. We characterized the magnetoelectric coupling of our heterostructure to demonstrate the static and dynamic magnetization tunability with an applied electric field. We utilized the time-resolved magneto-optic Kerr effect to understand the ultrafast demagnetization process under different applied electric fields and magnetic fields. The typically observed strain-induced magnetic easy-axis rotation in a ferromagnetic/ferroelectric heterostructure was also observed to tune the ultrafast demagnetization of FeGaB in our experiment. Additionally, we found that the magnetization rotation can be achieved with a lower electric field compared with static tuning without laser heating. Furthermore, we observed the hysteresis loops postultrafast demagnetization and found that the magnetoelectric heterostructure exhibits a mixture of volatile and nonvolatile behaviors. These findings shed light on the potential of our magnetoelectric heterostructure for ultrafast optomagnetic devices and electric field tuning of spintronic THz emitters.

Analysis and control of ultrafast demagnetization dynamics in ferrimagnetic Gdx(CoFe)1-x alloys

Deep understanding and powerful control of the magnetization dynamics is of great importance for developing high speed spintronic devices. In this work, laser-induced ultrafast demagnetization process of Gdx(CoFe)1-x alloy films has been extensively studied by using the fs-laser pump-probe technique. Interestingly, it is found that by adjusting the Gd content x and hence the intersublattice 3d-4f exchange coupling strength, the demagnetization dynamics varies from a fast one-stage decay to a continuous or a discrete two-stage decay, which is mainly resulted from the different energy transfer rates via the intersublattice exchange coupling and has been well simulated by considering the Elliott-Yafet type spin-flip scattering theory and the 4-Temperature model. Moreover, we observed that the second-stage decay time is strongly dependent on the external magnetic field, which is analyzed theoretically and suggests an origin of ultrafast giant magnetic cooling effect via the modulation of lattice-spin coupling. These studies are quite helpful for achieving the efficient manipulation of ultrafast magnetization behaviors.

Role of Spin–Orbit Coupling on Ultrafast Spin Dynamics in Nonmagnet/Ferromagnet Heterostructures

AbstractSpin–orbit coupling (SOC), the interaction between spin and orbital angular momentum of electrons, is imperative to control magnetic properties of nonmagnet (NM)/ferromagnet (FM) heterostructures and design energy‐efficient and faster spin‐based devices. Here, femtosecond pulsed laser‐induced time‐resolved magneto‐optical Kerr effect magnetometry is employed to investigate magnetization dynamics in different NM/Co20Fe60B20 heterostructures, where the NM layer varies as Cu, Ta, W, Pt, Ta/Ru/Ta, and Si/SiO2 (no underlayer) that differ in SOC strength. It is observed that there is a systematic variation in ultrafast demagnetization time (τm), fast remagnetization time (τr), and Gilbert damping parameter (α) with the SOC strength of the underlayer and an inverse relationship between α and τm, τr is established due to the dominant contribution of spin current transport in ultrafast demagnetization and fast remagnetization processes. The spin pumping formalism estimates the effective spin‐mixing conductance (Geff) for different interfaces, which signifies that the high SOC strength of underlayers results in high Geff indicating more efficient transport of spin current through it. This study suggests that the SOC strength of the NM underlayer plays a significant role in controlling the ultrafast demagnetization process through interfacial spin current transport in a NM/FM heterostructure which can be beneficial for the development of ultrafast spintronics devices.

Experimental observation of transition from type I to type II ultrafast demagnetization dynamics in chemically disordered Fe60Al40 thin film, driven by laser fluence

Understanding of the laser-induced ultrafast demagnetization dynamics is one of the most challenging and hot topics in magnetism research due to its potential applications in magnetic storage devices and the field of spintronics. Recently, a laser-induced switching of ferromagnetism, driven by a disorder–order transition on FeAl thin films, has been experimentally demonstrated. The switching of ferromagnetic ordering by ultrafast laser pulses in FeAl thin films may open new possible applications of this material such as magnetic data storage and manipulation. Since the speed of the magnetic switching of magnetic states in thin films is one of the critical parameters for these applications, here we used time resolved magneto-optical Kerr measurements to investigate the demagnetization dynamics of Fe60Al40 thin films at room temperature. We have for the first time observed a clear transition from one-step dynamics (type I) to two-step (type II) dynamics in the same material by increasing pump laser fluence. This experimental observation may give a strong confirmation that the ultrafast demagnetization process can be treated as a thermal process and is driven by the difference between temperatures of the electron and spin systems.

Ultrafast dynamics of 4f electron spins in TbFeCo film driven by inter-atomic 3d–5d–4f exchange coupling

The ultrafast demagnetization dynamics of 3d and 4f spins, respectively, in FeCo and Tb of TbFeCo alloy film are studied independently by employing a dual-color time-resolved magneto-optical Kerr spectroscopy. The demagnetization dynamics of 3d and 4f spins are independently probed, respectively, by 800 and 400 nm light. Two-step demagnetization dynamics are observed for both the 3d and 4f spins under the excitation of 800 nm laser. In particular, the onset of 4f spin dynamics presents a delayed time with respect to the one of 3d spin dynamics. Those results clearly reveal a strong inter-atomic 3d–5d–4f exchange coupling which drives the first-step subpicosecond ultrafast demagnetization process of 4f spins, and a spin(4f)-lattice coupling which drives the second-step slower demagnetization process of 4f spins. A numerical calculation based on four temperature model reproduces the coupling characteristics in the demagnetization dynamics, and reveals the energy evolution dynamics among the different subsystems. These results provide a direct demonstration of strong coupling dynamics between the two spin subsystems in rare earth-transition metal alloy occurring within subpicosecond timescale, and show a new approach for ultrafast control of 4f spins via an indirect excitation.

Revealing angular momentum transfer channels and timescales in the ultrafast demagnetization process of ferromagnetic semiconductors

Ultrafast control of magnetic order by light provides a promising realization for spintronic devices beyond Moore's Law and has stimulated intense research interest in recent years. Yet, despite 2 decades of debates, the key question of how the spin angular momentum flows on the femtosecond timescale remains open. The lack of direct first-principle methods and pictures for such process exacerbates the issue. Here, we unravel the laser-induced demagnetization mechanism of ferromagnetic semiconductor GaMnAs, using an efficient time-dependent density functional theory approach that enables the direct real-time snapshot of the demagnetization process. Our results show a clear spin-transfer trajectory from the localized Mn-d electrons to itinerant carriers within 20 fs, illustrating the dominant role of [Formula: see text] interaction. We find that the total spin of localized electrons and itinerant carriers is not conserved in the presence of spin-orbit coupling (SOC). Immediately after laser excitation, a growing percentage of spin-angular momentum is quickly transferred to the electron orbital via SOC in about 1 ps, then slowly to the lattice via electron-phonon coupling in a few picoseconds, responsible for the 2-stage process observed experimentally. The spin-relaxation time via SOC is about 300 fs for itinerant carriers and about 700 fs for Mn-d electrons. These results provide a quantum-mechanical microscopic picture for the long-standing questions regarding the channels and timescales of spin transfer, as well as the roles of different interactions underlying the GaMnAs demagnetization process.

Tuning the laser-induced ultrafast demagnetization of transition metals

The ultrafast demagnetization (UFD) dynamics of itinerant ferromagnets is theoretically investigated as a function of the characteristics of the initial laser excitation. A many-body pd-band Hamiltonian is considered which takes into account hybridizations, Coulomb interactions, spin-orbit interactions and the coupling to the laser field on the same electronic level. In this way, a fruitful connection is established between the non-adiabatic quantum dynamics and the well-known equilibrium statistical mechanics of itinerant-electron ferromagnets. The time evolution during and after the pulse absorption is determined exactly by performing numerical Lanczos propagations on a small cluster model with parameters appropriate for Ni. The most relevant laser parameters, namely, the fluence, wave length, polarization and pulse duration are varied systematically. The results show how they allow one to control the total absorbed energy, the spectral distribution of the initial excitation, and the subsequent magnetization dynamics. The calculations show that reasonable changes in these parameters do not affect the UFD dynamics qualitatively and have only a minor influence on the time scale which characterizes the initial demagnetization. In contrast, our model predicts that the degree of demagnetization correlates well with the average number of electrons excited by the laser or average number of absorbed photons. The theoretical results are discussed by comparing them with available experiments. From a fundamental perspective, the robustness of the ultrafast demagnetization effect is theoretically demonstrated, as a phenomenon reflecting the intrinsic dynamics of the metallic 3d valence electrons. A wide variety of well-focused possibilities of tailoring the efficacy of the ultrafast demagnetization process is thereby opened.

Relativistic theory of two-photon photoemission from ferromagnetic materials

A fully relativistic theory of angle-resolved two-photon photoemission for ferromagnetic materials is derived from the general theory of pump-probe photoemission that is based on the Keldysh formalism. Two-photon photoemission spectroscopy is a widely used analytical tool to study nonequilibrium phenomena in nonmagnetic as well as in magnetic solids, as for example ultrafast demagnetization processes. Our theoretical approach aims at a quantitative description of the time-dependent spectroscopic properties of specific solid systems under consideration, and it allows for the inclusion of static correlation effects via the local spin-density and dynamical mean-field theory electronic structure approach. To this end we follow Pendry's one-step theory of the photoemission process as closely as possible and make extensive use of concepts of relativistic multiple - scattering theory within the layer-Korringa-Kohn-Rostoker method in order to guarantee angular resolution in the spectroscopic calculations. As usual the final state of the two-photon photoemission process is represented by a time-reversed low-energy electron diffraction state. The formalism allows for a quantitative calculation of the time-dependent photocurrent for moderately correlated systems like simple ferromagnetic metals. A first application to the Fe(100) surface is discussed in detail.

Controlled coexcitation of direct and indirect ultrafast demagnetization in Co/Pd multilayers with large perpendicular magnetic anisotropy

Ever since its discovery in 1996, ultrafast demagnetization has ignited immense research interest due to its scientific rigor and technological potential. A flurry of recent theoretical and experimental investigations has proposed direct and indirect excitation processes in separate systems. However, it still lacks a unified mechanism and remains highly debatable. Here, we demonstrate that instead of either direct or indirect interaction, simultaneous and controlled excitation of both direct and indirect mechanisms of demagnetization is possible in multilayers composed of repeated Co/Pd bilayers. Moreover, we are able to modulate demagnetization time (from \ensuremath{\sim}350 to \ensuremath{\sim}750 fs) by fluence and thickness-dependent indirect excitation due to heat current flowing vertically downward from top layers, which is combined with an altogether different scenario of direct irradiation. Finally, by regulating the pump wavelength, we can effectively control the contribution of indirect process, which gives a confirmation to our understanding of the ultrafast demagnetization process.

Role of the heat effect and field effect in single femto-second laser pulse induced helicity-dependent all-optical magnetic switching

In the present work, we have investigated the role of helicity dependent heating (magnetic circular dichroism) and the effective field arising from the inverse Faraday effect in single femto-second laser pulse induced all-optical magnetic switching. We simulate the ultrafast dynamics of ferrimagnetic TbFe film based on the Landau-Lifshitz-Bloch (LLB) equation of motion, and especially, we use the heat pulse combined with the magnetic field pulse to simulate the effect of circularly polarized laser pulses. The results show that the mechanism of single-pulse helicity-dependent all-optical magnetic switching should be the combined results of heat and field effects. Furthermore, the duration of the field pulse should be longer than that of the heat pulse to enable to achieve the switching. To explain it, we separate the same duration of heat pulse and field pulse for different time intervals to distinguish the role played by laser heating and field action in the switching. The results demonstrate that no switching can be obtained, even at high pump fluence when there is no delay or a short delay of the field pulse to the heat pulse, because the field does not have a sufficiently strong effective action on the magnetization just after the excitation of heat pulse, and the sample is still in the ultrafast demagnetization process.

A multiscale model of the effect of Ir thickness on the static and dynamic properties of Fe/Ir/Fe films

The complex magnetic properties of Fe/Ir/Fe sandwiches are studied using a hierarchical multi-scale model. The approach uses first principles calculations and thermodynamic models to reveal the equilibrium spinwave, magnetization and dynamic demagnetisation properties. Finite temperature calculations show a complex spinwave dispersion and an initially counter-intuitive, increasing exchange stiffness with temperature (a key quantity for device applications) due to the effects of frustration at the interface, which then decreases due to magnon softening. Finally, the demagnetisation process in these structures is shown to be much slower at the interface as compared with the bulk, a key insight to interpret ultrafast laser-induced demagnetization processes in layered or interface materials.

Model of ultrafast demagnetization driven by spin-orbit coupling in a photoexcited antiferromagnetic insulator Cr2O3.

We theoretically study the dynamic time evolution following laser pulse pumping in an antiferromagnetic insulator Cr2O3. From the photoexcited high-spin quartet states to the long-lived low-spin doublet states, the ultrafast demagnetization processes are investigated by solving the dissipative Schrödinger equation. We find that the demagnetization times are of the order of hundreds of femtoseconds, in good agreement with recent experiments. The switching times could be strongly reduced by properly tuning the energy gaps between the multiplet energy levels of Cr3+. Furthermore, the relaxation times also depend on the hybridization of atomic orbitals in the first photoexcited state. Our results suggest that the selective manipulation of the electronic structure by engineering stress-strain or chemical substitution allows effective control of the magnetic state switching in photoexcited insulating transition-metal oxides.

Structural dynamics during laser-induced ultrafast demagnetization

The mechanism underlying femtosecond laser pulse induced ultrafast magnetization dynamics remains elusive despite two decades of intense research on this phenomenon. Most experiments focused so far on characterizing magnetization and charge carrier dynamics, while first direct measurements of structural dynamics during ultrafast demagnetization were reported only very recently. We here present our investigation of the infrared laser pulse induced ultrafast demagnetization process in a thin Ni film, which characterizes simultaneously magnetization and structural dynamics. This is achieved by employing femtosecond time resolved X-ray resonant magnetic reflectivity (tr-XRMR) as probe technique. The experimental results reveal unambiguously that the sub-picosecond magnetization quenching is accompanied by strong changes in non-magnetic X-ray reflectivity. These changes vary with reflection angle and changes up to 30$\%$ have been observed. Modeling the X-ray reflectivity of the investigated thin film, we can reproduce these changes by a variation of the apparent Ni layer thickness of up to 1$\%$. Extending these simulations to larger incidence angles we show that tr-XRMR can be employed to discriminate experimentally between currently discussed models describing the ultrafast demagnetization phenomenon.

Laser-induced ultrafast spin current pulses: a thermodynamic approach

The ultrafast demagnetization process allows for the generation of femtosecond spin current pulses. Here, we present a thermodynamic model of the spin current generation process, based on the chemical potential gradients as the driving force for the spin current. We demonstrate that the laser-induced spin current can be estimated by an easy to understand diffusion model.

Role of the Heat Sink Layer Ta for Ultrafast Spin Dynamic Process in Amorphous TbFeCo Thin Films

The ultrafast demagnetization processes (UDP) in Ta ([Formula: see text] nm)/TbFeCo (20 nm) films have been studied using the time-resolved magneto-optical Kerr effect (TRMOKE). With a fixed pump fluence of 2 mJ/cm2, for the sample without a Ta underlayer ([Formula: see text][Formula: see text]nm), we observed the UDP showing a two-step decay behavior, with a relatively longer decay time ([Formula: see text] around 3.0 ps in the second step due to the equilibrium of spin-lattice relaxation following the 4[Formula: see text] occupation. As a 10[Formula: see text]nm Ta layer is deposited, the two-step demagnetization still exists while [Formula: see text] decreases to [Formula: see text]1.9[Formula: see text]ps. Nevertheless, the second-step decay ([Formula: see text]) disappears as the Ta layer thickness is increased up to 20 nm, only the first-step UDP occurs within 500 fs, followed by a fast recovery process. The rapid magnetization recovery rate strongly depends on the pump fluence. We infer that the Ta layer provides conduction electrons involving the thermal equilibrium of spin-lattice interaction and serves as heat bath taking away energy from spins of TbFeCo alloy film in UDP.

Study of terahertz emission from nickel (Ni) films of different thicknesses using ultrafast laser pulses

The nickel (Ni) films of three different thicknesses, 65 nm, 135 nm and 1018 nm, were deposited by the RF magnetron sputtering technique. These films were subjected to femtosecond laser pulses of ~140 fs duration at 80 MHz repetition rate for generation of terahertz (THz) radiation. The obtained electromagnetic radiation was detected using photoconductive antennas by a sampling technique. The THz generation mechanism of Ni films is attributed to the ultrafast demagnetization process and its time depends on the Gilbert damping factor. The ascertained values of the Gilbert damping parameter of Ni films using ferromagnetic resonance (FMR) measurements lies between 0.011 and 0.0069. We have observed that the amplitude of emitted THz radiation from Ni films is proportional to Gilbert damping factor of films. The amplitude of emitted THz radiation from 65 nm Ni film is 1.19 and 1.66 times higher than 135 and 1018 nm films.