We have studied ultrafast photoinduced demagnetization in GaMnAs via two-color time-resolved magneto-optical Kerr spectroscopy. Below-bandgap midinfrared pump pulses strongly excite the valence band, while near-infrared probe pulses reveal sub-picosecond demagnetization that is followed by an ultrafast ($\sim$1 ps) partial recovery of the Kerr signal. Through comparison with InMnAs, we attribute the signal recovery to an ultrafast energy relaxation of holes. We propose that the dynamical polarization of holes through $p$-$d$ scattering is the source of the observed probe signal. These results support the physical picture of femtosecond demagnetization proposed earlier for InMnAs, identifying the critical roles of both energy and spin relaxation of hot holes.
We report a femtosecond response in photoinduced magnetization rotation in the ferromagnetic semiconductor GaMnAs, which allows for detection of a four-state magnetic memory at the femtosecond time scale. The temporal profile of this cooperative magnetization rotation exhibits a discontinuity that reveals two distinct temporal regimes, marked by the transition from a highly non-equilibrium, carrier-mediated regime within the first 200 fs, to a thermal, lattice-heating picosecond regime.
We report on the observation of ultrafast photo-enhanced ferromagnetism in GaMnAs. It is manifested as a transient magnetization increase on a 100-ps time scale, after an initial sub-ps demagnetization. The dynamic magnetization enhancement exhibits a maximum below the Curie temperature Tc and dominates the demagnetization component when approaching Tc. We attribute the observed ultrafast collective ordering to the p-d exchange interaction between photoexcited holes and Mn spins, leading to a correlation-induced peak around 20K and a transient increase in Tc.
We follow for the first time the evolution of the spin and orbital angular momentum of a thin Ni film during ultrafast demagnetization, by means of x-ray magnetic circular dichroism. Both components decrease with a 130 +/- 40 fs time constant upon excitation with a femtosecond laser pulse. Additional x-ray absorption measurements reveal an increase in the spin-orbit interaction by 6 +/- 2 % during this process. This is the experimental demonstration quantifying the importance of spin-orbit mediated processes during the demagnetization.
We use femtosecond time-resolved hard x-ray scattering to detect coherent acoustic phonons excited during ultrafast laser demagnetization of bcc Fe films. We determine the lattice strain propagating through the film through analysis of the oscillations in the x-ray scattering signal as a function of momentum transfer. The width of the strain wavefront is ~100 fs, similar to demagnetization timescales. First-principles calculations show that the high-frequency Fourier components of the strain, which give rise to the sharp wavefront, could in part originate from non-thermal dynamics of the latti…
Magnetic phenomena are ubiquitous in our surroundings and indispensable for modern science and technology, but it is notoriously difficult to change the magnetic order of a material in a rapid way. However, if a thin nickel film is subjected to ultrashort laser pulses, it can lose its magnetic order almost completely within merely femtosecond times. This phenomenon, in the meantime also observed in many other materials, has connected magnetism with femtosecond optics in an efficient, ultrafast and complex way, offering opportunities for rapid information processing or ultrafast spintronics at …
We directly measure the hole spin lifetime in ferromagnetic GaMnAs via time- and polarization-resolved spectroscopy. Below the Curie temperature Tc, an ultrafast photoexcitation with linearly-polarized light is shown to create a non-equilibrium hole spin population via the dynamical polarization of holes through p-d exchange scattering with ferromagnetically-ordered Mn spins, and we characterize their relaxation dynamics. The observed relaxation consists of a distinct three-step recovery : (i) femtosecond (fs) hole spin relaxation ~ $160-200 fs, (ii) picosecond (ps) hole energy relaxation ~ 1-…
In spintronics applications, ultrafast spin dynamics have to be controlled at femtosecond (fs) timescales via fs-laser radiation. At such ultrafast timescales, the effect of the Gilbert damping factor α on ultrafast demagnetization time should be considered. In previous explorations for the relationship between these two parameters, it was found that the theoretical calculations based on the local spin-flip scattering model do not agree with the experimental results. Here, we find that in Fe81Ga19(FeGa)/Ir20Mn80(IrMn) bilayers, the unconventional IrMn thickness dependence of α results from the…
We compare femtosecond pump-probe experiments in Ni and micromagnetic modelling based on the Landau-Lifshitz-Bloch equation coupled to a two-temperature model, revealing a predominant thermal ultrafast demagnetization mechanism. We show that both spin (femtosecond demagnetization) and electron-phonon (magnetization recovery) rates in Ni increase as a function of the laser pump fluence. The slowing down for high fluences arises from the increased longitudinal relaxation time.
Spin and orbital angular momenta are two intrinsic properties of an electron and are responsible for the physics of a solid. How the spin and orbital evolve with respect to each other on several hundred femtoseconds is largely unknown, but it is at the center of laser-induced ultrafast demagnetization. In this paper, we introduce a concept of the spin-orbital correlation diagram, where spin angular momentum is plotted against orbital angular momentum, much like the position-velocity phase diagram in classical mechanics. We use four sets of highly accurate time-resolved x-ray magnetic circular …
We report dynamics of the transient polar Kerr rotation (KR) and of the transient reflectivity induced by femtosecond laser pulses in ferromagnetic (Ga,Mn)As with no external magnetic field applied. It is shown that the measured KR signal consist of several different contributions, among which only the oscillatory signal is directly connected with the ferromagnetic order in (Ga,Mn)As. The origin of the light-induced magnetization precession is discussed and the magnetization precession damping (Gilbert damping) is found to be strongly influenced by annealing of the sample.
We use femtosecond optical pulses to induce, control and monitor magnetization precession in ferromagnetic Ga0.965Mn0.035As. At temperatures below ~40 K we observe coherent oscillations of the local Mn spins, triggered by an ultrafast photoinduced reorientation of the in-plane easy axis. The amplitude saturation of the oscillations above a certain pump intensity indicates that the easy axis remains unchanged above ~TC/2. We find that the observed magnetization precession damping (Gilbert damping) is strongly dependent on pump laser intensity, but largely independent on ambient temperature. We …
In pump-probe experiments an intensive laser pulse creates non-equilibrium excited electron distributions in the first few hundred femtoseconds after the pulse. The influence of non-equilibrium electron distributions caused by a pump laser on the apparent X-ray magnetic circular dichroism (XMCD) signal of Ni is investigated theoretically here for the first time, considering electron distributions immediately after the pulse as well as thermalized ones, that are not in equilibrium with the lattice or spin systems. The XMCD signal is shown not to be simply proportional to the spin momentum in th…
We theoretically study the dynamic time evolution following laser pulse pumping in an antiferromagnetic insulator Cr$_{2}$O$_{3}$. From the photoexcited high-spin quartet states to the long-lived low-spin doublet states, the ultrafast demagnetization processes are investigated by solving the dissipative Schrödinger equation. We find that the demagnetization times are of the order of hundreds of femtosecond, in good agreement with recent experiments. The switching times could be strongly reduced by properly tuning the energy gaps between the multiplet energy levels of Cr$^{3+}$. Furthermore, th…
We have measured the hot-electron induced demagnetization of a [Co/Pt]2 multilayer in M(x nm)/Cu(100 nm)/[Co(0.6 nm)/Pt(1.1 nm)]2 samples depending on the nature of the capping layer M and its thickness x. We found out that a Pt layer is more efficient than [Co/Pt]X, Cu or MgO layers in converting IR photon pulses into hot-electron pulses at a given laser power. We also found out that the maximum relative demagnetization amplitude is reached for M(x) = Pt (7 nm). Our experimental results show qualitative agreement with numerical simulations based on the superdiffusive spin transport model. We …
Electronic mechanism of sub-100-fs demagnetization induced by a femtosecond light pulse
Adam Mickiewicz University in Poznań（波兹南亚当·密茨凯维奇大学）；Deutsches Elektronen-Synchrotron DESY（德国电子同步加速器中心）；European XFEL GmbH（欧洲X射线自由电子激光有限公司）；Elettra-Sincrotrone Trieste S.C.p.A（的里雅斯特 Elettra-同步辐射研究中心）；University of Hamburg（汉堡大学）；TU Bergakademie Freiberg（弗莱贝格工业大学）；Polish Academy of Sciences（波兰科学院）
Irradiation with an ultrashort laser pulse can completely destroy the magnetic order of ferromagnetic thin films on the femtosecond timescale. This phenomenon holds great potential for ultrafast spintronics and information processing and is an active field of research. It is still an open question if the angular momentum of light can support this effect. While it has been shown that the spin of light only has a negligible influence, we experimentally demonstrate the influence of ultrashort laser pulses with orbital angular momentum (OAM) on the magnetization dynamics of a thin nickel film. Our…
We propose a semi-classical model for femtosecond-laser induced demagnetization due to spin-polarized excited electron diffusion in the super-diffusive regime. Our approach treats the finite elapsed time and transport in space between multiple electronic collisions exactly, as well as the presence of several metal films in the sample. Solving the derived transport equation numerically we show that this mechanism accounts for the experimentally observed demagnetization within 200fs in Ni, without the need to invoke any angular momentum dissipation channel.
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 em…
Following the demonstration of laser-induced ultrafast demagnetization in ferromagnetic nickel, several theoretical and phenomenological propositions have sought to uncover its underlying physics. In this work we revisit the three temperature model (3TM) and the microscopic three temperature model (M3TM) to perform a comparative analysis of ultrafast demagnetization in 20-nm-thick cobalt, nickel and permalloy thin films measured using an all-optical pump-probe technique. In addition to the ultrafast dynamics at the femtosecond timescales, the nanosecond magnetization precession and damping are…
Femtosecond x-ray magnetic circular dichroism was used to study the time-dependent magnetic moment of 4 fs electrons in the ferromagnets Gd and Tb, which are known for their different spin-lattice coupling. We observe a two-step demagnetization with an ultrafast demagnetization time of 750 fs identical for both systems and slower times which differ sizeably with 40 ps for Gd and 8 ps for Tb. We conclude that spin-lattice coupling in the electronically excited state is enhanced up to orders of magnitude compared to equilibrium.
From Ultrafast Demagnetization to Ultrafast Spintronics : a 30 years story
Université de Lorraine（洛林大学）；CNRS, Institut Jean Lamour（法国国家科学研究中心，让·拉穆尔研究所）；Department of Physics, Freie Universität Berlin（柏林自由大学物理系）；Center for Science and Innovation in Spintronics, Tohoku University（东北大学自旋电子学科学与创新中心）
A dependence of a relaxation rate on the shape of the demagnetized region for a longitudinal evolution of total magnetization to its equilibrium value following the ultrafast demagnetization is demonstrated. This shape-dependence is caused by a motion of the wave front inside the demagnetized region. The contribution of the wave front for spherically symmetric shape of the demagnetized region is up to 3 times and for cylindrically symmetric shape up to 2 times stronger than for one dimensional demagnetized region. This effect can be observed after the demagnetization by a tightly focused femto…
Engineering of magnetic materials for developing better spintronic applications relies on the control of two key parameters: the spin polarization and the Gilbert damping responsible for the spin angular momentum dissipation. Both of them are expected to affect the ultrafast magnetization dynamics occurring on the femtosecond time scale. Here, we use engineered Co2MnAlxSi1-x Heusler compounds to adjust the degree of spin polarization P from 60 to 100% and investigate how it correlates with the damping. We demonstrate experimentally that the damping decreases when increasing the spin polarizati…
The Elliott-Yafet mechanism is arguably the most promising candidate to explain the ultrafast demagnetization dynamics in ferromagnetic transition metals on timescales on the order of 100 femtoseconds. So far, only electron-phonon scattering has been analyzed as the scattering process needed to account for the demagnetization due to the Elliott-Yafet mechanism. We show for the first time that the electron-electron scattering contribution to the Elliott-Yafet mechanism has the potential to explain time-resolved magneto-optical Kerr effect measurements on thin magnetic cobalt and nickel films, w…
We report on {\it ab-initio} simulations of the quantum dynamics of electronic charge and spin when subjected to intense laser pulses. By performing separate calculations for a Ni thin film and bulk Ni, we conclude that surface effects have a dramatic influence on amplifying the laser induced demagnetization. We show that the reason for this amplification is due to increased spin-currents on the surface of the thin film. This enhancement is a direct consequence of the broken symmetry originating from the surface formation. We find that the underlying physics of demagnetization, during the earl…
Femtosecond laser induced ultrafast magnetization dynamics have been studied in multisublattice CoxDy1-x alloys. By performing element and time-resolved X-ray spectroscopy, we distinguish the ultrafast quenching of Co3d and Dy4f magnetic order when the initial temperatures are below (T=150K) or above (T=270K) the temperature of magnetic compensation (Tcomp). In accordance with former element-resolved investigations and theoretical calculations, we observe different dynamics for Co3d and Dy4f spins. In addition we observe that, for a given laser fluence, the demagnetization amplitudes and demag…
Ultrafast magnetization dynamics induced by femtosecond laser pulses have been measured in ferrimagnetic Co0.8Gd0.2, Co.74Tb.26 and Co.86Tb.14 alloys. Using element sensitivity of X-ray magnetic circular dichroism at the Co L3, Tb M5 and Gd M5 edges we evidence that the demagnetization dynamics is element dependent. We show that a thermalization time as fast as 280 fs is observed for the rare-earth in the alloy, when the laser excited state temperature is below the compensation temperature. It is limited to 500 fs when the laser excited state temperature is below the Curie temperature (Tc). We…
Interest in femtosecond demagnetization experiments was sparked by Bigot's discovery in 1995. These experiments unveil the elementary mechanisms coupling the electrons' temperature to their spin order. Even though first quantitative models describing ultrafast demagnetization have just been published within the past year, new calculations also suggest alternative mechanisms. Simultaneously, the application of fast demagnetization experiments has been demonstrated to provide key insight into technologically important systems such as high spin polarization metals, and consequently there is broad…
We investigate the temperature dependence of the coefficients in the Landau-Lifshitz-Gilbert equation in ferromagnetic GaMnAs by employing the Zener model. We first calculate the hole spin relaxation time based on the microscopic kinetic equation. We find that the hole spin relaxation time is typically several tens femtoseconds and can present a nonmonotonic temperature dependence due to the variation of the interband spin mixing, influenced by the temperature related Zeeman splitting. With the hole spin relaxation time, we are able to calculate the coefficients in the Landau-Lifshitz-Gilbert …
Revealing domain wall stability during ultrafast demagnetization
Max Planck Institute for Multidisciplinary Sciences（马克斯·普朗克多学科研究所）；Institute of Physics, University of Augsburg（奥格斯堡大学物理研究所）；School of Electrical Engineering, Tel Aviv University（特拉维夫大学电气工程学院）；th Physical Institute, University of Göttingen（哥廷根大学第四物理研究所）；Radboud University, Institute for Molecules and Materials（拉德堡德大学分子与材料研究所）
At the ultrafast limit of optical spin manipulation is the theoretically predicted phenomena of optical intersite spin transfer (OISTR), in which laser induced charge transfer between the sites of a multi-component material leads to control over magnetic order. A key prediction of this theory is that the demagnetization efficiency is determined by the availability of unoccupied states for intersite charge transfer. Employing state-of-the-art magneto-optical Kerr effect measurements with femtosecond time resolution, we probe this prediction via a systematic comparison of the ultrafast magnetic …
Laser-induced femtosecond demagnetization has attracted a broad attention as a possible candidate for information storage technology. However, whether or not lattice vibration directly participates in demagnetization has been highly controversial over a decade. A recent electron diffraction experiment attributed the demagnetization to the polarized phonon effect, but a similar x-ray diffraction experiment attributed it to the Einstein-de Haas effect. Common to both experiments is that neither the angular momentum of the lattice nor the rotation of the sample was directly probed. Here, we repor…
The ultrafast dynamics of magnetic order in a ferromagnet are governed by the interplay between electronic, magnetic and lattice degrees of freedom. In order to obtain a microscopic understanding of ultrafast demagnetization, information on the response of all three subsystems is required. A consistent description of demagnetization and microscopic energy flow, however, is still missing. Here, we combine a femtosecond electron diffraction study of the ultrafast lattice response of nickel to laser excitation with ab initio calculations of the electron-phonon interaction and energy-conserving at…
With the goal of pushing Spintronic devices towards faster and faster timescales, we demonstrate, using ab-intio time-dependent density functional theory simulations of bulk Heusler compounds subject to ultrashort intense laser pulses, that the local magnetic moment can increase or decrease in a few femtoseconds. This speed is due to the all optical nature of the process, which transfers spin moment from one sublattice to another. This transfer depends on easily tunable laser parameters. By comparing the spin dynamics of a variety of Heusler (or half-Heusler) compounds, we demonstrate that the…
Excitation of a metallic ferromagnet such as Ni with an intensive femtosecond laser pulse causes an ultrafast demagnetization within approximately 300 fs. It was proposed that the ultrafast demagnetization measured in femtosecond magneto-optical experiments could be due to relativistic light-induced processes. We perform an ab initio investigation of the influence of relativistic effects on the magneto-optical response of Ni. To this end, we develop, first, a response theory formulation of the additional appearing ultra-relativistic terms in the Foldy-Wouthuysen transformed Dirac Hamiltonian d…
Laser-induced femtosecond magnetism or femtomagnetism simultaneously relies on two distinctive contributions: (a) the optical dipole interaction (ODI) between a laser field and a magnetic system and (b) the spin expectation value change (SEC) between two transition states. Surprisingly, up to now, no study has taken both contributions into account simultaneously. Here we do so by introducing a new concept of the optical spin generator, a product of SEC and ODI between transition states. In ferromagnetic nickel, our first-principles calculation demonstrates that the larger the value of optical …
Stoner and Heisenberg excitations in magnetic materials are inherently different. The former involves an effective reduction of the exchange splitting, whereas the latter comprises excitation of spin-waves. In this work, we test the impact of these two excitations in the hybrid Stoner-Heisenberg system FePd. We present a microscopic picture of ultrafast demagnetization dynamics in this alloy, which represents both components of strong local exchange splitting in Fe, and induced polarization in Pd. We identify spin-orbit coupling and optical inter-site spin transfer as the two dominant factors …
Laser induced ultrafast demagnetization is the process whereby the magnetic moment of a ferromagnetic material is seen to drop significantly on a timescale of $10-100$s of femtoseconds due to the application of a strong laser pulse. If this phenomenon can be harnessed for future technology, it offers the possibility for devices operating at speeds several orders of magnitude faster than at present. A key component to successful transfer of such a process to technology is the controllability of the process, i.e. that it can be tuned in order to overcome the practical and physical limitations im…
We report a computational theoretical investigation of electron spin-flip scattering induced by the electron-phonon interaction in the transition-metal ferromagnets bcc Fe, fcc Co and fcc Ni. The Elliott-Yafet electron-phonon spin-flip scattering is computed from first-principles, employing a generalized spin-flip Eliashberg function as well as ab initio computed phonon dispersions. Aiming at investigating the amount of electron-phonon mediated demagnetization in femtosecond laser-excited ferromagnets, the formalism is extended to treat laser-created thermalized as well as nonequilibrium, nont…
We report on an element- and time-resolved investigation of femtosecond laser induced ultrafast dynamics of 3d and 4f spins in a ferrimagnetic Co80Dy20 alloy as a function of temperature. We observe an increase of the Co3d characteristic demagnetization time and a decrease of the Dy4f demagnetization time when the temperature is approaching the Curie temperature. It suggests that the critical slowing down regime, which affects the laser induced ultrafast dynamics in pure 3d transition metals and 4f rare-earth ferromagnetic layers, vanishes for the Dy sublattice in the CoDy alloy, in line with …
We have investigated ultrafast quenching of the Tb4f magnetic order in Co74Tb26 alloys, induced by femtosecond hot-electrons pulses. The hot-electron pulses were produced in specific non-magnetic capping layers by infrared fs laser pulses. Our experimental results show that sub-picosecond dynamics of Tb4f magnetic moments can be induced by non-thermal and thermal hot-electrons. We further demonstrate that the demagnetization efficiencies of non-thermal and thermal hot electrons are similar. However, the characteristic demagnetization times show values of 0.35 ps for non-thermal hot-electrons e…
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 r…
We report about the dephasing of the spins states in a Garnet film excited by femtosecond laser pulses. It is shown that magneto-optical four-wave-mixing signals are efficiently generated and controlled with an external static magnetic field. The self-diffraction of either two or three femtosecond pulses allows us determining independently the coherent response time ($T_2$ < 50 fs) and the population relaxation, which occurs in several steps. After the laser induced demagnetization, the spin-lattice relaxation is the dominant mechanism ($T_{1,spin-lat}$ < 1.35 ps) and the precession of the mag…
Ultrafast magnetization dynamics in metallic heterostructures consists of a combination of local demagnetization in the ferromagnetic constituent and spin-dependent transport contributions within and in between the constituents. Separation of these local and non-local contributions is essential to obtain microscopic understanding and for potential applications of the underlying microscopic processes. By comparing the ultrafast changes of the polarization rotation and ellipticity in the magneto-optical Kerr effect (MOKE) we observe a time-dependent magnetization profile M(z,t) in Co/Cu(001) fil…
Three-temperature atomistic spin-lattice dynamics in LAMMPS: a moment-consistent, fluctuation-dissipation-correct extension and its validation on ultrafast demagnetization and all-optical switching of GdFeCo
DGIST (Daegu Gyeongbuk Institute of Science and Technology)（大邱庆北科学技术院）；Kunsan National University（群山国立大学）；Chungbuk National University（忠北国立大学）
Laser induced ultrafast Co 3d and Ho 4f spin dynamics in CoHo ferrimagnetic alloys
Helmholtz-Zentrum Berlin für Materialien und Energie（柏林亥姆霍兹材料与能源中心）；Institut Jean Lamour, Université de Lorraine（洛林大学让·拉缪尔研究所）；Université de Strasbourg, CNRS, Institut de Physique et Chimie des Matériaux de Strasbourg（斯特拉斯堡大学、法国国家科学研究中心、斯特拉斯堡物理与化学材料研究所）
Single femtosecond pulse toggle switching of ferrimagnetic alloys is an essential building block for ultrafast spintronics. Very different element-specific demagnetization dynamics is believed to be a hard limit for switching in ferrimagnets. This suggests that ferrimagnets composed of two ions of different nature, such as rare earth transition metal alloys, are necessary for switching. However, experimental observation of toggle switching in Mn$_2$Ru$_x$Ga Heusler alloys, has contested this limit since Mn ions are of the same nature. To shed some light into this question, we present an atomis…
Magnetization manipulation in the absence of an external magnetic field is a topic of great interest, since many novel physical phenomena need to be understood and promising new applications can be imagined. Cutting-edge experiments have shown the capability to switch the magnetization of magnetic thin films using ultrashort polarized laser pulses. In 2007, it was first observed that the magnetization switching for GdFeCo alloy thin films was helicity-dependent and later helicity-independent switching was also demonstrated on the same material. Recently, all-optical switching has also been dis…
