Time-resolved Magneto-optical Kerr Effect Research Articles

Laser-induced coherent spin change due to spin-orbit coupling

Direct light-spin interactions, induced by spin-orbit coupling (SOC), enable ultrafast spin manipulation through coherent spin excitation, which is universal in magnetic materials due to the ubiquity of SOC. Despite earlier reports, coherent spin excitation has been overlooked, because of the challenges of interpreting magneto-optical Kerr effect (MOKE) signals and the assumption of optical dipole transitions. By combining time-resolved MOKE measurements with density functional theory calculations, we demonstrate that a transient MOKE peak during laser pulse includes coherent spin excitation. We observe a pronounced transient MOKE peak in Co/Pt bilayer and a weaker one in Co single layer across different probe beam wavelengths. Theoretical calculations reveal that the Co/Pt peak includes contributions from both coherent spin excitation and nonlocal transport whereas the Co peak contains the contribution from the coherent spin excitation exclusively, which is governed by anti-crossing physics. Our findings highlight the SOC-mediated coherent spin dynamics, advancing ultrafast spintronic technology.

Observation of Real-Time Spin-Orbit Torque Driven Dynamics in Antiferromagnetic Thin Film.

In the burgeoning field of spintronics, antiferromagnetic materials (AFMs) are attracting significant attention for their potential to enable ultra-fast, energy-efficient devices. Thin films of AFMs are particularly promising for practical applications due to their compatibility with spin-orbit torque (SOT) mechanisms. However, studying these thin films presents challenges, primarily due to the weak signals they produce and the rapid dynamics driven by SOT, that are too fast for conventional electric transport or microwave techniques to capture. The time-resolved magneto-optical Kerr effect (TR-MOKE) has been a successful tool for probing antiferromagnetic dynamics in bulk materials, thanks to its sub-picosecond (sub-ps) time resolution. Yet, its application to nanometer-scale thin films has been limited by the difficulty of detecting weak signals in such small volumes. In this study, the first successful observation of antiferromagnetic dynamics are presented in nanometer-thick orthoferrite films using the pump-probe technique to detect TR-MOKE signal. This paper report an exceptionally low damping constant of 1.5 × 10-4 and confirms the AFM magnonic nature of these dynamics through angular-dependent measurements. Furthermore, it is observed that electrical currents can potentially modulate these dynamics via SOT. Thefindings lay the groundwork for developing tunable, energy-efficient spintronic devices, paving the way for advancements in next-generation spintronic applications.

Development of L10-ordered FePt with low damping and large perpendicular magnetic anisotropy by engineering the nanostructure

THz spintronics is an emergent area of research aimed at bridging the gap between fifth- and sixth-generation wireless telecommunications by utilizing spintronic devices such as magnetic spin torque oscillators as a source of low powered THz emission. The realization of such devices using ferromagnetic metal thin films however requires magnetic materials with both large perpendicular magnetic anisotropy (PMA) and low Gilbert damping constants. In this Letter, we report on the development of L10-ordered FePt with an effective Gilbert damping constant as low as 0.033. Using time-resolved magneto-optical Kerr effect, we characterized the magnetization dynamics of continuous L10-ordered FePt grown on MgO and SrTiO3 substrates. By changing the substrate on which FePt is grown, the lattice mismatch and subsequent number of misfit dislocations at the interface and L10-ordering can be controlled. We found that fewer misfits and improved ordering in FePt lead to a reduced Gilbert damping constant due to reduced electron scattering but that FePt grown on SrTiO3 also shows robust perpendicular magnetic anisotropy. Importantly, these results demonstrate the ability to control the damping in FePt and similar materials by changing the number of misfit dislocations at the interface and the smaller damping in FePt opens up the possibility of using this material in spintronic materials in the THz wave range.

Above curie temperature ultrafast terahertz emission and spin current generation in a two-dimensional superlattice (Fe3GeTe2/CrSb)3

Abstract The increasing demand for denser information storage and faster data processing has fuelled a keen interest in exploring spin currents up to terahertz (THz) frequencies. The emergent two-dimensional (2D) intrinsic magnetic materials constitute a novel and highly controllable platform to access such femtosecond spin dynamics at atomic layer thickness. However, 2D van der Waals magnets are limited by their Curie temperatures (usually at low temperatures) to exhibit the functioning. Here, in a 2D superlattice (Fe3GeTe2/CrSb)3, we demonstrate ultrafast laser-induced spin current generation and THz radiation at room temperature, overcoming the challenge of its Curie temperature being only 206 K. In tandem with time-resolved magneto-optical Kerr effect measurements and first-principle calculations, we further elucidate the origin of the spin currents—a laser-enhanced proximity effect manifested as a laser-induced reduction of interlayer distance and enhanced electron exchange interactions, which causes transient spin polarization in the heterostructure. Our findings present an innovative, magnetic-element-free route for generating ultrafast spin currents in the 2D limit, underscoring the significant potential of laser THz emission spectroscopy in investigating laser-induced extraordinary spin dynamics.

Perpendicular magnetic anisotropy and Gilbert damping of MgO/Co100–x Fe x /Pt trilayers

Perpendicular magnetic anisotropy and Gilbert damping of MgO/Co100–x Fe x /Pt (x = 20, 50) trilayers before and after annealing at 200–400 °C were evaluated by hysteresis loop and time-resolved magneto-optical Kerr effect (TRMOKE) measurements. The anisotropy field of the trilayers increased with reducing the CoFe thickness, which reflects the anisotropy is originated from the interface. The annealing around 300 °C was effective to increase the anisotropy because the roughness of the MgO/CoFe interface was reduced by annealing. The effective Gilbert damping α eff also increased with reducing CoFe thickness and increasing annealing temperature. The increase of α eff was considered to be promoted by the interdiffusion between CoFe and Pt after annealing. Small α eff and large perpendicular anisotropy were confirmed to be obtained using Co50Fe50 compared to using Co80Fe20.

Role of Spin Transport on Ultrafast Spin Dynamics in Magnetic Weyl Semimetal (Co2MnGa)/Pt Heterostructures with High Spin‐Mixing Conductance

AbstractMagnetic Weyl semimetals are emerging as a new class of material systems for spintronics due to their intrinsic topological properties and the inherent interplay between topology and magnetism, giving rise to novel electronic states and spin‐orbital‐related effects. Here, ultrafast magnetization dynamics in the Weyl semimetal Heusler ferromagnet Co2MnGa is investigated, from femtosecond to nanosecond timescales, using time‐resolved magneto‐optical Kerr effect (TR‐MOKE) at room temperature. Ultrafast demagnetization and precessional dynamics corresponding to the Kittel and perpendicular standing spin‐wave (PSSW) modes of Co2MnGa, interfaced with Pt thin films of varying thickness, have been investigated. The exchange stiffness constant of Co2MnGa is determined, as well as detailed information about spin‐transport across the Co2MnGa/Pt interface. From the modulation of Gilbert damping of Co2MnGa with Pt thickness, a very high intrinsic spin‐mixing conductance, G↑↓ = 1.73 × 1016 cm−2 is extracted of the interface and the spin diffusion length of Pt is determined to be 2.9 ± 0.2 nm. The interfacial spin transparency is found to reach a sizeable value of ≈83%, in the perfect spin‐sink regime, suggesting the great potential of this heterostructure for spin‐orbitronics and devices relying on ultrafast magnetization dynamics.

Spin-wave emission using a V-shaped antenna

We investigated the dynamics of spin waves in micro-patterned Permalloy thin films using time-resolved magneto-optic Kerr effect microscopy (TR-MOKE). By applying an external magnetic field, we observe the field dependence of spin wave signals with picosecond resolution. Fourier transform analysis of the signals confirms their agreement with the dispersion relation, demonstrating the successful detection of propagating spin waves using the MOKE technique. Furthermore, we perform dynamic measurements of interfering spin waves generated by a V-shaped antenna. The experimental results reveal differences in spin wave amplitude at each detection point. In combination with simulation analysis based on wave propagation from the V-shaped antenna, we reproduced the experimental results and revealed the existence of a protective zone.

Picosecond Ultrasonics in Magnetic Topological Insulator MnBi2Te4.

MnBi2Te4 is a magnetic topological insulator with layered A-type antiferromagnetic order. It exhibits a rich layer- and magnetic-state dependent topological phase diagram; however, much about the coupling between spin, charge, and lattice remains to be explored. In this work, we report that MnBi2Te4 is an excellent acoustic phonon cavity by realizing phonon frequency combs using picosecond ultrasonics. With the generated acoustic phonon wavepackets, we demonstrate that the timing and phase of acoustic echoes can be used to detect the presence of stacking faults between van der Waals layers buried deep within the crystal. Furthermore, by implementing this nondestructive ultrafast optical measurement in conjunction with time-resolved magneto-optical Kerr effect experiments, we uncover that out-of-plane vibrations in MnBi2Te4 do not couple to the magnetic order, i.e. there is no appreciable magnetostriction. Our work points out how a well-developed technique can probe the structural defects and phonon pulse engineering in layered topological insulators.

Phase shift of coherent magnetization dynamics after ultrafast demagnetization in strongly quenched nickel thin films

The remagnetization process after ultrafast demagnetization can be described by relaxation mechanisms between the spin, electron, and lattice reservoirs. Thereby, collective spin excitations in form of spin waves and their angular momentum transfer play an important role on the longer timescales. In this work, we address the question whether the magnitude of demagnetization-the so-called quenching-affects the coherency and the phase of the excited spin waves. We present a study of coherent magnetization dynamics in thin nickel films after ultrafast demagnetization using the all-optical, time-resolved magneto-optical Kerr-effect technique. The largest coherent precession amplitude was observed for strongly quenched systems, indicating a well-defined precession phase for all pump pulses at a demagnetization of up to 90% in this system. Moreover, the phase of the excited spin-waves in Ni increases with the pump fluence, indicating a delayed start of the precession during the remagnetization. We compare these findings to recent studies in Ni80Fe20(permalloy), to evaluate the influence of the magneto-elastic coupling and non-linear spin-wave dynamics on the magnetization dynamics.

Distinguishing surface and bulk electromagnetism via their dynamics in an intrinsic magnetic topological insulator.

The indirect exchange interaction between local magnetic moments via surface electrons has been long predicted to bolster the surface ferromagnetism in magnetic topological insulators (MTIs), which facilitates the quantum anomalous Hall effect. This unconventional effect is critical to determining the operating temperatures of future topotronic devices. However, the experimental confirmation of this mechanism remains elusive, especially in intrinsic MTIs. Here, we combine time-resolved photoemission spectroscopy with time-resolved magneto-optical Kerr effect measurements to elucidate the unique electromagnetism at the surface of an intrinsic MTI MnBi2Te4. Theoretical modeling based on 2D Ruderman-Kittel-Kasuya-Yosida interactions captures the initial quenching of a surface-rooted exchange gap within a factor of two but overestimates the bulk demagnetization by one order of magnitude. This mechanism directly explains the sizable gap in the quasi-2D electronic state and the nonzero residual magnetization in even-layer MnBi2Te4. Furthermore, it leads to efficient light-induced demagnetization comparable to state-of-the-art magnetophotonic crystals, promising an effective manipulation of magnetism and topological orders for future topotronics.

Magnetization dynamics driven by displacement currents across a magnetic tunnel junction

Understanding the high-frequency transport characteristics of magnetic tunnel junctions (MTJs) is crucial for the development of fast-operating spintronics memories and radio frequency devices. Here, we present the study of a frequency-dependent capacitive current effect in CoFeB/MgO-based MTJs and its influence on magnetization dynamics using a time-resolved magneto-optical Kerr effect technique. In our device, operating at gigahertz frequencies, we find a large displacement current of the order of mA, which does not break the tunnel barrier of the MTJ. Importantly, this current generates an Oersted field and spin-orbit torque, inducing magnetization dynamics. Our discovery holds promise for building robust MTJ devices operating under high current conditions, also highlighting the significance of capacitive impedance in high-frequency magnetotransport techniques. Published by the American Physical Society 2024

Spin–wave dynamics in perpendicularly magnetized antidot multilayers

Using all-optical time-resolved magneto-optical Kerr effect measurements we demonstrate an efficient modulation of the spin–wave (SW) dynamics via the bias magnetic field orientation around nanoscale diamond shaped antidots that are arranged on a square lattice within a [Co(0.75 nm)/Pd(0.9 nm)]8 multilayer with perpendicular magnetic anisotropy (PMA). Micromagnetic modeling of the experimental results reveals that the SW modes in the lower frequency regime are related to narrow shell regions around the antidots, where in-plane (IP) domain structures are formed due to the reduced PMA, caused by Ga+ ion irradiation during the focused ion beam milling process of antidot fabrication. The IP direction of the shell magnetization undergoes a striking change with magnetic field orientation, leading to the sharp variation of the edge localized (shell) SW modes. Nevertheless, the coupling between such edge localized and bulk SWs for different orientations of bias field in PMA systems gives rise to interesting Physics and attests to new prospects for developing energy efficient and hybrid-system-based next-generation nanoscale magnonic devices.

Femtosecond Laser-Induced Transient Magnetization Enhancement and Ultrafast Demagnetization Mediated by Domain Wall Origami.

Femtosecond laser-induced ultrafast magnetization dynamics are all-optically probed for different remanent magnetic domain states of a [Co/Pt]22 multilayer sample, thus revealing the tunability of the direct transport of spin angular momentum across domain walls. A variety of different magnetic domain configurations (domain wall origami) at remanence achieved by applying different magnetic field histories are investigated by time-resolved magneto-optical Kerr effect magnetometry to probe the ultrafast magnetization dynamics. Depending on the underlying domain landscape, the spin-transport-driven magnetization dynamics show a transition from typical ultrafast demagnetization to being fully dominated by an anomalous transient magnetization enhancement (TME) via a state in which both TME and demagnetization coexist in the system. Thereby, the study reveals an extrinsic channel for the modulation of spin transport, which introduces a route for the development of magnetic spin-texture-driven ultrafast spintronic devices.

Speed limits of the laser-induced phase transition in FeRh

We use ultrafast x-ray diffraction and the polar time-resolved magneto-optical Kerr effect to study the laser-induced metamagnetic phase transition in two FeRh films with thicknesses below and above the optical penetration depth. In the thin film, we identify an intrinsic timescale for the light-induced nucleation of ferromagnetic (FM) domains in the antiferromagnetic material of 8ps, which is substantially longer than the time it takes for strain waves to traverse the film. For the inhomogeneously excited thicker film, only the optically excited near-surface part transforms within 8ps. For strong excitations, we observe an additional slow rise of the FM phase, which we experimentally relate to a growth of the FM phase into the depth of the layer by comparing the transient magnetization in frontside and backside excitation geometry. In the lower lying parts of the film, which are only excited via near-equilibrium heat transport, the FM phase emerges significantly slower than 8ps after heating above the transition temperature.

Evidence of Ferromagnetism and Ultrafast Dynamics of Demagnetization in an Epitaxial FeCl2 Monolayer.

The development of two-dimensional (2D) magnetism is driven not only by the interest of low-dimensional physics but also by potential applications in high-density miniaturized spintronic devices. However, 2D materials possessing a ferromagnetic order with a relatively high Curie temperature (Tc) are rare. In this paper, the evidence of ferromagnetism in monolayer FeCl2 on Au(111) surfaces, as well as the interlayer antiferromagnetic coupling of bilayer FeCl2, is characterized by using spin-polarized scanning tunneling microscopy. A Curie temperature (Tc) of ∼147 K is revealed for monolayer FeCl2, based on our static magneto-optical Kerr effect measurements. Furthermore, temperature-dependent magnetization dynamics is investigated by the time-resolved magneto-optical Kerr effect. A transition from one- to two-step demagnetization occurs as the lattice temperature approaches Tc, which supports the Elliott-Yafet spin relaxation mechanism. The findings contribute to a deeper understanding of the underlying mechanisms governing ultrafast magnetization in 2D ferromagnetic materials.

Terahertz electric-field-driven dynamical multiferroicity in SrTiO3

The emergence of collective order in matter is among the most fundamental and intriguing phenomena in physics. In recent years, the dynamical control and creation of novel ordered states of matter not accessible in thermodynamic equilibrium is receiving much attention1–6. The theoretical concept of dynamical multiferroicity has been introduced to describe the emergence of magnetization due to time-dependent electric polarization in non-ferromagnetic materials7,8. In simple terms, the coherent rotating motion of the ions in a crystal induces a magnetic moment along the axis of rotation. Here we provide experimental evidence of room-temperature magnetization in the archetypal paraelectric perovskite SrTiO3 due to this mechanism. We resonantly drive the infrared-active soft phonon mode with an intense circularly polarized terahertz electric field and detect the time-resolved magneto-optical Kerr effect. A simple model, which includes two coupled nonlinear oscillators whose forces and couplings are derived with ab initio calculations using self-consistent phonon theory at a finite temperature9, reproduces qualitatively our experimental observations. A quantitatively correct magnitude was obtained for the effect by also considering the phonon analogue of the reciprocal of the Einstein–de Haas effect, which is also called the Barnett effect, in which the total angular momentum from the phonon order is transferred to the electronic one. Our findings show a new path for the control of magnetism, for example, for ultrafast magnetic switches, by coherently controlling the lattice vibrations with light.

Spatial control of hybridization-induced spin-wave transmission stop band

Spin-wave (SW) propagation close to the hybridization-induced transmission stop band is investigated within a trapezoid-shaped 200 nm thick yttrium iron garnet film using time-resolved magneto-optic Kerr effect microscopy and broadband spin-wave spectroscopy, supported by micromagnetic simulations. The gradual reduction of the effective field within the structure leads to local variations of the SW dispersion relation and results in a SW hybridization at a fixed position in the trapezoid where the propagation vanishes, since the SW group velocity approaches zero. By tuning external field or frequency, spatial control of the spatial stop band position and spin-wave propagation is demonstrated and utilized to gain transmission control over several microstrip lines.

Ultrafast magnetization enhancement via the dynamic spin-filter effect of type-II Weyl nodes in a kagome ferromagnet.

The magnetic type-II Weyl semimetal (MWSM) Co3Sn2S2 has recently been found to host a variety of remarkable phenomena including surface Fermi-arcs, giant anomalous Hall effect, and negative flat band magnetism. However, the dynamic magnetic properties remain relatively unexplored. Here, we investigate the ultrafast spin dynamics of Co3Sn2S2 crystal using time-resolved magneto-optical Kerr effect and reflectivity spectroscopies. We observe a transient magnetization behavior, consisting of spin-flipping dominated fast demagnetization, slow demagnetization due to overall half-metallic electronic structures, and an unexpected ultrafast magnetization enhancement lasting hundreds of picoseconds upon femtosecond laser excitation. By combining temperature-, pump fluence-, and pump polarization-dependent measurements, we unambiguously demonstrate the correlation between the ultrafast magnetization enhancement and the Weyl nodes. Our theoretical modelling suggests that the excited electrons are spin-polarized when relaxing, leading to the enhanced spin-up density of states near the Fermi level and the consequently unusual magnetization enhancement. Our results reveal the unique role of the Weyl properties of Co3Sn2S2 in femtosecond laser-induced spin dynamics.

Photoinduced Nonthermal Reduction of the Coercive Field in FePt and FePt0.84Rh0.16 Epitaxial Thin Films in the L10 Phase

The time-resolved magneto-optical Kerr effect in epitaxial thin films of the FePt compound and the FePt0.84Rh0.16solid solution with the perpendicular magnetic anisotropy on MgO (001) substrates has been studied. The evolution of hysteresis loops at short (100 fs–1 ns) and long (1–20 ms) time scales after the excitation by a femtosecond light pulse has been studied. Long-lived nonthermal reduction of the coercive field has been detected. The coercive field is recovered in several milliseconds. It has been proposed to explain the observed phenomenon by the excitation of high-Q-factor acoustic resonances in the substrate/film system and to the strong magnetoelastic interaction in FePt and FePt0.84Rh0.16films.

All-optical observation of giant spin transparency at the topological insulator BiSbTe1.5Se1.5/Co20Fe60B20 interface

The rise of three-dimensional topological insulators as an attractive playground for the observation and control of various spin-orbit effects has ushered in the field of topological spintronics. To fully exploit their potential as efficient spin-orbit torque generators, it is crucial to investigate the efficiency of spin injection and transport at various topological insulator/ferromagnet interfaces, as characterized by their spin-mixing conductances and interfacial spin transparencies. Here, we use all-optical time-resolved magneto-optical Kerr effect magnetometry to demonstrate efficient room-temperature spin pumping in Sub/BiSbTe1.5Se1.5(BSTS)/Co20Fe60B20(CoFeB)/SiO2 thin films. From the modulation of Gilbert damping with BSTS and CoFeB thicknesses, the spin-mixing conductances of the BSTS/CoFeB interface and the spin diffusion length in BSTS are determined. For BSTS thicknesses far exceeding the spin diffusion length, in the so-called “perfect spin sink” regime, we obtain an interfacial spin transparency as high as 0.9, promoting such systems as scintillating candidates for spin-orbitronic devices.