Demagnetization Time Research Articles

Applying Machine Learning to Elucidate Ultrafast Demagnetization Dynamics in Ni and Ni80Fe20

Understanding the correlation between fast and ultrafast demagnetization (UFD) processes is crucial for elucidating the microscopic mechanisms underlying UFD, which is pivotal for various applications in spintronics. Initial theoretical models attempt to establish this correlation but face challenges due to the complex interplay of physical phenomena. To address this, a variety of machine learning (ML) methods are employed, including supervised learning regression algorithms and symbolic regression (SR), to analyze limited experimental data and derive meaningful mathematical expressions between demagnetization time (τM) and the Gilbert damping factor (α). The results reveal that polynomial regression and K‐nearest neighbors algorithms perform best in predicting τM. Additionally, variable‐selection sure‐independence‐screening‐and‐sparsifying‐operator (VS‐SISSO) as a SR method suggests a direct correlation between τM and α for Ni and Ni80Fe20, indicating spin‐flip scattering predominantly influences the UFD mechanism. The developed models demonstrate promising predictive capabilities, validated against independent experimental data. Comparative analysis between different materials underscores the significant impact of material properties on UFD behavior. This study underscores the potential of ML in unraveling complex physical phenomena and offers valuable insights for future research in ultrafast magnetism.

The rheological behavior of aqueous magnetic fluids over a wide range of shear rates

Abstract A rheological measurement system was constructed to investigate the rheological behavior of magnetic fluids under a wide range of shear rates, and its feasibility was verified. The system is capable of measuring a shear rate range spanning five orders of magnitude, with a maximum shear rate of 106 s−1. It was utilized to study the time required for aqueous magnetic fluids in a magnetic field to reach a steady state, taking into account the coupling effect of the flow field and magnetic field. Additionally, the time needed for the magnetic fluids to return to their initial state after demagnetization was also measured. Based on these measurements, the rheological behavior of magnetic fluids with varying concentrations and magnetic field directions was studied. Results indicate that the residence time of the magnetic fluids in the magnetic field and the de-magnetization time have almost no effect on their viscosity. When the magnetic field direction is perpendicular to the flow direction, regardless of concentration, aqueous magnetic fluids exhibit shear thinning behavior; when it is parallel to the flow direction, high-concentration aqueous magnetic fluids show shear thickening, while low-concentration ones behave as Newtonian fluids. In this study’s shear rate range, no Newtonian regions were found in either high- or low-shear rate regions.

Tuning ultrafast demagnetization dynamics in FePdPt ternary alloy films: the role of spin–orbit coupling

The influence of spin–orbit coupling (SOC) on ultrafast demagnetization dynamics is still an obscure issue. Ferromagnetic ternary alloy film containing Platinum element has the advantage that SOC strength can be regulated flexibly by varying the component ratio, thus is an excellent platform for investigating the dynamical effect of SOC. Here the fluence and component dependences of ultrafast demagnetization dynamics in ternary alloy L10-Fe0.5(Pd1-xPtx)0.5 films are studied by using time-resolved magneto-optical Kerr spectroscopy. An excitation-fluence dependent transition of demagnetization dynamics shows the Elliott-Yafet (EY) spin-flip scattering as the dominant mechanism of ultrafast demagnetization. We analyze the dynamics by a compatible fitting model and extract multiple characteristic time constants. It is clearly demonstrated that the gradual enhancement of SOC in fact decreases the spin-flip scattering and slow down the demagnetization process. We further show that the demagnetization time τm scales with the SOC strength ξ approximately under a dependence of τm∝ξ1/3, while a linear dependence exists between the spin–lattice relaxation time τs-l and ξ-6. These findings clarify the role of SOC in EY-dominated demagnetization dynamics and demonstrate an approach for tuning the speeds and duration of ultrafast demagnetization by regulating SOC strength.

Ultrafast spin dynamics: From femtosecond magnetism to attosecond magnetism

Ultrafast spin dynamics is the study of the evolution of spin degrees of freedom on a time scale from picoseconds to attoseconds after being excited by an external field. With the development of laser technology, ultrafast spin dynamics has presented new opportunities for realizing ultrafast spintronic devices since 1996. However, despite decades of development, many aspects of femtosecond magnetism remain unclear. Understanding the parameters of these ultrafast spin dynamics processes requires experiments on an even faster timescale. Attosecond magnetism and the interaction of attosecond laser pulses with magnetic materials can reveal spin dynamics on a sub-femtosecond to attosecond time scale. In this review, we first introduce the significant research progress, including the mechanisms of ultrafast demagnetization, all-optical switching, ultrafast spin currents, and terahertz waves. Secondly, we analyze the problems in ultrafast spin dynamics, such as the unclear physical mechanisms of ultrafast demagnetization, the uncertain relationship between magnetic damping and ultrafast demagnetization time, and the unexplored anisotropic ultrafast demagnetization. Thirdly, we discuss the opportunities and challenges in attosecond magnetism. Finally, we analyze and discuss the future development and prospects of ultrafast spin dynamics.

A Novel-Driven Scheme Regarding to Current Dynamics Enhancement for Switched Reluctance Motor System

Aiming at lowering the cost and enhancing the current dynamics for switched reluctance motor (SRM) system, a novel driven scheme is proposed by combing the self-boost voltage converter (SBVC) and chopping limit active regulation (CLAR) current control strategy. First, the working modes of SBVC are analyzed during the one-phase and two-phase operation situations. And the DC-link voltage pump-up mechanism and capacitor parameters selection method are developed. Then, the CLAR current control strategy is performed to adjust the excitation voltage and demagnetization voltage and then shape the current waveform. Owing to the proposed current profile generation technique, the excitation time and demagnetization time can be actively adjusted to enhance the current dynamics under different mission profiles. Compared to the existing current dynamics enhancement strategy, the proposed operation strategy achieves the lower cost, better current tracking performance, lower torque ripple and wider speed range. Simulations and experiments are performed on a four-phase 8/6 SRM to verify the validity of the proposed operation strategy in different mission profiles.

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.

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.

Investigation of ultrafast demagnetization and Gilbert damping and their correlation in different ferromagnetic thin films grown under identical conditions

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 recorded at various pump excitation fluences revealing a fluence-dependent enhancement in both the demagnetization times and the damping factors. We confirm that the Curie temperature to magnetic moment ratio of a given system acts as a figure of merit for the demagnetization time, while the demagnetization times and damping factors show an apparent sensitivity to the density of states at the Fermi level for a given system. Further, from numerical simulations of the ultrafast demagnetization based on both the 3TM and the M3TM, we extract the reservoir coupling parameters that best reproduce the experimental data and estimate the value of the spin flip scattering probability for each system. We discuss how the fluence-dependence of inter-reservoir coupling parameters so extracted may reflect a role played by nonthermal electrons in the magnetization dynamics at low laser fluences.

Ultrafast behavior of induced and intrinsic magnetic moments in CoFeB/Pt bilayers probed by element-specific measurements in the extreme ultraviolet spectral range

The ultrafast and element-specific response of magnetic systems containing ferromagnetic $3d$ transition metals and $4d/5d$ heavy metals is of interest both from a fundamental as well as an applied research perspective. However, to date no consensus about the main microscopic processes describing the interplay between intrinsic $3d$ and induced $4d/5d$ magnetic moments upon femtosecond laser excitation exist. In this work we study the ultrafast response of CoFeB/Pt bilayers by probing element-specific, core-to-valence-band transitions in the extreme ultraviolet spectral range using high harmonic radiation. We show that the combination of magnetic scattering simulations and analysis of the energy- and time-dependent magnetic asymmetries allows us to accurately disentangle the element-specific response in spite of overlapping Co and Fe ${\mathit{M}}_{2,3}$ as well as Pt ${\mathit{O}}_{2,3}$ and ${\mathit{N}}_{7}$ resonances. We find a considerably smaller demagnetization time constant as well as much larger demagnetization amplitudes of the induced moment of Pt compared to the intrinsic moment of CoFeB. Our results are in agreement with enhanced spin-flip probabilities due to the high spin-orbit coupling localized at the heavy metal Pt, as well as with the recently formulated hypothesis that a laser-generated, incoherent magnon population within the ferromagnetic film leads to an overproportional reduction of the induced magnetic moment of Pt.

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.

Accelerated Ultrafast Magnetization Dynamics at Graphene/CoGd Interfaces.

Atomically thin graphene layers can act as a spin-sink material when adjacent to a nanoscale magnetic surface. The enhancement in the extrinsic spin-orbit coupling (SOC) strength of graphene plays an important role in absorbing the spin angular momentum injected from the magnetic surface after perturbation with an external stimulus. As a result, the dynamics of the excited spin system is modified within the magnetic layer. In this paper, we demonstrate the modulation of ultrafast magnetization dynamics at graphene/ferrimagnet interfaces using the time-resolved magneto-optical Kerr effect (TRMOKE) technique. Magnetically modified interfaces with a systematic increase in the number of graphene layers coupled with the 10 nm-thick Co74Gd26 layer are studied. We find that the variation in the dynamical parameters, i.e., ultrafast demagnetization time, remagnetization times, decay time, effective damping, precessional frequency, etc., observed at different time scales is interconnected. The demagnetization time and decay time for the ferrimagnet become approximately two times faster than the corresponding intrinsic values. We found a possible correlation between the demagnetization time and damping. The effect is more pronounced for the interfaces with monolayer graphene and graphite. The spin-mixing conductance is found to be approximately 0.8 × 1015 cm-2. The effect of SOC, pure spin current, the appearance of structural defects, and thermal properties at the graphene/ferrimagnet interface are responsible for the modifications of several dynamical parameters. This work demonstrates some important properties of the graphene/ferrimagnet interface which may unravel the possibilities of designing spintronic devices with elevated performance in the future.

Ultrafast magnetic scattering on ferrimagnets enabled by a bright Yb-based soft x-ray source

Development of ultrafast table-top x-ray sources that can map various spin, orbital, and electronic configurations and reordering processes on their natural time and length scales is an essential topic for modern condensed matter physics as well as ultrafast science. In this work, we demonstrate spatiotemporally resolved resonant magnetic scattering (XRMS) to probe the inner-shell 4d electrons of a rare-earth (RE) composite ferrimagnetic system using a bright > 200 e V soft x-ray high harmonic generation (HHG) source, which is relevant for future energy-efficient, high-speed spintronic applications. The XRMS is enabled by direct driving of the HHG process with power-scalable, high-energy Yb laser technology. The optimally phase-matched broadband plateau of the HHG offers a record photon flux ( > 2 × 1 0 9 p h o t o n s / s / 1 % bandwidth) with excellent spatial coherence and covers the entire resonant energy range of RE’s N 4 , 5 edges. We verify the underlying physics of our x-ray generation strategy through the analysis of microscopic and macroscopic processes. Using a CoTb alloy as a prototypical ferrimagnetic system, we retrieve the spin dynamics, and resolve a fast demagnetization time of 500 ± 126 f s , concomitant with an expansion of the domain periodicity, corresponding to a domain wall velocity of ∼ 750 m / s . The results confirm that, far from cross-contamination of low-energy absorption edges in multi-element systems, the highly localized states of 4 d electrons associated with the N 4 , 5 edges can provide high-quality core-level magnetic information on par with what can be obtained at the M edges, which is currently accessible only at large-scale x-ray facilities. The analysis also indicates the rich material-, composition-, and probing-energy-dependent driving mechanism of RE-associated multicomponent systems. Considering the rapid emergence of high-power Yb lasers combined with novel nonlinear compression technology, this work indicates potential for next-generation high-performance soft x-ray HHG-based sources in future extremely photon-hungry applications on the table-top scale, such as probing electronic motion in biologically relevant molecules in their physiological environment (liquid phase), and advanced coherent imaging of nano-engineered devices with 5 ∼ 8 n m resolution.

Role of spin-lattice coupling in ultrafast demagnetization and all optical helicity-independent single-shot switching in Gd1−x−yTbyCox alloys

All optical switching (AOS) of magnetization exhibits a high potential for ultrafast and energy-efficient memory applications. Many works have been carried out in the area of AOS, including its observation in a wide variety of ferromagnetic or ferrimagnetic materials, and the exploration of the parameters for the achievements of AOS such as the laser fluence and helicity, and duration of laser pulses. A large majority of all optical helicity-independent single-shot switching (AO-HIS) has been observed in Gd-based rare-earth transition-metal ferrimagnets. It is then necessary to explore the unique role of Gd in AO-HIS mechanism, compared with other rare-earth elements. Here, we engineered ${\mathrm{Gd}}_{1\ensuremath{-}x\ensuremath{-}y}{\mathrm{Tb}}_{y}{\mathrm{Co}}_{x}$ alloys and investigated the influence of the Tb concentration on the magnetization dynamics via static Kerr microscope and time-resolved magneto-optical Kerr effect (TR-MOKE) measurements. The ultrafast demagnetization time at low fluence is found to be independent of Tb concentration, while both the range of laser fluence and pulse duration allowing for AO-HIS becomes narrower with increasing the Tb concentration. The TR-MOKE signal $\mathrm{\ensuremath{\Delta}}{\mathrm{\ensuremath{\Theta}}}_{K}/{\mathrm{\ensuremath{\Theta}}}_{K_\mathrm{sat}}\ensuremath{\sim}10\phantom{\rule{0.16em}{0ex}}\mathrm{ps}$ after the laser pulse excitation decreases with increasing either the Tb concentration or the pulse duration. The fact that AO-HIS is prohibited by increasing the Tb content is explained by considering a larger damping for Tb than Gd in atomistic simulations. Our results are well explained by the fact that angular momentum can be transferred from Gd to Co resulting in the magnetization switching, whereas for Tb it is dissipated through the lattice due to the large spin-orbit coupling, instead of being transferred between Tb and Co.

Novel Deperming Protocols to Reduce Demagnetizing Time and Improve the Performance for the Magnetic Silence of Warships

Magnetic silence of warships is necessary to prevent damage caused by the magnetic mines detecting the magnetic field of the warship. Anhysteretic and Deperm-ME protocols are used to reduce permanent magnetization among magnetic signals. However, they have some disadvantages. Therefore, this paper proposes an effective deperming protocol that is easily controlled and reduces the demagnetization time. A protocol composed of two Anhysteretic protocols is presented using the Preisach model to easily manage and ensure excellent performance. Each stage has its own advantages by considering the Preisach density distribution. In Stage 1, the existing magnetic history is erased, and the demagnetization time is reduced. In Stage 2, the demagnetization performance is improved. The effectiveness of the protocol was verified via simulations using the Preisach model and experiments using a specimen. When the proposed protocol was applied, the results were excellent when applying Anhysteretic and Deperm-ME. In addition, even if the number of magnetic fields was reduced by 4 and 8 in the proposed protocol, the demagnetization result was maintained. Therefore, if the proposed protocol is applied, excellent demagnetization results can be obtained and the time required to perform demagnetization can be reduced, thereby improving the operational capability of the warship.

Ultrafast magnetization dynamics in the half-metallic Heusler alloy Co2FeAl

We report on optically induced, ultrafast magnetization dynamics in the Heusler alloy ${\mathrm{Co}}_{2}\mathrm{FeAl}$, probed by time-resolved magneto-optical Kerr effect. Experimental results are compared to results from electronic structure theory and atomistic spin-dynamics simulations. Experimentally, we find that the demagnetization time (${\ensuremath{\tau}}_{M}$) in films of ${\mathrm{Co}}_{2}\mathrm{FeAl}$ is almost independent of varying structural order, and that it is similar to that in elemental $3d$ ferromagnets. In contrast, the slower process of magnetization recovery, specified by ${\ensuremath{\tau}}_{R}$, is found to occur on picosecond time scales, and is demonstrated to correlate strongly with the Gilbert damping parameter ($\ensuremath{\alpha}$). Based on these results we argue that for ${\mathrm{Co}}_{2}\mathrm{FeAl}$ the remagnetization process is dominated by magnon dynamics, something which might have general applicability.

Ultrafast demagnetization in NiCo2O4 thin films probed by time-resolved microscopy

Using a time-resolved magneto-optical Kerr effect microscope, we observed ultrafast demagnetization of inverse-spinel-type ferrimagnet NiCo2O4 (NCO) epitaxial thin films with perpendicular magnetic anisotropy. This microscope uses a pump-probe method, where the sample is pumped at 1030 nm, and magnetic domain images are acquired via magneto-optical Kerr effect microscopy at 515 nm (the second harmonic). We observed the dynamics of the magnetic domain of the NCO thin film via laser irradiation and obtained a demagnetization time constant of approximately 0.4 ps. This time constant was significantly smaller than the large time constants reported for other half-metallic oxides. This timescale of ∼0.4 ps agrees with the spin polarization of ∼0.7 determined by tunnel magnetoresistance [Shen et al., Appl. Phys. Lett. 117, 042408 (2020)].

Giant anisotropic Gilbert damping versus isotropic ultrafast demagnetization in monocrystalline Co50Fe50 films

The magnetization orientation dependence of Gilbert damping and ultrafast demagnetization were investigated in single-crystalline ${\mathrm{Co}}_{50}{\mathrm{Fe}}_{50}$ films grown on a MgO(100) substrate by the time-resolved magneto-optical Kerr effect technique. The intrinsic Gilbert damping coefficient extracted from the time evolution of magnetization precessions shows a remarkably large anisotropy with a ratio of more than 300% for magnetization orientations along the $\ensuremath{\langle}100\ensuremath{\rangle}$ and $\ensuremath{\langle}110\ensuremath{\rangle}$ axes. Such a large anisotropy of Gilbert damping persists to high frequencies up to $50\phantom{\rule{4pt}{0ex}}\mathrm{GHz}$, where the effect of two-magnon scattering is suppressed. In contrast to the anisotropic Gilbert damping, the ultrafast demagnetization time and ratio for different in-plane magnetization orientations are nearly isotropic with the magnetization orientation. Although anisotropic Gilbert damping can be explained by the variation of spin-orbit coupling with the magnetization orientations, our results demonstrate that the role of spin-orbit coupling in the high nonequilibrium state after ultrafast laser excitation is isotropic in driving ultrafast spin-flip processes.

Spin Dynamics Slowdown near the Antiferromagnetic Critical Point in Atomically Thin FePS3.

Two-dimensional (2D) magnetic materials have attracted much recent interest with unique properties emerging at the few-layer limit. Beyond the reported impacts on the static magnetic properties, the effects of reducing the dimensionality on the magnetization dynamics are also of fundamental interest and importance for 2D device development. In this report, we investigate the spin dynamics in atomically thin antiferromagnetic FePS3 of varying layer numbers using ultrafast pump-probe spectroscopy. Following the absorption of an optical pump pulse, the time evolution of the antiferromagnetic order parameter is probed by magnetic linear birefringence. We observe a strong divergence in the demagnetization time near the Néel temperature. The divergence can be characterized by a power-law dependence on the reduced temperature, with an exponent decreasing with sample thickness. We compare our results to expectations from critical slowing down and a two-temperature model involving spins and phonons and discuss the possible relevance of spin-substrate phonon interactions.

Anisotropic ultrafast spin dynamics in epitaxial cobalt

We investigate the ultrafast spin dynamics in an epitaxial hcp(11¯00) cobalt thin film. By performing pump–probe magneto-optical measurements with the magnetization along either the easy or hard magnetic axis, we determine the demagnetization and recovery time for the two axes. We observe an average of 33% slower dynamics along the easy magnetization axis, which we attribute to magneto-crystalline anisotropy of the electron–phonon coupling, supported by our ab initio calculations. This points toward an unambiguous and previously undisclosed role of anisotropic electron–lattice coupling in ultrafast magnetism.

Ultrafast Spin Dynamics of Electrochemically Grown Heusler Alloy Films

The electrochemical growth of Heusler alloy film with good morphological quality and crystalline order using single-crystalline substrate is demonstrated. Static magneto optical Kerr effect studies are employed to reveal the surface magnetization reversal of the films. An understanding of the intrinsic nature of the magnetization dynamics in this class of electrochemically grown materials is presented using time-resolved magneto optical Kerr effect measurements, under femtosecond laser excitation. Excitation laser fluence dependence study reveals the ultrafast demagnetization time, fast remagnetization time, and magnetic damping parameter as well as their correlation. © 2021 American Chemical Society. All rights reserved.