Laser-induced Magnetization Dynamics Research Articles

Optically controlling the competition between spin flips and intersite spin transfer in a Heusler half-metal on sub-100-fs time scales.

The direct manipulation of spins via light may provide a path toward ultrafast energy-efficient devices. However, distinguishing the microscopic processes that can occur during ultrafast laser excitation in magnetic alloys is challenging. Here, we study the Heusler compound Co2MnGa, a material that exhibits very strong light-induced spin transfers across the entire M-edge. By combining the element specificity of extreme ultraviolet high-harmonic probes with time-dependent density functional theory, we disentangle the competition between three ultrafast light-induced processes that occur in Co2MnGa: same-site Co-Co spin transfer, intersite Co-Mn spin transfer, and ultrafast spin flips mediated by spin-orbit coupling. By measuring the dynamic magnetic asymmetry across the entire M-edges of the two magnetic sublattices involved, we uncover the relative dominance of these processes at different probe energy regions and times during the laser pulse. Our combined approach enables a comprehensive microscopic interpretation of laser-induced magnetization dynamics on time scales shorter than 100 femtoseconds.

Lattice structure dependence of laser-induced ultrafast magnetization switching in ferrimagnets

The experimental discovery of single-pulse ultrafast magnetization switching in ferrimagnetic alloys, such as GdFeCo and MnRuGa, opened the door to a promising route toward faster and more energy efficient data storage. A recent semi-phenomenological theory has proposed that a fast, laser-induced demagnetization below a threshold value puts the system into a dynamical regime where angular momentum transfer between sublattices dominates. Notably, this threshold scales inversely proportional to the number of exchange-coupled nearest neighbors considered in the model, which in the simplest case is directly linked to the underlying lattice structure. In this work, we study the role of the lattice structure on the laser-induced ultrafast magnetization switching in ferrimagnets by complementing the phenomenological theory with atomistic spin dynamics computer simulations. We consider a spin model of the ferrimagnetic GdFeCo alloy with increasing number of exchange-coupled neighbors. Within this model, we demonstrate that the laser-induced magnetization dynamics and switching depend on the lattice structure. Furthermore, we determine that the critical laser energy for switching reduces for decreasing number of exchange-coupled neighbors.

Comparative study of femtosecond laser-induced ultrafast magnetization dynamics in soft ferromagnetic ultra-thin alloy

The typical demagnetization mechanism is applicable in magneto-optic recording systems on a nanoseconds timescale. Interest in this area has expanded rapidly since understanding the physics of ultrafast magnetization processes by experiment are an acutely challenging task. As a result, in order to explain the quenching of magnetization by laser heating in ferromagnetic alloys such as permalloy, we executed a new theoretical study on permalloy, as well as Ni and Fe ultra-thin films with thickness varying from 1 nm to 5 nm. In particular, we demonstrate that the magnetization decays in a timescale of about 0.1 picosecond by the microscopic three-temperature model. Our free electron model-based theoretical model represents that electron–phonon coupling coefficient and demagnetization time in the thin-films are strongly sensitive to film thickness, pulse duration and laser fluence, respectively. In particular, we show that the rapid demagnetization (100 fs) is due to electron-magnon excitation. This feature improves data processing speed in communication systems and the recording industry.

Ultrafast magnetization and energy flow in the laser-induced dynamics of transition metal compounds

An electronic theory of the laser-induced ultrafast magnetization dynamics in transition-metal alloys is presented. A many-body model Hamiltonian is considered that incorporates the fundamental electronic hopping, local Coulomb interaction, and spin-orbit coupling (SOC) on the same footing. Exact time propagations are performed on a tetrahedral cluster, from which the time dependencies of the local spin moments, orbital occupations, and single-particle energies of homogeneous systems and binary alloys are obtained. The consequences of inhomogeneities in the laser absorption and in the SOC strengths are investigated giving emphasis to the nature of spin-density transfers between the alloy components. A local perspective on the optically induced spin transfer is proposed in terms of which two main steps emerge: a dominantly local optical excitation followed by hopping-driven spin-density transfers among the different alloy components. The conjoint action of the spin-orbit interactions at the origin of local spin flips and spin-to-orbital angular-momentum transfer and the interatomic hoppings responsible for charge, spin, and energy flows between different sublattices is demonstrated. Transient and steady-state dynamical regimes are identified that result in delays in the onset of the local demagnetizations and in ultrafast redistributions of the spin. The central role of electron delocalization and electronic hopping in the spin-density redistribution and demagnetization of the different alloy components is demonstrated.

Spin and Orbital Symmetry Breakings Central to the Laser-Induced Ultrafast Demagnetization of Transition Metals

The role of spin and orbital rotational symmetry on the laser-induced magnetization dynamics of itinerant-electron ferromagnets was theoretically investigated. The ultrafast demagnetization of transition metals is shown to be the direct consequence of the fundamental breaking of these conservation laws in the electronic system, an effect that is inherent to the nature of spin-orbit and electron-lattice interactions. A comprehensive symmetry analysis is complemented by exact numerical calculations of the time evolution of optically excited ferromagnetic ground states in the framework of a many-body electronic Hamiltonian. Thus, quantitative relations are established between the strength of the interactions that break the rotational symmetries and the time scales that are relevant for the magnetization dynamics.

All-optical nonlinear chiral ultrafast magnetization dynamics driven by circularly polarized magnetic fields

Abstract Ultrafast laser pulses provide unique tools to manipulate magnetization dynamics at femtosecond timescales, where the interaction of the electric field usually dominates over the magnetic field. Recent proposals using structured laser beams have demonstrated the possibility to produce regions where intense oscillating magnetic fields are isolated from the electric field. In these conditions, we show that technologically feasible tesla-scale circularly polarized high-frequency magnetic fields induce purely precessional nonlinear magnetization dynamics. This fundamental result not only opens an avenue in the study of laser-induced ultrafast magnetization dynamics, but also sustains technological implications as a route to promote all-optical non-thermal magnetization dynamics both at shorter timescales – towards the sub-femtosecond regime – and at THz frequencies.

Laser-Induced Magnetization Dynamics in Si-Doped Yttrium-Iron Garnet Film

We experimentally demonstrate that the excitation of a silicon-doped yttrium-iron garnet film by femtosecond laser pulses triggers a magnetization precession with an amplitude determined by the external magnetization direction. The maximum efficiency is achieved at the pump wavelength corresponding to the absorption maximum due to doping with silicon ions. Based on the azimuthal dependences of the precession amplitude and frequency, it is shown that the magnetization dynamics is induced by a thermal disruption of the magnetocrystalline anisotropy. By modeling hysteresis loops, it was found that the silicon doping leads to a decrease in the value of the exchange interaction in the film and an increase in the anisotropy field.

Energy exchange dependent transient ferromagnetic like state of ultrafast magnetization dynamics

The study of laser-induced ultrafast magnetization dynamics is crucial for the development of information recording technology. Due to the complex mechanism, there is still a lack of comprehensive understanding for ultrafast magnetization dynamics. As an essential stage of laser-induced ultrafast magnetization switching process, the transient ferromagnetic like state (TFLS), has attracted much attention. Different from other studies on TFLS through the difference of magnetization dynamics between rare-earth and transition-metal, our study mainly focuses on the influence of energy injection and relaxation on TFLS in the process of ultrafast magnetization dynamics. The influence of various parameters on the formation of energy exchange dependent TFLS is studied. The results of simulation well support our view. Understanding the mechanism behind the TFLS is of great significance to promote the application of laser-induced ultrafast magnetization switching.

Heisenberg representation of nonthermal ultrafast laser excitation of magnetic precessions

We derive the Heisenberg representation of the ultrafast inverse Faraday effect that provides the time evolution of magnetic vectors of a magnetic system during its interaction with a laser pulse. We obtain a time-dependent effective magnetic operator acting in the Hilbert space of the total angular momentum that describes a process of nonthermal excitation of magnetic precessions in an electronic system by a circularly polarized laser pulse. The magnetic operator separates the effect of the laser pulse on the magnetic system from other magnetic interactions. The effective magnetic operator provides the equations of motion of magnetic vectors during the excitation by the laser. We show that magnetization dynamics calculated with these equations is equivalent to magnetization dynamics calculated with the time-dependent Schr\"odinger equation, which takes into account the interaction of an electronic system with the electric field of light. We model and compare laser-induced precessions of magnetic sublattices of an easy-plane and an easy-axis antiferromagnetic systems. Using these models, we show how the ultrafast inverse Faraday effect induces a net magnetic moment in antiferromagnets and demonstrate that a crystal field environment and the exchange interaction play essential roles for laser-induced magnetization dynamics even during the action of a pump pulse. Using our approach, we show that light-induced precessions can start even during the action of the pump pulse with a duration several tens times shorter than the period of induced precessions and affect the position of magnetic vectors after the action of the pump pulse.

Ultrafast Laser Modulation of Local Magnetization Orientation in Perpendicularly Exchange-Coupled Bilayer

Laser-induced magnetization dynamics in a perpendicularly exchange-coupled TbFeCo/GdFeCo bilayer film are studied by using pump-probe magneto-optical Kerr spectroscopy. An ultrafast modulation effect on local magnetization orientation is observed. Such ultrafast magnetization reorientation in the GdFeCo layer is revealed to be triggered by the femtosecond laser pulse and driven by the effective exchange field. These processes occur within a timescale of hundreds of picoseconds, in which the field- and fluence-dependent dynamical behaviors are demonstrated. In addition, an atomistic Heisenberg model is proposed for studying the laser-induced magnetization dynamics by using micromagnetic simulation. The simulated results agree with the experimental phenomena and further reveal the underlying mechanism. These results show an approach for ultrafast manipulation of the local magnetization orientation in perpendicularly exchange-coupled structures.

Sub-100 femtosecond time scale spin dynamics in epitaxial Fe3O4 thin film

We have investigated the ultrafast laser-induced magnetization and reflectivity dynamics in the Fe3O4 thin film using all-optical pump–probe measurements. The high-quality of our sample is verified by the temperature-dependent static magnetic properties, showing a clear Verwey transition. Although the Fe3O4 is predicted to be a half-metal, the demagnetization process upon laser incident shows a transition-metal-like behavior. The demagnetization characteristic time has been found to be in sub-100 fs (fs) time scale, suggesting the dominant interaction of hole spin-flip processes. The spin relaxation process has been found to be around 600 fs, which is attributed to the electron–phonon coupling in accordance to the observed two electron–phonon coupling modes in reflectivity dynamics. Our results provide new insight into the ultrafast laser-induced spin dynamics in half-metallic ferrimagnet.

S−d model for local and nonlocal spin dynamics in laser-excited magnetic heterostructures

We discuss a joint microscopic theory for the laser-induced magnetization dynamics and spin transport in magnetic heterostructures based on the $s$-$d$ interaction. Angular momentum transfer is mediated by scattering of itinerant $s$ electrons with the localized ($d$ electron) spins. We use the corresponding rate equations and focus on a spin one-half $d$ electron system, leading to a simplified analytical expression for the dynamics of the local magnetization that is coupled to an equation for the non-equilibrium spin accumulation of the $s$ electrons. We show that this description converges to the microscopic three-temperature model in the limit of a strong $s$-$d$ coupling. The equation for the spin accumulation is used to introduce diffusive spin transport. The presented numerical solutions show that during the laser-induced demagnetization in a ferromagnetic metal a short-lived spin accumulation is created that counteracts the demagnetization process. Moreover, the spin accumulation leads to the generation of a spin current at the interface of a ferromagnetic and non-magnetic metal. Depending on the specific magnetic system, both local spin dissipation and interfacial spin transport are able to enhance the demagnetization rate by providing relaxation channels for the spin accumulation that is build up during demagnetization in the ferromagnetic material.

Laser-induced magnetization dynamics in a van der Waals ferromagnetic Cr2Ge2Te6 nanoflake

Laser-induced magnetization dynamics is quantitatively investigated in a van der Waals ferromagnetic Cr2Ge2Te6 nanoflake by means of time-resolved Faraday rotation. Under ferromagnetic resonance conditions, the angular dependence of spin precession dynamics gives rise to a perpendicular magnetic anisotropy with an effective field of 125 ± 8 mT. We further determine the field dependence of the effective damping coefficient, which is dominated by the inhomogeneous broadening of magnetic anisotropy in the regime of a small magnetic field while it diminishes to an intrinsic value of 0.006 ± 0.002 at high fields.

Ultrafast Magnetization Dynamics in Metallic Amorphous Ribbons with a Giant Magnetoimpedance Response

We report on the dynamics of laser-induced magnetization in magnetic ribbons using the time-resolved magneto-optical Kerr effect. Damped oscillations representing precessional motion of the magnetization vector are observed in a $\mathrm{Co}$-rich ribbon. The frequency and amplitude of these oscillations depend on the fluence of the pump laser pulse and the external magnetic field. With an anisotropy factor defined by the effective field of the magnetic anisotropy, we demonstrate that the frequency of the ferromagnetic mode activated by the laser pulses is close to the frequency of the giant magnetoimpedance response. These findings provide an opportunity to investigate the magnetoimpedance effect in the gigahertz frequency range using ultrashort laser pulses and are particularly interesting for the development of broadband sensor devices. Furthermore, our results link laser-induced magnetization dynamics with magnetoresistive sensing techniques, thus opening up the supergigahertz domain to magnetoimpedance studies.

Laser-induced magnetization precession parameters dependence on Pt spacer layer thickness in mixed magnetic anisotropies Co/Pt/Co trilayer

Laser-induced magnetization dynamics was investigated in Co/Pt/Co trilayer, with 1 nm thick Co-sublayers, as a function of Pt-spacer layer thickness (dPt) using time-resolved magneto-optical Kerr effect. The magnetization precession parameters: frequency (f), amplitude (A) and effective damping parameter (αeff) dependencies on the external magnetic field (H) were determined as a function of dPt. Three different regions of Pt layer thickness: 0–1.3 nm, 1.3–2.5 nm, and 2.5–4 nm, denoted as regions I, II, and III, respectively, exhibiting distinctly different magnetic properties, were distinguished. It is found that as dPt increases reorientation of Co-magnetizations from in-plane to perpendicular direction occurs at dPt around 1.2 nm. For dPt>2.5 nm magnetization reorientation from parallel to the orthogonal directions due to mixed anisotropies in Co-layers appears. It is found that in the whole dPt range single magnetization precession frequency occurs, despite different effective magnetic anisotropies of Co-sublayers. Simulation of the magnetization precession parameters was performed in an effective magnetic layer model. We found that the observed A(H) behavior is well simulated within the model in region I and III. The αeff(H) correlations with A(H) in different dPt regions are discussed.

Comparing all-optical switching in synthetic-ferrimagnetic multilayers and alloys

We present an experimental and theoretical investigation of all-optical switching by single femtosecond laser pulses. Our experimental results demonstrate that, unlike rare-earth transition-metal ferrimagnetic alloys, $\mathrm{Pt}/\mathrm{Co}/{[\mathrm{Ni}/\mathrm{Co}]}_{N}/\mathrm{Gd}$ can be switched in the absence of a magnetization compensation temperature, indicative for strikingly different switching conditions. In order to understand the underlying mechanism, we model the laser-induced magnetization dynamics in Co/Gd bilayers and GdCo alloys on an equal footing, using an extension of the microscopic three-temperature model to multiple magnetic sublattices and including exchange scattering. In agreement with our experimental observations, the model shows that Co/Gd bilayers can be switched for a thickness of the Co layer far away from compensating the total Co and Gd magnetic moment. We identify the switching mechanism in Co/Gd bilayers as a front of reversed Co magnetization that nucleates near the Co/Gd interface and propagates through the Co layer driven by exchange scattering.

The role of intermixing in all-optical switching of synthetic-ferrimagnetic multilayers

We present a theoretical study of single-pulse all-optical switching (AOS) in synthetic-ferrimagnetic multilayers. Specifically, we investigate the role of interface intermixing in switching Co/Gd bilayers. We model the laser-induced magnetization dynamics in Co/Gd bilayers using the microscopic three-temperature model for layered magnetic systems. Exchange scattering is included, which mediates angular momentum transfer between the magnetic sublattices. In this work, each layer is represented by one atomic monolayer of a GdCo alloy with an arbitrary Co concentration, allowing Co/Gd bilayers with an intermixed interface to be modelled. Our results indicate that within the model intermixing of the Co/Gd interface reduces the threshold fluence for AOS significantly. We show that intermixing does not qualitatively affect the switching mechanism and leads to an increase of the propagation speed of the switching front.

Annealing effect on laser-induced magnetization dynamics in Co/Ni-based synthetic antiferromagnets with perpendicular magnetic anisotropy

We report a comprehensive study of annealing treatment modulation on the laser-induced ultrafast magnetic behaviors in antiferromagnetically exchange-coupled [Ni/Co]4/Ru/[Co/Ni]3 multilayers with perpendicular magnetic anisotropy (PMA). Magnetic hysteresis loops indicate that the uniaxial PMA field Hkeff decreases monotonously with the increase in annealing temperature Ta, but the variation of interlayer coupling field Hex is rather complicated. Time-resolved magnetic-optical Kerr effect (MOKE) measurements demonstrate that the laser-excited demagnetization and precession process relies significantly on Ta. Upon laser impulsion, the MOKE signal immediately shows a nonchanging transient increase and decrease with H increasing for low Ta, but only the ultrafast decreasing behavior for high Ta. From the subsequent dynamic precession spectra, the optical and acoustic precession modes are identified. By fitting the field-dependent frequency curves via the deduced dispersion relations, both Hkeff and Hex are determined and their variation trends agree well with the results from the static magnetic measurement. Moreover, it is found that the critical field where the ultrafast signal decrease occurs is dependent on the co-effect of Hkeff and Hex, whereas the maximum field at which the optical mode precession disappears shares the same trend as Hex. The magnetic damping of acoustic mode is seen to increase with Ta due to the increased inhomogeneities. Our findings provide a deep understanding of the magnetic properties in synthetic antiferromagnetic multilayers with high annealing temperatures, which will be helpful for designing advanced spintronic devices.

A novel method for time-resolved measurement of magnetization dynamics induced by femtosecond laser pulse in highly absorbing and metallic layer coated thin films based on a magnetic loop antenna

Femtosecond laser-induced magnetization dynamics is a popular research topic, due to its potential applications in fast magnetic storage devices and the field of spintronics. In the article we present a novel technique for time-resolved measurement of femtosecond laser-induced magnetization dynamics, based on inductive sensing by a loop antenna. It allows performing investigation of magnetization dynamics in highly absorbing samples, samples with patterned surfaces and samples coated by a thin metallic layer, which are difficult to study using optical techniques. To test the technique, both thick and thin films made of highly absorbing and diffusing materials, were used. Results of tests for several ferromagnetic materials including Gadolinium are discussed in the article. The test results prove that the method provides a simple tool to observe and study magnetization dynamics phenomena in the vicinity of the materials’ Curie temperature.

Time-Resolved Imaging of Magnetoelastic Waves by the Cotton-Mouton Effect

Time-resolved magneto-optical (TRMO) imaging with ultrashort laser pulses now allows direct observation of the excitation, propagation, and relaxation dynamics of magnetization, with the details of excitation still being studied. This article shows how to image the in-plane component of magnetization, modulated by propagating spin waves, via the birefringence that arises in a transverse magnetic field. The authors use this method to resolve the contribution from magnetoelastic coupling in an out-of-plane-magnetized film. This technique extends the scope of TRMO imaging for investigating laser-induced magnetization dynamics, which will enable work in spintronics.