Time-resolved Magneto-optical Kerr Effect Technique Research Articles

Enhancing spin-transfer torque in magnetic tunnel junction devices: Exploring the influence of capping layer materials and thickness on device characteristics

We have developed and optimized two categories of spin-ransfer torque magnetic tunnel junctions (STT-MTJs) that exhibit a high tunnel magnetoresistance ratio, low critical current, high outputpower in the micro-watt range, and auto-oscillation behavior. These characteristics demonstrate the potential of STT-MTJs for low-power, high-speed, and reliable spintronic applications, including magnetic memory, logic, and signal processing. The only distinguishing factor between the two categories, denoted as A-MTJs and B-MTJs, is the composition of their free layers, two CoFeB/0.21 Ta/6 CoFeSiB for A-MTJs and two CoFeB/0.21 Ta/7 NiFe for B-MTJs. Our study reveals that B-MTJs exhibit lower critical currents for auto-oscillation than A-MTJs. We found that both stacks have comparable saturation magnetization and anisotropy field, suggesting that the difference in auto-oscillation behavior is due to the higher damping of A-MTJs compared to B-MTJs. To verify this hypothesis, we employed the all-optical time-resolved magneto-optical Kerr effect technique, which confirmed that STT-MTJs with lower damping exhibited auto-oscillation at lower critical current values. Additionally, our study aimed to optimize the STT-MTJ performance by investigating the impact of the capping layer on the device’s response to electronic and optical stimuli.

Concomitant modulation of interlayer exchange coupling and Gilbert damping in Fe/CoO with spin conductor Ag layer at interface

We report on the modulation of interlayer exchange coupling (IEC) between the ferromagnet (FM) Fe and insulating antiferromagnet (AFM) CoO and its impact on the Gilbert damping by a time-resolved magneto-optical Kerr effect technique. By inserting a wedge spin conductor Ag layer at the interface of Fe/CoO, it is revealed that both uniform spin precession frequency and Gilbert damping constant of Fe film decrease with increasing Ag thickness up to ∼2 nm, and above that with eliminated IEC, these two parameters reach the intrinsic values of the Fe film. The precession frequency and damping also show similar variation tendency with temperature for attenuated IEC. These results prove that the exchange coupling at the FM–AFM interface plays an essential role for the spin angular momentum transfer to the insulating AFM layer, and the spin pumping is insignificant even for the very thin spin conductor layer. Our findings may have general insights into the role of the FM/AFM interface in the magnetization dynamics and spin angular momentum transfer for future spintronic applications.

Ultra-long spin relaxation in two-dimensional ferromagnet Cr2Ge2Te6 flake

By utilizing time-resolved magneto-optical Kerr effect technique, we investigated both demagnetization and remagnetization processes of a typical two-dimensional (2D) van der Waals (vdW) ferromagnetic (FM) semiconductor Cr2Ge2Te6 (CGT) and two comparative Te-based crystals, i.e. three-dimensional (3D) FM metal Cr3Te4 (CT) and 2D vdW FM metal Fe3GeTe2 (FGT). Different from 3D CT showing one-step laser-induced demagnetization, two-steps demagnetization behavior is dominated in 2D vdW FGT and CGT, which indicates that the 2D vdW magnets have relatively weaker electron-spin coupling. Of particular interest, we discovered an ultra-long remagnetization behavior in the 2D vdW CGT flake. The laser pumping induced spin demagnetization would not recover even in the time scale of 3500 ps (experimental setup limitation). To the best of our knowledge, this is the longest remagnetization process observed so far and it might be one of the particular characterizations of 2D vdW ferromagnet semiconductors. Based on the modified three temperature model, the key roles played by the crystal dimensionality and thermal diffusion anisotropy were revealed in the spin dynamics of 2D vdW magnets. Our findings would not only open the door to the thoroughly understanding of the origin of the magnetism in 2D FM materials but also undoubtedly find suitable applications in spintronics and magnonics devices.

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.

Magnetic anisotropy manipulation and interfacial coupling in Sm3Fe5O12 films and CoFe/Sm3Fe5O12 heterostructures* *Project supported by the National Key Research and Development Program of China (Grant Nos. 2017YFA0303603 and 2016YFA0401803), the National Natural Science Foundation of China (Grant Nos. U2032218, 11574316, 11874120, 61805256, and 11904367), the Plan for Major Provincial Science & Technology Project (Grant No. 202003a05020018), and the Key Research

The magnetic anisotropy manipulation in the Sm3Fe5O12 (SmIG) films and its effect on the interfacial spin coupling in the CoFe/SmIG heterostructures were studied carefully. By switching the orientation of the Gd3Ga5O12 substrates from (111) to (001), the magnetic anisotropy of obtained SmIG films shifts from in-plane to out-of-plane. Similar results can also be obtained in the films on Gd3Sc2Ga3O12 substrates, which identifies the universality of such orientation-induced magnetic anisotropy switching. Additionally, the interfacial spin coupling and magnetic anisotropy switching effect on the spin wave in CoFe/SmIG magnetic heterojunctions have also been explored by utilizing the time-resolved magneto–optical Kerr effect technique. It is intriguing to find that both the frequency and effective damping factor of spin precession in CoFe/SmIG heterojunctions can be manipulated by the magnetic anisotropy switching of SmIG films. These findings not only provide a route for the perpendicular magnetic anisotropy acquisition but also give a further path for spin manipulation in magnetic films and heterojunctions.

Structural Phase-Dependent Giant Interfacial Spin Transparency in W/CoFeB Thin-Film Heterostructures.

Pure spin current has transformed the research field of conventional spintronics due to its various advantages, including energy efficiency. An efficient mechanism for generation of pure spin current is spin pumping, and high effective spin-mixing conductance (Geff) and interfacial spin transparency (T) are essential for its higher efficiency. By employing the time-resolved magneto-optical Kerr effect technique, we report here a giant value of T in substrate/W (t)/Co20Fe60B20 (d)/SiO2 (2 nm) thin-film heterostructures in the beta-tungsten (β-W) phase. We extract the spin diffusion length of W and spin-mixing conductance of the W/CoFeB interface from the variation of damping as a function of W and CoFeB thickness. This leads to a value of T = 0.81 ± 0.03 for the β-W/CoFeB interface. A stark variation of Geff and T with the thickness of the W layer is obtained in accordance with the structural phase transition and resistivity variation of W with its thickness. Effects such as spin memory loss and two-magnon scattering are found to have minor contributions to damping modulation in comparison to the spin pumping effect which is reconfirmed from the unchanged damping constant with the variation of Cu spacer layer thickness inserted between W and CoFeB. The giant interfacial spin transparency and its strong dependence on crystal structures of W will be important for future spin-orbitronic devices based on pure spin current.

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.

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.

Role of magnetic anisotropy in the ultrafast magnetization dynamics of Gd-Fe thin films of different thicknesses

The time-resolved magneto-optical Kerr effect technique has been exploited to study the magnetization dynamics over femtosecond to nanosecond timescale for Gd-Fe thin films with different thicknesses. The precessional magnetization dynamics is studied by varying the magnitude and orientation of the bias magnetic field from nearly in-plane to nearly out-of-plane (OOP) direction. The analysis of bias magnetic field dependence of precessional frequency has revealed the development of OOP anisotropy when the film thickness was increased from 20 to 100 nm and this is in good agreement with the static magnetic properties of the films. Moreover, the 100-nm-thick film has shown a signature of perpendicular standing spin wave modes along with the uniform Kittel mode. The evolution of their frequency, decay time, and damping with the magnitude and orientation of bias magnetic field is analyzed in detail, showing possible energy transfer between the perpendicular standing spin wave modes and the uniform Kittel mode. Interestingly, the effective damping could be tuned over a broader range of 0.02 to 0.15, which shows the potential for applications in miniaturized and fast magnetic storage, magnetic memory, and magnonic devices.

Piezostrain modulation of magnetic damping in MBE-grown epitaxial Co2FeAl/GaAs heterostructure

The electric voltage manipulated magnetic dynamic properties of Co2FeAl/GaAs heterostructure bonded on PbZr1−xTixO3 piezoelectric substrate were investigated using the time-resolved magneto-optical Kerr effect technique at room temperature. The butterfly-like response of precession frequency dependence on the applied electric field was observed with no external magnetic field applied, indicating efficient manipulation of precession frequency solely by piezostrain. The magnetic damping of Co2FeAl was observed to be enhanced (decreased) under the strengthened compressive (tensile) strain modulation. The observed efficient tuning of both precession frequency and extrinsic damping constant at low magnetic fields is attributed to piezostrain modulated magnetic anisotropy value and its inhomogeneous distribution. When the magnetic dynamics is governed by the external magnetic field at high-field regime, electric field tunability of precession frequency and damping constant becomes weaker and the intrinsic damping constant shows no modulation. The electric voltage tuning of magnetic damping and the associated physical mechanism presented in this work can help designing strain-mediated low-energy cost spintronic devices.

Interlayer modulation on the dynamic magnetic properties of L1-FePt/NM/[CoNi]5 composite film structures

The magnetization precession dynamics is systematically studied in L10-FePt/NM/[CoNi]5 exchange-coupled films using the time-resolved magneto-optical Kerr effect technique, where the nonmagnetic interlayer is Ta or Pt. It is demonstrated that the precession frequency f decreases dramatically with the increasing interlayer thickness of Ta (tTa) or Pt (tPt). By fitting the f ∼ H curves of different tTa with the coherent precession model including the interfacial coupling interaction, the exchange coupling strength as well as the magnetization orientations of FePt and [CoNi]5 are determined. Meanwhile, it is found the magnetic damping constant α can be modulated considerably as well by adjusting the interlayer thickness. For the Ta interlayer, the α value decreases gradually with the increase in tTa as the result of decreased coupling strength, while it shows a monotonic increasing behavior for the Pt layer. The increased α with tPt can be attributed to the enhanced spin pumping contribution from Pt, which greatly exceeds the decreasing tendency caused by the reduced interfacial coupling. This study provides deep insights into the dynamic behaviors of the exchange-coupled magnetic system, which are valuable to achieve effective control of the ultrafast magnetization dynamics for spintronic applications.

All-optical detection of interfacial spin transparency from spin pumping in β-Ta/CoFeB thin films.

Generation and utilization of pure spin current have revolutionized energy-efficient spintronic devices. Spin pumping effect generates pure spin current, and for its increased efficiency, spin-mixing conductance and interfacial spin transparency are imperative. The plethora of reports available on generation of spin current with giant magnitude overlook the interfacial spin transparency. Here, we investigate spin pumping in β-Ta/CoFeB thin films by an all-optical time-resolved magneto-optical Kerr effect technique. From variation of Gilbert damping with Ta and CoFeB thicknesses, we extract the spin diffusion length of β-Ta and spin-mixing conductances. Consequently, interfacial spin transparency is derived as 0.50 ± 0.03 from the spin Hall magnetoresistance model for the β-Ta/CoFeB interface. Furthermore, invariance of Gilbert damping with Cu spacer layer thickness inserted between β-Ta and CoFeB layers confirms the absence of other interface effects including spin memory loss. This demonstrates a reliable and noninvasive way to determine interfacial spin transparency and signifies its role in generation of pure spin current by spin pumping effect.

탄소가 도핑 된 FePt 박막에서의 펨토 초 펄스 레이저에 의한 자기 소거와 회복 분석

After preparing carbon-doped FePt films by dc magnetron sputtering, we observed ultrafast demagnetization and its recovery by means of a time-resolved magneto-optical Kerr effect technique. We confirm that the degree of ordering is decreased and coercivity is changed, as the carbon concentration increases. All samples are demagnetized within ~5 ps after the femtosecond laser pulse heated the sample. Interestingly, ultrafast relaxation time, which indicates fast magnetization recovery, increases as the carbon concentration increases due to the low spin-orbit coupling of carbon.

The Study of the Delay Kerr Loops Induced by Femtosecond Laser Pulses in GdFeCo Film

To study the evolution of the coercivity and magnetization induced by femtosecond laser pulses in GdFeCo film at distinct pump–probe delays and reveal the laser-induced magnetization reversal process, the static delay Kerr loops were measured by the traditional time-resolved magneto-optical Kerr effect technique. The results showed that the loops contained memory effect from external field history and could not reflect the real change of coercivity and magnetization after laser pump. To eliminate the memory effect, a home-made experimental setup of photomagnetic synchronized time-resolved magneto-optical Kerr effect technology was used to measure the dynamic delay Kerr loops and reveal the real evolution of the coercivity and magnetization after laser pump, whose results are of great importance in real magneto-optical recording application.

Magneto-optical observation of four-wave scattering in a 15-nm Ni81Fe19film during large-angle magnetization precession

Large-angle magnetization precession induced by a short pulsed magnetic field in a 15-nm thick Ni${}_{81}$Fe${}_{19}$ film is observed by use of a time-resolved magneto-optical Kerr effect technique with sensitivity to all three components of the magnetization vector. A reduction of the magnitude of the magnetization vector $|\mathbf{M}|$ during large-angle precession is inferred and indicates incoherent dynamics due to excitation of traveling spin waves with wavelengths smaller than the diameter of the sampling area, that is, the laser spot size for the magneto-optical measurements. The reduction in $|\mathbf{M}|$ depends on the magnetic bias field ${H}_{\mathrm{bias}}$, which can be qualitatively understood by comparison to the theoretical threshold for observing four-magnon scattering in a small time interval $t$. We estimate that two-magnon scattering cannot explain the experimental results.

Spin Dynamics of B2 and L21-Ordered Co2FeAl0.5Si0.5 Heusler Alloy Films

Measurements of magnetization precession are performed for L21 and B2-ordered Co2FeAl0.5Si0.5 full Heusler alloy films using the time-resolved magneto-optical Kerr effect technique in the out-of-plan configuration. Dependence of the precession frequency on the external magnetic field can be fitted by the Landau-Lifshitz-Gilbert equation. The chirp parameters are close to each other for the L21 and B2 structures, and decrease with the external magnetic field increasing. The relaxation rate of the B2-ordered film is smaller than that of the L21 one.