Magneto-optical Kerr Effect Measurements Research Articles

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.

Modified Martin-Puplett interferometer for magneto-optical Kerr effect measurements at sub-THz frequencies.

We present a magneto-optical Kerr effect (MOKE) spectrometer based on a modified Martin-Puplett interferometer, utilizing continuous wave sub-THz low-power radiation in a broad frequency range. This spectrometer is capable of measuring the frequency dependence of the MOKE response function, both the Kerr rotation and ellipticity, simultaneously, with accuracy limited by a sub-milliradian threshold, without the need for a reference measurement. The instrument's versatility allows it to be coupled to a cryostat with optical windows, enabling studies of a variety of quantum materials such as unconventional superconductors, two-dimensional electron gas systems, quantum magnets, and other systems showing optical Hall response at sub-Kelvin temperatures and in high magnetic fields. We demonstrate the functionality of the MOKE spectrometer using an undoped InSb wafer as a test sample.

Magneto-optical Kerr effect (MOKE) and magnetic force microscopy (MFM) studies on Cr/Ni nanodot arrays deposited using innovative nano-stencil method

Nanodot arrays of Nickel (Ni) on Chromium (Cr), [Cr in two forms i. e. (Cr dot) and Cr (layer)] were fabricated by electron beam evaporation technique using nano-stencil. The deposition of a periodic dot pattern was confirmed through atomic force microscopy (AFM) imaging. The nano magnetic properties of the Cr/Ni nanodot arrays were probed using both Magnetic Force Microscopy (MFM) and Magneto-Optical Kerr Effect (MOKE) magnetometer. MOKE measurements unveiled saturated hysteresis with the exchange bias effect for the in-plane configuration and a minor hysteresis loop for the out-of-plane configuration in the Cr/Ni nanodot system. The experimental results were found to be in good agreement with the theoretical Stoner-Wohlfarth (SW) model. Additionally, the exchange bias effect was also observed in the Cr (thin film)/Ni (nanodot) system for both the in-plane and out-of-plane configurations, showing the dominating behavior of the Cr layer over the Ni dots due to its continuous layer and higher thickness. The observation of exchange bias properties was also verified through MFM measurements conducted with both positive and negative in-plane magnetic fields. Hence, this straightforward nano-stencil method holds significant promise for the streamlined production of patterned AFM/FM arrays enabling the comprehensive study of exchange bias phenomena. The magnetic properties of the Cr/Ni structure can be manipulated by depositing Cr either in nanodot form or in continuous layer form.

Features of itinerant magnetism in Fe0.6Al0.4 film with B2-like short-range order

The complex relationship between spontaneous magnetization (M s) and structural parameters in Fe–Al systems should be attributed to the B2-like short-range order (SRO) and itinerancy-induced sensitivity of magnetism in B2 FeAl. We focused on the role of SRO in the magnetism of Fe0.6Al0.4 film with the B2-like SRO. The high-field susceptibility (χHF) was observed through magnetometry, X-ray magnetic circular dichroism, and magneto-optical Kerr effect measurement, through which different χHF values were obtained. The M s–T curves deviated from the Brillouin function, and it can be interpreted following the spin-wave theory with a low spin-wave stiffness of 0.75 meV nm2. Based on the observed χHF and M s trends, the magnetic moment in the B2-like SRO was found to be induced by the exchange field from the surrounding ferromagnetic region. The coexistence of weak and strong ferromagnetic regions can be a possible reason for the complex magnetism in the Fe–Al system.

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.

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.

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.

Measurement of the coupling of magnetism with electricity or light irradiation in BiFeO3 using the Kerr angle

The magneto-optical Kerr effect (MOKE) in monodomain bismuth ferrite (BiFeO3) thin films was investigated under applied electric fields and light irradiation. The field-swept MOKE measurements show that the Kerr rotation of BiFeO3 changes under applied electric fields or light irradiation. The piezoresponse force microscopy measurements found that the variation in electrical polarization at the BiFeO3 surface was similar under an applied electric field and ultraviolet irradiation, confirming that the Kerr rotation of BiFeO3 can be controlled using both electric fields and light irradiation. This paper presents a method to couple a magnetic field with an electric field or light irradiation via the Kerr angle measurements of BiFeO3, providing a concept for fabricating multifunctional devices in oxide electronics.

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.

Magneto-optic Kerr Effect Measurement of TbMn6Sn6 at mK Temperature

Novel electron states stabilized by Coulomb interactions attract tremendous interests in condensed matter physics. These states are studied by corresponding phase transitions occurring at extreme conditions such as mK temperatures and high magnetic field. In this work, we introduce a magneto-optical Kerr effect measurement system to comprehensively explore these phases in addition to conventional transport measurement. This system, composed of an all-fiber zero-loop Sagnac interferometer and in situ piezo-scanner inside a dilution refrigerator, operates below 100 mK, with a maximum field of 12 Tesla and has a resolution as small as 0.2 μrad. As a demonstration, we investigate TbMn6Sn6, where the manganese atoms form Kagome lattice that hosts topological non-trivial Dirac cones. We observed two types of Kerr signals, stemming from its fully polarized ferromagnetic ground state and positive charged carriers within the Dirac-like dispersion.

High-precision measurement of the complex magneto-optical Kerr effect using weak measurement

The present paper introduces a quantum weak measurement (WM) scheme for the measurement of the complex magneto-optical Kerr effect (MOKE). We achieve the simultaneous measurement of the Kerr rotation angle and the ellipticity in a single WM process by utilizing two auxiliary pointers derived from the same meter state. The experimental measurement precision for both the Kerr rotation angle and the ellipticity is capable of reaching 10−4 deg. This technique is also employed for the determination of the complex magneto-optical constant Q. The proposed method overcomes the limitation of acquiring the complex magneto-optical Kerr parameters through a multi-step measurement process, which was previously encountered. This breakthrough holds immense significance for efficiently measuring and applying the complex MOKE with high precision and cost-effectiveness.

Highly Efficient Spin-Orbit Torque Switching in Bi2Se3/Fe3GeTe2 van der Waals Heterostructures.

Topological insulators (TIs) have shown promise as a spin-generating layer to switch the magnetization state of ferromagnets via spin-orbit torque (SOT) due to charge-to-spin conversion efficiency of the TI surface states that arises from spin-momentum locking. However, when TIs are interfaced with conventional bulk ferromagnetic metals, the combination of charge transfer and hybridization can potentially destroy the spin texture and hamper the possibility of accessing the TI surface states. Here, we fabricate an all van der Waals (vdW) heterostructure consisting of molecular beam epitaxy grown bulk-insulating Bi2Se3 and exfoliated 2D metallic ferromagnet Fe3GeTe2 (FGT) with perpendicular anisotropy. By detecting the magnetization state of the FGT via anomalous Hall effect and magneto-optical Kerr effect measurements, we determine the critical switching current density for magnetization switching to be Jc ≈ 1.2 × 106 A/cm2, the lowest reported for the switching of a perpendicular anisotropy ferromagnet using Bi2Se3. From second harmonic Hall measurements, we further determine the SOT efficiency (ξDL) to be in the range of 1.8 ± 0.3 and 1.4 ± 0.08 between 5 and 150 K, comparable to the highest values reported for Bi2Se3. Our density functional theory calculations find that the weak interlayer interactions at the Bi2Se3/FGT interface lead to a weakened dipole at the interface and suppress the proximity induced magnetic moment on Bi2Se3. This enables direct access to the TI surface states contributed by the first quintuple layer, where the spins are singly degenerate with significant net in-plane spin polarization. Our results highlight the clear advantage of all-vdW heterostructures with weak interlayer interactions that can enhance SOT efficiency and minimize critical current density, an important step toward realizing next generation low-power nonvolatile memory and spintronic devices.

Current-Induced Magnetization Switching in Light-Metal-Oxide/Ferromagnetic-Metal Bilayers via Orbital Rashba Effect.

The orbital angular momentum (OAM) generation as well as its associated orbital torque is currently a matter of great interest in spin-orbitronics and is receiving increasing attention. In particular, recent theoretical work predicts that the oxidized light metal Cu can serve as an efficient OAM generator through its surface orbital Rashba effect. Here, for the first time, the crucial current-induced magnetic-field-free in-plane magnetization reversal is experimentally demonstrated in CoFeB/CuOx bilayers without any heavy elements. We show that the critical current density can be comparable to that of strong spin-orbit coupling systems with heavy metals (Pt and Ta) and that the magnetization reversal mechanism is governed by coherent rotation in the grains through the second-harmonic and magneto-optical Kerr effect measurements. Our results indicate that light metal oxides can play an equally important role as heavy metals in magnetization reversal, broadening the choice of materials for engineering spintronic devices.

Effect of thermal annealing on the magnetization reversal and spin dynamics in ferrimagnetic TbCo thin films

“Ferrimagnetic rare earth-transition metal alloys are promising materials for high-speed and low-power spintronic device applications due to their wide tunability in magnetic properties as a function of temperature and composition. Here, we report the influence of thermal annealing on the magnetization reversal and spin dynamics of ferrimagnetic Tb10Co90 thin films. We have used the co-sputtering technique for the deposition of two different thicknesses (20 nm and 25 nm) of the TbCo films. Annealing of the samples has been carried out in the vacuum at 350 °C for 1 h. The magnetization reversal and dynamic properties of the as-deposited and annealed films are studied using the magneto-optical Kerr effect and broadband ferromagnetic resonance measurements at room temperature, respectively. The hysteresis loops reveal an anisotropic reversal process. The anisotropy field reduces from 66 Oe to 38 Oe for 20-nm-thick films and 98 Oe to 31 Oe for 25-nm-thick films upon annealing. Consequently, thermal annealing has led to a large reduction (∼70 %) in inhomogeneous linewidth broadening of the ferromagnetic resonance modes. Gilbert damping constant is found to decrease from 0.027 to 0.021 for 20-nm-thick films, whereas it decreases from 0.043 to 0.019 for 25-nm-thick films. The results have been attributed to the enhanced crystallinity of the films as observed in the structural analysis using grazing incidence X-ray diffraction.”

Ultrafast laser-induced precession dynamics in perpendicular artificial ferrimagnetic [D022-Mn3Ga/Co2MnSi]5 superlattices

Artificial ferrimagnetic [D022-Mn3Ga/Co2MnSi]N superlattices ([Mn3Ga/CMS]N SLs) combining perpendicular magnetic anisotropy as well as exceptional thermal and magnetic stability hold promises in functional spintronic devices. However, the relevant precession dynamics are still lacking. Here, we report on the magnetic dynamic properties in [Mn3Ga/CMS]5 SLs investigated by the time-resolved magneto-optical Kerr effect (TRMOKE) measurements. The magnetization precession process and magnetic damping constant (α0) of [Mn3Ga/CMS]5 SLs rely heavily on the thickness of Mn3Ga layer (tMn3Ga). In addition, α0 is found to be higher with increasing tMn3Ga, but is not simply scaled by the uniaxial magnetic anisotropy (Ku), which can be ascribed to the contribution of spin–orbit interaction combined with the additional contributions, like spin-pumping. Furthermore, a large Ku value of 1.33 Merg/cm3 and a low α0 of 0.022 have been simultaneously obtained in [Mn3Ga/CMS]5 SL with tMn3Ga = 1.5 nm. This study contributes to the design of high-performance spintronic devices based on [Mn3Ga/CMS]N SLs.

Nonvolatile magnetization switching in a single-layer magnetic topological insulator

Magnetization in a ferromagnetic layer could be manipulated by the spin-orbit torque whose generation commonly relies on the spin-orbit coupling from the adjacent heavy-metal layer within the bilayer. The fact that the magnetic topological insulator possesses both the ferromagnetic order with perpendicular anisotropy and inherent spin-orbit coupling inspires to realize such a torque-induced magnetization switching without forming any heterostructure with other materials. Here, only using a single layer of magnetically-doped topological insulator Cr:(Bi,Sb)2Te3, we realize a magnetization switching only by applying a large dc current. Assisted by the magnetic history, such a switching behaves nonvolatile under zero field but becomes volatile otherwise, as consistently shown by magnetoelectric transports and magneto-optical Kerr effect measurements. Static and quasistatic current are found to be equivalent for the switching. We propose that this switching may associate with the torque resulted from the spin-orbit coupling and the compositional asymmetry in the Cr-profile of the single layer.

Effect of TaN buffer layer on the sensitivity of ASIC-integrated AMR sensors

This study investigates the impact of a TaN buffer layer on the performance of ASIC-integrated Anisotropic Magnetoresistive (AMR) position sensors. Our findings demonstrate that the inclusion of a TaN buffer layer significantly enhances the sensitivity and stability of the sensors, with a measured sensitivity of 0.678 ± 0.03 Gauss/mV. In contrast, devices without the buffer layer exhibit a notable decline in sensitivity, decreasing to 0.165 ± 0.02 Gauss/mV after 450℃ annealing. In addition, AMR sensors fabricated on non-ASIC substrates showed no such deterioration. Magneto-optical Kerr effect (MOKE) measurements reveal that the change in sensitivity is primarily attributed to variations in the anisotropy field. We propose that the increased anisotropy field is a result of magnetostatic energy induced by alterations in the PECVD-deposited SiO2 surface granular structure, supported by surface morphology characterization and X-ray diffraction experiments. These findings highlight the critical role of the TaN buffer layer in optimizing the performance and stability of ASIC-integrated AMR sensors.

Field-free spin-orbit torque switching in interlayer exchange coupled Co/Ta/CoTb

This study investigates a T-type field-free spin-orbit torque device with an in-plane magnetic layer coupled to a perpendicular magnetic layer via a non-magnetic spacer. The device utilizes a Co/Ta/CoTb structure, in which the in-plane Co layer and the perpendicular CoTb layer are ferromagnetically (FM) coupled through the Ta spacer. ‘T-type’ refers to the magnetization arrangement in the FM/spacer/FIM structure, where the magnetization in FM is in-plane, while in FIM, it is out-of-plane. This configuration forms a T-shaped arrangement for the magnetization of the two magnetic layers. Additionally, ‘interlayer exchange coupling (IEC)’ denotes the interaction between the two magnetic layers, which is achieved by adjusting the material and thickness of the spacer. Our results show that an in-plane effective field from the IEC enables deterministic current-induced magnetization switching of the CoTb layer. The field-driven and the current-driven asymmetric domain wall motion are observed and characterized by magneto-optic Kerr effect measurements. The functionality of multistate synaptic plasticity is demonstrated by understanding the relationship between the anomalous Hall resistance and the applied current pulses, indicating the potential for the device in spintronic memory and neuromorphic computing.

Ta interfaced CoFeB: Role of CoFeB thickness and thermal annealing in modification of structural and magnetic properties

CoFeB interfaced with heavy metals is a potential system for spintronic applications. In the present work, evolution of magnetic and structural properties of CoFeB interfaced with Ta have been studied as function of increasing thickness of magnetic film and increasing annealing temperature. While the evolution of structural properties was investigated using X-ray scattering measurements i.e., X-ray reflectivity (XRR) and grazing incidence X-ray Diffraction (GIXRD), the modification of magnetic properties were studied using vibrating sample magnetometer (VSM) and magneto-optical Kerr effect (MOKE) measurements. The detailed XRR analysis shows the diffusion of Boron to the bottom Ta layer with annealing, which increases significantly for post-crystallized phase. Such enriched B diffusion also increases the thickness of interfacial magnetic dead layer as confirmed from VSM results. The synchrotron XRD show an increased in-plane CoFe grain size relative to out-of-plane grain size, while CoFeB is interfaced with Ta. MOKE measurements confirm the presence of stress-induced uniaxial magnetic anisotropy in as-deposited CoFeB films, which reduces for Ta-interfaced CoFeB. Vacuum annealing well above the crystallization temperature eliminates such magnetic anisotropy with concurrent increase of magnetic coercivity (Hc) because of increasing grain size. CoFeB interfaced with Ta shows an abrupt enhancement of Hc, as compared to CoFeB without Ta interface.

Interfacial electronic structure modulated magnetic properties in Ta/CoFeB/Ta multilayers

Thinfilm heterostructure Ta/CoFeB/TaOx can exhibit chiral magnetic textures such as skyrmions as well as spin-orbit torque induced zero field magnetization switching, which offers great promise for next-generation information storage technologies. Here, we provide a strong correlation between the structural and magnetic properties in as-deposited and annealed Ta (1 nm)/ CoFeB (1.5 nm)/ TaOx (1 nm) trilayer and Ta (1 nm)/ CoFeB (1.5 nm)/ Ta (t)/ CoFeB (1.5 nm)/ TaOx (1 nm)] multilayered stack with t = 0.5, 1 nm. Interestingly, x-ray reflectivity results indicate improvement in the interface quality post annealing up to 300 °C. While the thinner Ta spacer layer (0.5 nm) in the stack causes the formation of smaller interface width, the thicker Ta spacer (1 nm) results in relatively larger interface width. Annealing results in the enhancement of saturation magnetization (Ms) and uniaxial magnetic anisotropy (UMA) and the extent of variation is mostly determined by thickness of the spacer Ta layer. Magneto-optical Kerr effect measurement suggests the existence of strong two-fold UMA in the film stack with Ta spacer layer 0.5 nm, however, its strength diminishes as the Ta spacer layer thickness is increased to 1.0 nm. Furthermore, the in-depth chemical/electronic investigations using x-ray photoelectron spectroscopy technique reveal the formation of additional chemical states such as TaB and TaOB at the interfaces. Remarkably, Boron diffusion and likely oxygen migration from the Fe-oxide/Co-oxide in CoFeB to neighboring Ta layers improves the metallic nature of Fe and Co due to annealing. Note that the thicker Ta spacer has a significant impact on the chemical transformation at the interface in comparison to that of thinner Ta spacer layer. Overall, in these Ta/CoFeB multilayered stacks, the thickness of the Ta spacer layer and annealing temperature play a crucial role in controlling interface quality that eventually governs the magnetic characteristics. These results provide new insight into the interfacial electronic structure modifications which are essential for controlling DMI and stabilization of skyrmions in Ta/CoFeB multilayers for future spintronic device applications.