Time-resolved Magneto-optical Kerr Effect Research Articles

Dynamical behaviour of ultrathin [CoFeB (tCoFeB)/Pd] films with perpendicular magnetic anisotropy

CoFeB-based ultrathin films with perpendicular magnetic anisotropy are promising for different emerging technological applications such as nonvolatile memories with low power consumption and high-speed performance. In this work, the dynamical properties of [CoFeB (tCoFeB)/Pd (10 Å)]5 multilayered ultrathin films (1 Å ≤ tCoFeB ≤ 5 Å) are studied by using two complementary methods: time-resolved magneto-optical Kerr effect and broadband ferromagnetic resonance. The perpendicular magnetization is confirmed for multilayers with tCoFeB ≤ 4 Å. The effective perpendicular magnetic anisotropy reaches a clear maximum at tCoFeB = 3 Å. Further increase of CoFeB layer thickness reduces the perpendicular magnetic anisotropy and the magnetization became in-plane oriented for tCoFeB ≥ 5 Å. This behaviour is explained by considering competing contributions from surface and magnetoelastic anisotropies. It was also found that the effective damping parameter αeff decreases with CoFeB layer thickness and for tCoFeB = 4 Å reaches a value of ~ 0.019 that is suitable for microwave applications.

Spin injection characteristics of Py/graphene/Pt by gigahertz and terahertz magnetization dynamics driven by femtosecond laser pulse

Spin transport characteristics of graphene have been extensively studied so far. The spin transport along the c-axis is however reported by rather limited number of papers. We have studied spin transport characteristics through graphene along the c-axis with permalloy(Py)/graphene(Gr)/Pt by gigahertz (GHz) and terahertz (THz) magnetization dynamics driven by femtosecond laser pulses. The relatively simple sample structure does not require electrodes on the sample. The graphene layer was prepared by chemical vapor deposition and transferred on Pt film. The quality of the graphene layer was characterized by Raman microscopy. Time-resolved magneto-optical Kerr effect is used to characterize gigahertz magnetization dynamics. Magnetization precession is clearly observed both for Pt/Py and Pt/Gr/Py. The Gilbert damping constant of Pt/Py was 0.015, indicating a spin pumping effect from Py to Pt. The Gilbert damping constant of Pt/Gr/Py was found to be 0.011, indicating that the graphene layer blocks spin injection. We also performed the measurement of THz emission for Pt/Py and Pt/Gr/Py. While a THz emission is clearly observed for Pt/Py, a substantial reduction of THz emission is observed for Pt/Gr/Py. With these two different experiments, and highly anisotropic resistivity of graphite, we conclude that the vertical spin transport is strongly suppressed by the graphene layer.

Resonant amplification of intrinsic magnon modes and generation of new extrinsic modes in a two-dimensional array of interacting multiferroic nanomagnets by surface acoustic waves.

Using time-resolved magneto-optical Kerr effect (TR-MOKE) microscopy, we demonstrate surface-acoustic-wave (SAW) induced resonant amplification of intrinsic spin-wave (SW) modes, as well as generation of new extrinsic or driven modes at the SAW frequency, in a densely packed two-dimensional array of elliptical Co nanomagnets fabricated on a piezoelectric LiNbO3 substrate. This system can efficiently serve as a magnonic crystal (MC), where the intrinsic shape anisotropy and the strong inter-element magnetostatic interaction trigger the incoherent precession of the nanomagnets' magnetization in the absence of any bias magnetic field, giving rise to the 'intrinsic' SW modes. The magnetoelastic coupling leads to a rich variety of SW phenomena when the SAW is launched along the major axis of the nanomagnets, such as 4-7 times amplification of intrinsic modes (at 3, 4, 7 and 10 GHz) when the applied SAW frequencies are resonant with these frequencies, and the generation of new extrinsic modes at non-resonant SAW frequencies. However, when the SAW is launched along the minor axis, a dominant driven mode appears at the applied SAW frequency. This reveals that the magnetoelastic coupling between SW and SAW is anisotropic in nature. Micromagnetic simulation results are in qualitative agreement with the experimental observations and elucidate the underlying dynamics. Our findings lay the groundwork for bias-field free magnonics, where the SW behavior is efficiently tuned by SAWs. It has important applications in the design of energy efficient on-chip microwave devices, SW logic, and extreme sub-wavelength ultra-miniaturized microwave antennas for embedded applications.

Femtosecond laser-induced spin dynamics in single-layer graphene/CoFeB thin films.

Graphene/ferromagnet hybrid heterostructures are important building blocks of spintronics due to the unique ability of graphene to transport spin current over unprecedented distances and possible increase in its spin-orbit coupling due to proximity and hybridization. Here, we present magnetization dynamics over a femtosecond to nanosecond timescale by employing an all-optical time-resolved magneto-optical Kerr effect technique in single-layer graphene (SLG)/CoFeB thin films with varying CoFeB thickness and compared them with reference CoFeB thin films without an SLG underlayer. Gilbert damping variation with CoFeB thickness is modelled to extract spin-mixing conductance for the SLG/CoFeB interface and isolate the two-magnon scattering contribution from spin pumping. In SLG/CoFeB, we have established an inverse relationship between ultrafast demagnetization time (τm) and the Gilbert damping parameter (α) induced by interfacial spin accumulation and pure spin-current transport via a spin pumping mechanism. This systematic study of ultrafast demagnetization in SLG/CoFeB heterostructures and its connection with magnetic damping can help to design graphene-based ultrahigh-speed spintronic devices.

Materials Engineering Enabled by Time-Resolved Magneto-Optical Kerr Effect for Spintronic Applications

As an updated version of the ultrafast pump–probe laser technique, the time-resolved magneto-optical Kerr effect (TR-MOKE) methodology enables the detection of magnetization dynamics with superb te...

Controllable intrinsic Gilbert damping in Pt buffered [Co/Ni]n multilayers with enhanced perpendicular magnetic anisotropy

We systematically investigate the magnetization precession processes of post-prepared and after-annealing Pt buffered [Co/Ni]n multilayers with adjustable perpendicular magnetic anisotropy (PMA) by time-resolved magneto-optical Kerr effect. The PMA of the [Co/Ni]n multilayers could be modulated by changing the number of Co/Ni periods n, Pt thickness tPt and annealing temperature Ta, respectively. With a fixed tPt, both uniaxial magnetic anisotropy Ku and intrinsic damping constant α0 could be modulated by the variation of n. We observed that α0 is linearly proportional to Ku, since both of them originate from the spin–orbit coupling (SOC). For a fixed n, with the increase of tPt, Ku exhibits a non-monotonic change behavior, but α0 increases monotonously. It is found that α0 shows a nonlinear positve correlation to Ku. It suggests that the SOC is not the only factor affecting the damping process. Finally, for the specific sample, both α0 and Ku also display the different variation behaviors with increasing Ta. In this case, the changes in film microstructure caused by the annealing have different contributions to Ku and α0, resulting in a negative correlation between Ku and α0. The findings could be helpful to design the potential application of [Co/Ni]n multilayers system in STT-MRAM with high density, low power and high thermal stability.

Tuning of the Magnetic Damping Parameter by Varying Cr Composition in Fe1–xCrx Alloy*Supported by the National Key Research and Development Program of China (Grant No. 2016YFA0300803), the National Natural Science Foundation of China (Grant Nos. 61427812 and 11774160), the Natural Science Foundation of Jiangsu Province of China (Grant No. BK20192006), the Fundamental Research Funds for the Central Universities (Grant No.

We investigate the magnetic damping parameter of Fe1 – xCrx thin films using the time-resolved magneto-optical Kerr effect technique. It is demonstrated that the overall effective damping parameter is enhanced with the increasing Cr concentration. The effective damping at high field α0 is found to be significantly enhanced when increasing the Cr concentration with the α0 = 0.159 in the Fe45Cr55 enhanced by 562% compared with that of α0 = 0.024 in the pure Fe film. This study provides a new approach of controlling the effective damping parameter with a desired magnitude via varying Cr composition.

All-optical probe of magnetization precession modulated by spin–orbit torque

Laser-induced magnetization precession modulated by an in-plane direct current was investigated in a W/CoFeB/MgO micron-sized strip using an all-optical time-resolved magneto-optical Kerr effect microscope. We observed a relatively large change in the precession frequency, owing to a current-induced spin–orbit torque. The generation efficiency of the spin–orbit torque was evaluated as −0.35 ± 0.03, which was in accordance with that evaluated from the modulation of damping. This technique may become an alternate method for the evaluation of spin–orbit torque.

Ultrafast laser induced precessional dynamics in antiferromagnetically coupled ferromagnetic thin films

Antiferromagnetically coupled ferromagnetic thin films have recently attracted significant attention in magnonics because of the possibility to tune the spin-wave dispersion by altering the interlayer exchange coupling. To implement such coupled films in magnonic devices, a detailed understanding of the precessional dynamics of magnetization in such systems is required. Here, we present a systematic characterization of the precessional dynamics for systems with the layer magnetization going from nearly antiparallel to parallel alignment in a magnetic field. Experimentally, we have measured the ultrafast-laser-induced magnetization precession in samples with different interlayer exchange coupling strengths using the time-resolved magneto-optical Kerr effect. In our measurements, in addition to the acoustic and optical modes, an extra mode is observed that is due to the laser-induced decoupling of the two ferromagnetic layers. The observed precessional dynamics is in good agreement with our theoretical model based on the Landau-Lifshitz-Gilbert equation, and can be separated into three different field regions determined by the relationship between the Zeeman energy and the energy associated with the interlayer exchange coupling. This systematic paper, which gives a detailed description of how the interlayer exchange coupling and Zeeman energy influence the precessional dynamics, provides an important guide to implement antiferromagnetically coupled films in functional magnonic devices.

Towards ultraefficient nanoscale straintronic microwave devices

We investigate the ultrafast magnetization dynamics of individual nanomagnets resonantly excited by surface acoustic waves (SAWs) generated nonlocally by nonmagnetic phononic gratings. Using a time-resolved magneto-optic Kerr effect microscope, we report the dependence of the magnetoelastic resonance on SAW wavelength and nanomagnet size. Our measurements show that the precession amplitude of single nanomagnets increases monotonically and nonlinearly with decreasing sample size. In addition, for nanomagnets below a critical size we find that the oscillation amplitude increases with SAW frequency. Field-swept measurements of the magnetization dynamics reveal that the damping of the magnetoelastic resonance also decreases with the size of the magnet, ultimately reaching a minimum value determined by the Gilbert damping. This work shows that acoustically driven spin dynamics possess favorable scaling characteristics, which supports the notion that efficient, high-frequency (10--100 GHz) nanoscale straintronic devices limited only by $\ensuremath{\alpha}$ are feasible.

Large anisotropy of magnetic damping in amorphous CoFeB films on GaAs(001)

Amorphous CoFeB films grown on GaAs(001) substrates demonstrating significant in-plane uniaxial magnetic anisotropy were investigated by vector network analyzer ferromagnetic resonance. Distinct in-plane anisotropy of magnetic damping, with a largest maximum–minimum damping ratio of about 109%, was observed via analyzing the frequency dependence of linewidth in a linear manner. As the CoFeB film thickness increases from 3.5 nm to 30 nm, the amorphous structure for all the CoFeB films is maintained while the magnetic damping anisotropy decreases significantly. In order to reveal the inherent mechanism responsible for the anisotropic magnetic damping, studies on time-resolved magneto-optical Kerr effect and high resolution transmission electron microscopy were performed. Those results indicate that the in-plane angular dependent anisotropic damping mainly originates from two-magnon scattering, while the Gilbert damping keeps almost unchanged.

Ultrafast dynamics of exchange stiffness in Co/Pt multilayer

The exchange stiffness coefficient, Aex, represents the strength of direct exchange interactions among neighboring spins. Aex is linked to most of the magnetic properties such as skyrmion formation, magnetic vortex, magnetic domain wall width, and exchange length. Hence, the quantification of Aex is essential to understanding fundamental magnetic properties, but little is known for the dynamics of Aex on a sub-picosecond timescale. We report the ultrafast dynamcis of Aex in an ordered magnetic state in Co/Pt ferromagnetic multilayer. Time-resolved magneto-optical Kerr effect and reflectivity measurements were analyzed for various pump fluences. We reveal that the significant dynamical reduction of Aex is responsible for the dramatic increase of remagnetization time for high fluences. The analysis shows that Aex dynamically varies, strongly affecting overall ultrafast demagnetization/remagnetization process. The investigation demonstrates the possibility of Aex engineering in femtosecond timescale and thereby provides a way to design ultrafast spintronic devices.

Off-stoichiometry effect on magnetic damping in thin films of Heusler alloy Co2MnSi

We investigated the effect of off-stoichiometry on the Gilbert magnetic damping constant (\ensuremath{\alpha}) of Heusler alloy $\mathrm{C}{\mathrm{o}}_{2}\mathrm{MnSi}$ (CMS) thin films by employing time-resolved magneto-optical Kerr effect (TR-MOKE) measurements along with first-principles calculations based on the linear-response theory for magnetic damping. Because of the contribution of extrinsic damping arising from two-magnon scattering, the effective \ensuremath{\alpha} (${\ensuremath{\alpha}}_{\mathrm{eff}}$) extracted from the TR-MOKE responses showed dependences on the in-plane magnetic-field angle (${\ensuremath{\theta}}_{\mathrm{H}}$) and the field strength ($H$). Then, we obtained the smallest ${\ensuremath{\alpha}}_{\mathrm{eff}}$ (${\ensuremath{\alpha}}_{0}$) for each sample under the most reduced contribution from two-magnon scattering realized through varying ${\ensuremath{\theta}}_{\mathrm{H}}$ and $H$ values, which is the closest value to \ensuremath{\alpha}. The thus obtained ${\ensuremath{\alpha}}_{0}$ values of epitaxially grown off-stoichiometric $\mathrm{C}{\mathrm{o}}_{2}\mathrm{M}{\mathrm{n}}_{\ensuremath{\beta}}\mathrm{S}{\mathrm{i}}_{\ensuremath{\gamma}}$ ($\ensuremath{\gamma}=0.82$) films with various (Mn + Si) compositions, $(\ensuremath{\beta}+\ensuremath{\gamma})$, decreased with increasing $(\ensuremath{\beta}+\ensuremath{\gamma})$ from ${\ensuremath{\alpha}}_{0}=0.0057$ for (Mn + Si)-deficient $(\ensuremath{\beta}+\ensuremath{\gamma})=1.44$ to ${\ensuremath{\alpha}}_{0}=0.0036$ for $(\ensuremath{\beta}+\ensuremath{\gamma})=1.90$ being close to the stoichiometric one of $(\ensuremath{\beta}+\ensuremath{\gamma})=2.0$ at 300 K. It was also demonstrated that a half-metallic (Mn+Si)-rich CMS film with $\ensuremath{\beta}=1.30$ and $\ensuremath{\gamma}=0.90$ showed a low ${\ensuremath{\alpha}}_{0}$ of 0.0035. The dependence of ${\ensuremath{\alpha}}_{0}$ on $(\ensuremath{\beta}+\ensuremath{\gamma})$ in $\mathrm{C}{\mathrm{o}}_{2}\mathrm{M}{\mathrm{n}}_{\ensuremath{\beta}}\mathrm{S}{\mathrm{i}}_{\ensuremath{\gamma}}$ ($\ensuremath{\gamma}=0.82$) was well explained by the first-principles calculations. Through the systematic investigations of off-stoichiometric CMS with various values of $(\ensuremath{\beta}+\ensuremath{\gamma})$, it was clarified that the total density of states (DOS) at the Fermi level, $D({E}_{\mathrm{F}})$, plays the key role for determining the damping constant of CMS. Furthermore, it was revealed that the reduced minority-spin DOS at ${E}_{\mathrm{F}}$, caused by decreasing harmful $\mathrm{C}{\mathrm{o}}_{\mathrm{Mn}}$ antisites, is essential for reducing the damping constant of CMS. These findings demonstrate that appropriately controlling off-stoichiometry and film composition is thus promising for achieving half-metallicity and a low \ensuremath{\alpha} simultaneously for CMS thin films.

Modulation of magnetic damping in antiferromagnet/CoFeB heterostructures

The modulation of antiferromagnetic (AFM) material and thermal annealing treatment on the magnetic damping of various AFM/CoFeB (CFB) samples have been systematically studied with the time-resolved magneto-optical Kerr effect. It is found the saturated magnetic damping factor αs increases considerably after introducing a thin MnIr or MnPt AFM layer. As a thin Al spacer of tAl = 0–2 nm is inserted, αs is found to decrease rapidly, reaching nearly the same value as that of the single CFB film at tAl = 2 nm. The result suggests that the AFM layer is not a good spin sink material and the surprisingly strong decrease in the damping factor is mainly attributed to the reduced direct exchange coupling between CoFe and Mn spins at the AFM/FM interface. Moreover, in spite of the exchange bias effect occurring or not, a similar monotonic increasing trend of αs with the increasing AFM layer thickness is observed for the as-deposited and annealed AFM/CFB samples, indicating that the enhanced magnetic damping at an elevated annealing temperature is mainly related to the increased interface roughness and atomic diffusion. These findings provide deeper insights into the role of the AFM/FM interface in magnetization dynamics, which will be helpful for future spintronic applications.

Ultrafast Magnetization Precession in Perpendicularly Magnetized L10-MnAl Thin Films with Co2MnSi Buffer Layers**Supported by the National Key R&D Program of China (Grant No. 2018YFB0407601), the Key Research Project of Frontier Science of Chinese Academy of Sciences (Grant Nos. QYZDY-SSW-JSC015 and XDPB12), and the National Natural Science Foundation of China (Grant Nos. 11834013, 11874349, and 11774339).

Perpendicularly magnetized L10-MnAl thin films with Co2MnSi buffer layers were prepared on GaAs (001) substrates by molecular-beam epitaxy (MBE). The samples with high crystalline quality show a maximum uniaxial perpendicular magnetic anisotropy constant of 1.4 × 107 erg/cm3. Ultrafast spin dynamics with a magnetization precession frequency up to 200 GHz was investigated by using time-resolved magneto-optical Kerr effect (TRMOKE) measurements, from which the Gilbert damping constant α of epitaxial L10-MnAl thin films is evaluated to be less than 0.0175. This work provides an important reference for analyzing the current-induced magnetization switching process in MnAl-based spintronic devices.

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.

Parametric Amplification of Magnons in Synthetic Antiferromagnets

We demonstrate the parametric amplification of an acoustic magnon mode induced by an optical magnon mode in synthetic antiferromagnets, which was achieved by using the all-optical pump-probe time-resolved magneto-optical Kerr effect. The acoustic and optical modes with low and high frequencies, respectively, are clearly observed under different field directions and pump-laser powers. For a relatively high laser power, the acoustic mode shows a temporal increase in amplitude in the time domain; this is observed when the acoustic mode frequency is approximately half the frequency of the optical mode. Correspondingly, we also observe a large enhancement in the spectral density of the acoustic mode in the frequency domain. These data are supported by a numerical simulation using a macrospin model; furthermore, the optical mode amplitude threshold for achieving the acoustic mode amplification is also discussed. The parametric effect in synthetic antiferromagnets demonstrated here can be applied to nanoscale parametric amplifiers and oscillators of magnons, which are the building blocks for spintronic and magnonic computing beyond von Neumann architectures.

Interlayer transmission of magnons in dynamic spin valve structures

Magnonic devices are promising alternatives to conventional charge-current-driven spintronic devices. As the basic unit of spintronic devices, the spin valve is of limited use in magnonics because its dynamics is rarely studied. Here, we investigate the interlayer transmission of magnons in dynamic spin valve structures using the time-resolved magneto-optical Kerr effect. Interaction between magnons and the interfacial dissipation are studied by comparing three samples with different spin valve structures. Magnons with different intrinsic frequencies have strong interactions. In contrast, magnons with similar intrinsic frequencies have relatively weak interactions. Interfacial dissipations of magnons are increased by rare earth insertion, which can reduce the interactions between magnons indirectly. This work extends the application of spin valve structures to magnonic devices beyond their conventional use.

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.

Gilbert Damping in Annealed Perpendicular Exchange Biased CoFeB/Pt/MnIr Multilayers

Exchange bias and intrinsic Gilbert damping [Formula: see text] are two important parameters in magnetic multilayers. To explore the correlation between exchange coupling and damping, the samples Ta (2.6[Formula: see text]nm)/Pt (2.5[Formula: see text]nm)/Co[Formula: see text]Fe[Formula: see text]B[Formula: see text] (0.63[Formula: see text]nm)/Pt (0.5[Formula: see text]nm)/Mn[Formula: see text]Ir[Formula: see text] ([Formula: see text])/Ta (4[Formula: see text]nm) are deposited by using pulsed-DC magnetron sputtering. The static and dynamic magnetic responses with different thickness of antiferromagnetic MnIr are investigated using vibrating sample magnetometer (VSM) and the pump-probe time-resolved magneto-optical Kerr effect (TR-MOKE). The exchange bias initially increases and then decreases with increasing thickness of MnIr layer, which is associated with the interfacial exchange coupling. The dynamic results demonstrate that the Gilbert damping of CoFeB/Pt/MnIr can increase by up to 40% compared with CoFeB/Pt. Based on the fact that the spin pumping has been excluded because of the Pt interlayer, we attribute the enhancement in Gilbert damping to the exchange coupling between AFM and FM.