Ultrafast Magnetization Research Articles

Polarization-dependent subpicosecond demagnetization in iron garnets

Controlling the magnetization dynamics at the fastest speed is a major issue of fundamental condensed matter physics and its applications for data storage and processing technologies. It requires a deep understanding of the interactions between the degrees of freedom in solids, such as spin, electron, and lattice as well as their responses to external stimuli. In this paper, we systematically investigate the fluence dependence of ultrafast magnetization dynamics induced by below-bandgap ultrashort laser pulses in the ferrimagnetic insulators ${\mathrm{Bi}}_{x}{\mathrm{Y}}_{3\ensuremath{-}x}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ with $1\ensuremath{\le}{x}_{\mathrm{Bi}}\ensuremath{\le}3$. We demonstrate subpicosecond demagnetization dynamics in this material followed by a very slow remagnetization process. We prove that this demagnetization results from an ultrafast heating of iron garnets by two-photon absorption (TPA), suggesting a phonon-magnon thermalization time of $\ensuremath{\le}0.6$ ps. We explain the slow remagnetization timescale by the low phonon heat conductivity in garnets. Additionally, we show that the amplitudes of the demagnetization, optical change, and lattice strain can be manipulated by changing the ellipticity of the pump pulses. We explain this phenomenon considering the TPA circular dichroism. These findings open exciting prospects for ultrafast manipulation of spin, charge, and lattice dynamics in magnetic insulators by ultrafast nonlinear optics.

Heat-conserving three-temperature model for ultrafast demagnetization in nickel

Multireservoir models are widely used for modeling and interpreting ultrafast magnetization dynamics. Here we introduce an alternative formulation to existing three-temperature models for the treatment of spin, electron, and lattice temperatures in magnetization dynamics simulations. In contrast to most existing models of calculations of energy transfer between reservoirs in these types of simulations, the heat distribution of the spin and lattice subsystems is evaluated during the simulation instead of being calculated a priori. The model is applied to investigate the demagnetization and remagnetization of fcc Ni, when subjected to a strong laser pulse. In particular, our model results in a fast interplay between the electron and spin subsystems which reproduces the main features of experimental data for fcc Ni significantly better than most reported three-temperature models. We also show that the way in which the electron, spin, and lattice heat capacities are described can have a significant impact on the simulated ultrafast dynamics. By introducing spin-lattice couplings in the simulation, it is shown that these explicit interactions only have a marginal impact on the magnetization dynamics of fcc Ni, albeit it is more pronounced for higher laser pulse powers.

Anisotropic laser-pulse-induced magnetization dynamics in van der Waals magnet Fe3GeTe2

Femtosecond laser-pulse excitation provides an energy efficient and fast way to control magnetization at the nanoscale, providing great potential for ultrafast next-generation data manipulation and nonvolatile storage devices. Ferromagnetic van der Waals materials have garnered much attention over the past few years due to their low dimensionality, excellent magnetic properties, and large response to external stimuli. Nonetheless, their behaviour upon fs laser-pulse excitation remains largely unexplored. Here, we investigate the ultrafast magnetization dynamics of a thin flake of Fe3GeTe2 (FGT) and extract its intrinsic magnetic properties using a microscopic framework. We find that our data is well described by our modeling, with FGT undergoing a slow two-step demagnetization, and we experimentally extract the spin-relaxation timescale as a function of temperature, magnetic field and excitation fluence. Our observations indicate a large spin-flip probability in agreement with a theoretically expected large spin–orbit coupling, as well as a weak interlayer exchange coupling. The spin-flip probability is found to increase when the magnetization is pulled away from its quantization axis, opening doors to an external control over the spins in this material. Our results provide a deeper understanding of the dynamics van der Waals materials upon fs laser-pulse excitation, paving the way towards two-dimensional materials-based ultrafast spintronics.

Ultrafast Demagnetization Control in Magnetophotonic Surface Crystals.

Magnetic memory combining plasmonics and magnetism is poised to dramatically increase the bit density and energy efficiency of light-assisted ultrafast magnetic storage, thanks to nanoplasmon-driven enhancement and confinement of light. Here we devise a new path for that, simultaneously enabling light-driven bit downscaling, reduction of the required energy for magnetic memory writing, and a subtle control over the degree of demagnetization in a magnetophotonic surface crystal. It features a regular array of truncated-nanocone-shaped Au-TbCo antennas showing both localized plasmon and surface lattice resonance modes. The ultrafast magnetization dynamics of the nanoantennas show a 3-fold resonant enhancement of the demagnetization efficiency. The degree of demagnetization is further tuned by activating surface lattice modes. This reveals a platform where ultrafast demagnetization is localized at the nanoscale and its extent can be controlled at will, rendering it multistate and potentially opening up so-far-unforeseen nanomagnetic neuromorphic-like systems operating at femtosecond time scales controlled by light.

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.

Controlling High-Frequency Spin-Wave Dynamics Using Double-Pulse Laser Excitation

Manipulating spin waves is highly required for the development of innovative data transport and processing technologies. Recently, the possibility of triggering high-frequency standing spin waves in magnetic insulators using femtosecond laser pulses was discovered, raising the question about how one can manipulate their dynamics. Here we explore this question by investigating the ultrafast magnetization and spin-wave dynamics induced by double-pulse laser excitation. We demonstrate a suppression or enhancement of the amplitudes of the standing spin waves by precisely tuning the time delay between the two pulses. The results can be understood as the constructive or destructive interference of the spin waves induced by the first and second laser pulses. Our findings open exciting perspectives towards generating single-mode standing spin waves that combine high frequency with large amplitude and low magnetic damping.

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.

Light-induced magnetization driven by interorbital charge motion in the spin-orbit assisted Mott insulator α−RuCl3

The authors show that an ultrafast photoinduced magnetization results from coherent charge motion between different orbitals of Ru${}^{{3}^{+}}$ ions in the honeycomb-lattice spin-liquid candidate \ensuremath{\alpha}-RuCl${}_{3}$.

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.

Magnetic resonance of plasmonic modulation and switchable routing in gated graphene waveguides and their robustness and broadband tunability

We analytically derived an explicit condition for high-contrast plasmonic modulation and switchable routing in coupled graphene waveguides consisting of two coupled gated graphene sheets sandwiching a gyrotropic medium. The analytical condition intuitively predicts that by manipulating the graphene chemical potential and the gyration of the in-between medium we can combine robustness of the plasmonic routing to the deviation of design and material parameters due to imperfect fabrication and harsh environment with the extreme high contrast. Also, it predicts broadband tunability of the high-contrast plasmonic routing. The intuitive analytical condition reveals that the high contrast is achievable at a resonance gyration due to a magnetic resonance of nonreciprocal directional coupling. The magnetic resonance of nonreciprocity breaks a currently held preconception that the stronger magnetic field induces the stronger magnetically induced nonreciprocity. Numerical simulations demonstrate the robustness of the magnetic resonance of nonreciprocity and its tunability over a one-octave spanning spectral range. In practice, the chemical potential and the gyration can be manipulated by controlling the gate voltage of the graphene and the external magnetic field, respectively. Our findings would provide a robust and broadband tunable tool for high-contrast plasmonic modulator and switchable router and for probing of ultrafast magnetization dynamics.

Theory of ultrafast magnetization of nonmagnetic semiconductors with localized conduction bands

The magnetization of a nonmagnetic semiconductor by femtosecond light pulses is crucial to achieve an all-optical control of the spin dynamics in materials and to develop faster memory devices. However, the conditions for its detection are largely unknown. In this paper, we identify the criteria for the observation of ultrafast magnetization and critically discuss the difficulties hindering its experimental detection. We show that ultrafast magnetization of a nonmagnetic semiconductor can be observed in compounds with very localized conduction band states and more delocalized valence bands, such as in the case of a p-d charge transfer gap. By using constrained and time-dependent density functional theory simulations, we demonstrate that a transient ferrimagnetic state can be induced in diamagnetic semiconductor ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$ via ultrafast pulses at realistic fluences. The ferrimagnetic state has opposite magnetic moments on vanadium (conduction) and oxygen (valence) states. Our methodology outruns the case of ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$ as it identifies the key requirements for a computational screening of ultrafast magnetism in nonmagnetic semiconductors.

Progress in ultrafast spintronics research

Ultrafast spintronics is one of the most promising fields for information processing technology innovation. When compared with traditional electronic devices, new spin-based devices have advantages such as low power consumption, high speed, and nonvolatility. Exploring novel physical phenomena and mechanisms relating to electronic spins significantly promotes the development of next-generation spintronic devices. In this review, we outline a sequence of advances in the fields of ultrafast magnetics and spintronics. First, a quick overview of spintronics and the discovery of ultrafast demagnetization induced by a femtosecond laser is provided. The proposed physical mechanisms and theories of ultrafast spin dynamics are then summarized, including the phenomenological three-temperature model, computations based on the time-dependent density functional theory, the Elliott-Yafet spin-flip mechanism, and the nonlocal superdiffusive spin transport model. Following that, the most recent experimental and theoretical studies on all-optical switching of ferrimagnetic and ferromagnetic metals are thoroughly reviewed. Furthermore, the discovery of hot-electron spin transport induced by ultrashort lasers as well as its influence on ultrafast magnetization dynamics, ultrafast spin transfer torque, spin injection into semiconductors, and spin accumulation and dissipation in hot-electron transport are described. The advancements in the optimization and manipulation of spintronics-based terahertz emitters are reviewed and analyzed. Finally, we discuss the current challenges in ultrafast spintronics research as well as possible future studies.

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.

All-Optical Switching on the Nanometer Scale Excited and Probed with Femtosecond Extreme Ultraviolet Pulses

Ultrafast control of magnetization on the nanometer length scale, in particular all-optical switching, is key to putting ultrafast magnetism on the path toward future technological application in data storage technology. However, magnetization manipulation with light on this length scale is challenging due to the wavelength limitations of optical radiation. Here, we excite transient magnetic gratings in a GdFe alloy with a periodicity of 87 nm by the interference of two coherent femtosecond light pulses in the extreme ultraviolet spectral range. The subsequent ultrafast evolution of the magnetization pattern is probed by diffraction of a third, time-delayed pulse tuned to the Gd N-edge at a wavelength of 8.3 nm. By examining the simultaneously recorded first and second order diffractions and by performing reference real-space measurements with a wide-field magneto-optical microscope with femtosecond time resolution, we can conclusively demonstrate the ultrafast emergence of all-optical switching on the nanometer length scale.

Strong momentum-dependent electron–magnon renormalization of a surface resonance on iron

The coupling of electrons to spin excitations and the generation of magnons is essential for spin mixing in the ultrafast magnetization dynamics of 3d ferromagnets. Although magnon energies are generally much larger than phonon energies, until now their electronic band renormalization effect in 3d ferromagnets suggests a significantly weaker quasiparticle interaction. Using spin- and angle-resolved photoemission, we show an extraordinarily strong renormalization leading to two-branch splitting of an iron surface resonance at ∼200 meV. Its strong magnetic linear dichroism unveils the magnetic nature and momentum dependence of the energy renormalization. By determining the frequency- and momentum-dependent self-energy due to generic electron–boson interaction to compute the resultant electron spectral function, we suggest that the surface-state splitting can be described by strong coupling to an optical spin wave in an iron thin film.

Laser MBE growth of cobalt-platinum multilayers with perpendicular magnetic anisotropy

Periodic multilayers with perpendicular magnetic anisotropy (PMA) attract a lot of attention nowadays being fundamentally interesting from the point of view of interface magnetism. They also provide a suitable test bench for the studies of spatially resolved magnetization dynamics at picosecond time scale conducted at x-ray free electron laser (XFEL) facilities. In the present paper we have developed a novel laser molecular beam epitaxy (LMBE) approach to grow Co/Pt multilayers on crystalline Al2O3 (0001) substrates. By applying Kerr-effect magnetometry to the samples fabricated at different conditions, we have found it essential for the appearance of PMA, to perform metal deposition in 0.03 mbar of argon and to keep Co layer thickness below 4 Å. Along with magnetic studies, we have investigated crystal structure and depth resolved layer composition of the fabricated Co/Pt multilayers by applying electron diffraction, x-ray diffraction and x-ray reflectometry. The combined atomic-force/magnetic-force microscopy studies have revealed a very low surface roughness of down to 1 Å (for the samples grown at properly optimized growth conditions) and a pronounced labyrinth pattern of magnetic domains with a periodicity of 250 nm appropriate for XFEL studies of ultrafast optical magnetization. The presented LMBE approach to fabricate magnetically ordered heterostructures with a pronounced PMA is supposed to be an important milestone on the way of facilitating future design of nanomaterials for applications in magnetic memories and for fundamental studies of ultrafast magnetization dynamics.

Asynchronous current-induced switching of rare-earth and transition-metal sublattices in ferrimagnetic alloys.

Ferrimagnetic alloys are model systems for understanding the ultrafast magnetization switching in materials with antiferromagnetically coupled sublattices. Here we investigate the dynamics of the rare-earth and transition-metal sublattices in ferrimagnetic GdFeCo and TbCo dots excited by spin-orbit torques with combined temporal, spatial and elemental resolution. We observe distinct switching regimes in which the magnetizations of the two sublattices either remain synchronized throughout the reversal process or switch following different trajectories in time and space. In the latter case, we observe a transient ferromagnetic state that lasts up to 2 ns. The asynchronous switching of the two magnetizations is ascribed to the master-agent dynamics induced by the spin-orbit torques on the transition-metal and rare-earth sublattices and their weak antiferromagnetic coupling, which depends sensitively on the alloy microstructure. Larger antiferromagnetic exchange leads to faster switching and shorter recovery of the magnetization after a current pulse. Our findings provide insight into the dynamics of ferrimagnets and the design of spintronic devices with fast and uniform switching.

Tunneling Magnetoresistance in Noncollinear Antiferromagnetic Tunnel Junctions.

Antiferromagnetic (AFM) spintronics has emerged as a subfield of spintronics driven by the advantages of antiferromagnets producing no stray fields and exhibiting ultrafast magnetization dynamics. The efficient method to detect an AFM order parameter, known as the Néel vector, by electric means is critical to realize concepts of AFM spintronics. Here, we demonstrate that noncollinear AFM metals, such as Mn_{3}Sn, exhibit a momentum dependent spin polarization which can be exploited in AFM tunnel junctions to detect the Néel vector. Using first-principles calculations, we predict a tunneling magnetoresistance (TMR) effect as high as 300% in AFM tunnel junctions with Mn_{3}Sn electrodes, where the junction resistance depends on the relative orientation of their Néel vectors and exhibits four nonvolatile resistance states. We argue that the spin-split band structure and the related TMR effect can also be realized in other noncollinear AFM metals like Mn_{3}Ge, Mn_{3}Ga, Mn_{3}Pt, and Mn_{3}GaN. Our work provides a robust method for detecting the Néel vector in noncollinear antiferromagnets via the TMR effect, which may be useful for their application in AFM spintronic devices.

Nanostructured Spintronic Emitters for Polarization-Textured and Chiral Broadband THz Fields

Spintronic THz emitters (STEs) rely on spin current acceleration or decay, resulting in a burst of broadband THz emission, offering a new route to THz optics. Here, we demonstrate novel metastructures of STEs for molding the vectorial focal distribution of the emitted THz radiations. The metastructures also allow controlling the characteristics of local fields. Performing combined micromagnetic-electromagnetic simulations, we demonstrate the generation of broadband THz fields with designed vectorial, chiral, magnetic, or topological features and discuss the potential of these fields for studying magnetic, multiferroic, and chiral structures. The STEs metastructure is shown to produce higher-order electric and magnetic multipoles as well as localized, longitudinal, broadband magnetoelectric THz pulses. Furthermore, subwavelength features of the near fields at a distance less than 10 μm can be tuned by steering the magnetization of the metastructure. The results point to a new type of engineered STEs for the generation of structured broadband THz fields, with potential applications in the fields of optoelectronics, optics, and ultrafast magnetism.

Influence of diffusive transport on ultrafast magnetization dynamics

We study the influence of transport effects on time- and space-resolved magnetization dynamics in a laser-excited thick nickel film. We explicitly include diffusive heat transport and spin-resolved charge transport as well as Seebeck and Peltier effects and calculate the dynamics of spin-dependent electronic temperatures, chemical potentials, lattice temperatures, and magnetization. We find that transport has an influence on the magnetization dynamics closer to the excited surface as well as in regions deeper than the penetration depth of the laser. We reveal that, for higher absorbed fluences and in the presence of transport, thick magnetic films show a quenching time nearly independent of depth, though the magnitude of quenching is depth-dependent.