Kerr Effect Imaging Research Articles

Spin‐Orbit Torque Switching of Magnetization in Ultra‐Thick Ferromagnetic Layers

AbstractCurrent‐induced magnetization switching via spin‐orbit torque (SOT) holds great potential for applications in high‐speed and energy‐efficient magnetic memory and logic devices. In the extensively studied heavy metal/ferromagnet (HM/FM) SOT heterostructures, the thickness of the FM layer is typically restricted to a few nanometers or less due to the rapid spin dephasing, making it challenging to implement thermally stable memory cells with high density. In this study, it is demonstrated that this thickness constraint can be significantly alleviated by utilizing an oxide ferromagnet La0.67Sr0.33MnO3 (LSMO). Through electrical transport and magnetic optical measurements, it is found that the SOT can switch the magnetization in Pt/LSMO heterostructures even at an LSMO thickness of 35 nm, which is one order of magnitude larger than that for metallic FMs, such as CoFeB. Furthermore, based on the FM thickness dependence of the switching current and the domain switching type revealed by magnetic optical Kerr effect imaging (MOKE), a possible picture is proposed to describe the SOT switching in Pt/LSMO, which highlights the critical role of the domain wall propagation in the vertical direction. The work provides valuable insights into the behavior of SOT switching in ultra‐thick FM films, offering new possibilities for their practical applications.

Spin–Orbit Torque and Current‐Driven Switching in Pt100‐yTby/Co/AlOx Trilayers

AbstractTo decrease the energy consumption for the electrical manipulation of magnetization, the enhancement of the spin Hall effect through alloying is widely investigated, but the use of rare earth elements is rarely mentioned. This work reports the modification of the spin Hall effect on Pt by doping rare earth Tb atoms. The spin–orbit torque (SOT) performance is significantly enhanced in Pt100‐yTby alloyed heavy metal (HM) layer. Compared with the Tb‐free sample, the damping‐like effective field per unit current density increases to 1.9 times in the samples with Tb content between 5% and 10%. The critical current density for magnetization reversal is greatly reduced by 65% in a device with Pt87Tb13 HM layer and the in‐plane assistant field as small as ±20 Oe is sufficient for the deterministic switching in the same device. By magneto‐optical Kerr effect imaging, it is confirmed that the increased in‐plane field can effectively compensate the Dzyaloshinskii–Moriya interaction (DMI), which not only helps to reduce the critical current, but also facilitates the domain wall motion and is beneficial for the switching process. All results show that the Pt‐Tb alloy is a competitive candidate for low‐power spintronic devices.

Key Parameters for Detectivity Improvement of Low Noise Anisotropic Magnetoresistive Sensors Made of La2/3Sr1/3MnO3 Single Layers on Vicinal Substrates

The current trend in magnetoresistive sensors development is to increase the sensitivity of single sensing elements by using multilayer structures and to design them into arrays. Such arrays are designed to compensate the excess low frequency noise of individual elements, which limits their magnetic resolution. Here, we report the modeling, design, and fabrication of single layer anisotropic magnetoresistive (AMR) sensors using low noise epitaxial La2/3Sr1/3MnO3 (LSMO) oxide thin films deposited on vicinal SrTiO3 substrates. The fabrication process is simple, and the operation of the sensor is based on a step-induced uniaxial magnetic anisotropy, described using the Stoner–Wohlfarth model. A coherent magnetization reversal process is observed by magneto-optical Kerr effect imaging. A good agreement between experimental data and the expected sensor response confirms the correct operation of the device. Three main fabrication parameters, namely the vicinal angle of the substrate, the deposition temperature, the thin film thickness, and their effects on film anisotropy field and device detectivity have been studied. Detectivity levels as low as 1.4 nT Hz–1/2 at 1 Hz and 240 pT Hz–1/2 in the white noise region are achieved with a single Wheatstone bridge element operating at 310 K. Compared to GMR and AMR sensors, these results are promising for further development and for their use as single layer LSMO low field AMR sensors, including applications as implantable biomedical devices.

Perpendicular Magnetization Switching Driven by Spin‐Orbit Torque for Artificial Synapses in Epitaxial Pt‐Based Multilayers

AbstractPerpendicular magnetization switching driven by spin‐orbit torque (SOT) exhibits nonvolatility and adjustability, which has great potential applications in magnetic random‐access memory and neuromorphic computing. In this work, the SOT efficiency in the Pt (001)/NiFe (Py) and Pt (111)/Py bilayers is first investigated, where the single crystal Pt films and polycrystalline Py films are grown by molecular beam epitaxy and magnetron sputtering, respectively. The (001)‐oriented Pt sample shows a larger SOT efficiency than that of the (111)‐oriented one, which is mainly attributed to the facet‐dependent intrinsic spin Hall effect of the Pt layer related to the Berry curvature of the electrical band structure. Then, the epitaxial Pt (001)‐based perpendicularly magnetized multilayers are designed to study the perpendicular magnetization switching driven by SOT. A continuous and stable reversal is successfully achieved, providing a conceivable candidate for reliable and variable imitation of the artificial synapses. The magneto–optical Kerr effect imaging proves that the sustainable change of magnetization state originates from the multiple site domain nucleation and growth in the ferromagnetic layer. This work provides an efficient method to enhance the SOT efficiency, as well as employs the epitaxial thin films in artificial synaptic devices for neuromorphic computing.

Estimation of a surface magnetization direction of thin cylinders by magnetooptical Kerr effect

Magneto-optical Kerr effect (MOKE) imaging is a versatile technique for probing a surface magnetization of magnetic materials. Here, we investigate the influence of the polarizers' rotation on the resulting magnetooptical contrast of thin magnetic wires with cylindrical geometry. Employing MOKE microscopy, we image the magnetooptical contrast change and quantify the contribution of the spatially curved cylindrical surface to the overall magnetic contrast. The calculated magnetooptical contrast difference for various extinction angles between polarizer and analyzer provide an estimation of the surface magnetization orientation at curved surfaces. The magnetooptical analysis scheme can be extended to other non-planar magnetic surfaces.

Direct Visualization of a Domain Wall Pinning by Time-Resolved Microscopy in Amorphous Glass-Coated Microwires

Amorphous glass-coated microwires with positive magnetostriction are composite material with very fast domain wall (DW) propagation that can reach up to 18 km/s. Previously it was shown that such high DW velocity can be explained by the tilted orientation of a DW in static or quasi-static experimental conditions. Here we perform time-resolved observation of a DW in propagation. The shape of a DW is studied via magneto-optical Kerr effect (MOKE) imaging. Time-resolved MOKE microscope operating in the single-shot regime and a static mode is used to ascertain the role of surface pinning on DW velocity fluctuation. It is shown that a tilted DW undergoes complex DW distortions on a local scale.

Fully Compensated Synthetic Antiferromagnets with Pronounced Anomalous Hall and Magneto-Optical Responses

We report the realization of fully compensated synthetic antiferromagnets (SAFs) in $\mathrm{Pt}/\mathrm{Co}/\mathrm{Ru}/{\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Tb}}_{x}$ multilayers with perpendicular magnetic anisotropy, pronounced anomalous Hall resistances and magneto-optical responses. In particular, the properties of SAFs are systematically investigated through optimizing the thicknesses of the $\mathrm{Co}$ layer, the $\mathrm{Ru}$ spacer, and the concentration (x) of the ferrimagnet (FIM) ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Tb}}_{x}$ layers. The incorporation of the FIM ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Tb}}_{x}$ films into SAFs accelerates the search for a magnetic multilayer with fully compensated magnetism and exhibiting pronounced anomalous Hall resistance and magneto-optical signals. These advantages enable spin-orbit-torque switching and magneto-optical Kerr effect imaging experiments on the fully compensated SAFs to be readily investigated. Through incorporating FIMs into SAFs, we believe that the present material system could provide a promising platform not only for adequately addressing the fully compensated spin dynamics of SAFs at room temperature, but also for developing intriguing SAF-based spin-orbitronic devices.

Compensated Ferrimagnet Based Artificial Synapse and Neuron for Ultrafast Neuromorphic Computing

AbstractSpintronic devices are considered a possible solution for the hardware implementation of artificial synapses and neurons, as a result of their non‐volatility, high scalability, complementary metal‐oxide‐semiconductor transistor compatibility, and low power consumption. As compared to ferromagnets, ferrimagnet‐based spintronics exhibits equivalently fascinating properties that have been witnessed in ultrafast spin dynamics, together with efficient electrical or optical manipulation. Their applications in neuromorphic computing, however, have still not been revealed, which motivates the present experimental study. Here, by using compensated ferrimagnets containing Co0.80Gd0.20 with perpendicular magnetic anisotropy, it is demonstrated that the behavior of spin‐orbit torque switching in compensated ferrimagnets could be used to mimic biological synapses and neurons. In particular, by using the anomalous Hall effect and magneto‐optical Kerr effect imaging measurements, the ultrafast stimulation of artificial synapses and neurons is illustrated, with a time scale down to 10 ns. Using experimentally derived device parameters, a three‐layer fully connected neural network for handwritten digits recognition is further simulated, based on which, an accuracy of more than 93% could be achieved. The results identify compensated ferrimagnets as an intriguing candidate for the ultrafast neuromorphic spintronics.

Magnetic Properties and Growth‐Induced Anisotropy in Yttrium Thulium Iron Garnet Thin Films

AbstractRare‐earth iron garnets (REIG) have recently become the materials platform of choice for spintronic studies on ferrimagnetic insulators. However, thus far the materials studied have mainly been REIG with a single rare earth species such as thulium, yttrium, or terbium iron garnets. In this study, magnetometry, ferromagnetic resonance, and magneto‐optical Kerr effect imaging is used to explore the continuous variation of magnetic properties as a function of composition for YxTm3−x iron garnet (YxTm3−xIG) thin films grown by pulsed laser deposition on gadolinium gallium garnet substrates. It is reported that the tunability of the magnetic anisotropy energy, with full control achieved over the type of anisotropy (from perpendicular, to isotropic, to an in‐plane easy axis) on the same substrate. In addition, a nonmonotonic composition‐dependent anisotropy term is reported, which is ascribed to growth‐induced anisotropy similar to what is reported in garnet thin films grown by liquid‐phase epitaxy. Ferromagnetic resonance shows linear variation of the damping and the g‐factor across the composition range, consistent with prior theoretical work. Domain imaging reveals differences in reversal modes, remanant states, and domain sizes in YxTm3−x iron‐garnet thin films as a function of anisotropy.

Direct-Write of Nanoscale Domains with Tunable Metamagnetic Order in FeRh Thin Films.

We have directly written nanoscale patterns of magnetic ordering in FeRh films using focused helium-ion beam irradiation. By varying the dose, we pattern arrays with metamagnetic transition temperatures that range from the as-grown film temperature to below room temperature. We employ transmission electron microscopy, X-ray diffraction, and temperature-dependent transport measurements to characterize the as-grown film, and magneto-optic Kerr effect imaging to quantify the He+ irradiation-induced changes to the magnetic order. Moreover, we demonstrate temperature-dependent optical microscopy and conductive atomic force microscopy as indirect probes of the metamagnetic transition that are sensitive to the differences in dielectric properties and electrical conductivity, respectively, of FeRh in the antiferromagnetic (AF) and ferromagnetic (FM) states. Using density functional theory, we quantify strain- and defect-induced changes in spin-flip energy to understand their influence on the metamagnetic transition temperature. This work holds promise for in-plane AF-FM spintronic devices, by reducing the need for multiple patterning steps or different materials, and potentially eliminating interfacial polarization losses due to cross material interfacial spin scattering.

Rapid Kerr imaging characterization of the magnetic properties of two-dimensional ferromagnetic Fe3GeTe2

Van der Waals (vdW) ferromagnetic materials have attracted considerable attention in the nanomaterial community, which could provide a unique platform to study magnetism at the nanoscale. Along this direction, many interesting results have been reported, including the electric field control of magnetism and topological spin textures. In this report, we present a rapid and spatially resolved imaging method to study the dimensionality-dependent magnetic properties of Fe3GeTe2 (FGT) nanoflakes. Our method is named as polar magneto-optical Kerr imaging microscopy magnetometry (p-MIMM), which is made possible by analyzing the intensity evolution of wide-field polar magneto-optical Kerr effect (MOKE) images that were collected by varying magnetic fields, thicknesses, and temperatures. In particular, spatially resolved MOKE hysteresis loops can be acquired in the FGT nanoflakes with a submicrometer resolution. By analyzing the evolution of the relative (saturated) MOKE intensity as a function of temperature, we further study the critical exponent and universality class and its dependence on the FGT nanoflake thickness. Combining the polar MOKE images with the calculated MOKE hysteresis loops, a detailed magnetic phase diagram summarizing an evolution of the stripe domain, single domain, and paramagnetic state is further validated. Our results suggest that the wide-field p-MIMM can be conveniently used for rapidly examining the magnetic properties of versatile vdW magnetic materials.

Quantitative mapping of stray field planar component by tracking singular points in metallic indicator film

In this work we obtained experimental and simulated images of in-plane component of the stray field created by a permanent magnet using indicator film technique and magneto-optical Kerr effect imaging system. The applied uniform magnetic field and the presence of the source singular point leads to the appearance of the saddle point. The increase of magnetic field leads to convergence of the two points and they annihilation at a critical field. If a relatively weak magnetic field comparable to the coercivity is applied, magnetooptical image with noticeable hysteresis is formed. For the external magnetic field range exceeding the coercivity, we demonstrated the possibility to derive the topography of the stray field created by the permanent magnet by tracking the position of the singular points.

The role of uniaxial magnetic anisotropy distribution on domain wall tilting in amorphous glass-coated microwires

Magnetic properties of highly magnetostrictive amorphous glass-coated microwires are strongly correlated to the presence of a glass coating that introduces a spatially inhomogeneous stress field distribution. We investigate the influence of mechanical stresses on the inclination of magnetic domain walls in magnetic microwires. Magneto-optical Kerr effect imaging is used to compare the tilted orientation of the domain wall shape in as-cast and annealed microwires. Angular dependencies of magnetization loops measured by alternating gradient field magnetometry reveal that the change of domain wall tilting with annealing is related to the decrease of magnetic anisotropy with axial orientation. Finally, micromagnetic simulations are used to show that sufficiently high uniaxial magnetic anisotropy gives rise to the presence of observed charged domain walls with tilted orientation.

VI3 -a New Layered Ferromagnetic Semiconductor.

2D materials are promising candidates for next-generation electronic devices. In this regime, insulating 2D ferromagnets, which remain rare, are of special importance due to their potential for enabling new device architectures. Here the discovery of ferromagnetism is reported in a layered van der Waals semiconductor, VI3 , which is based on honeycomb vanadium layers separated by an iodine-iodine van der Waals gap. It has a BiI3 -type structure ( , No.148) at room temperature, and the experimental evidence suggests that it may undergo a subtle structural phase transition at 78 K. VI3 becomes ferromagnetic at 49 K, below which magneto-optical Kerr effect imaging clearly shows ferromagnetic domains, which can be manipulated by the applied external magnetic field. The optical bandgap determined by reflectance measurements is 0.6 eV, and the material is highly resistive.

Magneto-optic Kerr effect CCD imaging with polarization modulation technique

We have developed a magneto-optic Kerr effect (MOKE) imaging system with a charge-coupled-device (CCD) camera by using the rotating compensator technique. We chose optimal conditions of the rotation frequency of the compensator with stable rotation along with a CCD camera frame rate that allowed precise control of the exposure timing in order to link with the angle of the compensator. Precise timing management of the CCD exposure enables us to carry out repeated experiments, which greatly improves the signal-to-noise ratio of the longitudinal MOKE signal. We applied the technique to the material characterization of the Ni81 Fe19 thin film and its microstructure, and succeeded in evaluating the spatial variation of the complex magneto-optic constant Q of the sample. Because of its attractive advantages such as high-speed and compactness, the present method provides a novel platform for investigating the domain structures in various magnetic materials.

Spin-orbit torques in perpendicularly magnetized Ir22Mn78/Co20Fe60B20/MgO multilayer

The current-induced spin-orbit torques (SOTs) in the perpendicularly magnetized Ir22Mn78/Co20Fe60B20/MgO structures are investigated. The damping- and field-like torques are characterized using a harmonic technique. The spin Hall angle of Ir22Mn78 is determined to be θSHE = +0.057 ± 0.002. The SOT-driven magnetization switching is also demonstrated with the assistance of an external in-plane field. Furthermore, the magneto-optical Kerr effect imaging experiments show that the magnetization switching is realized through domain nucleation and domain wall motion. These results may promise potential practical applications in high-performance SOT devices based on the antiferromagnetic materials.

Helicity and field dependent magnetization dynamics of ferromagnetic Co/Pt multilayers

We present helicity and field dependent magnetization dynamics of ferromagnetic Co/Pt multilayers, suitable for all-optical helicity-dependent switching. Employing single-shot time-resolved magneto-optical Kerr effect imaging, our study demonstrates an ultra-fast quenching of the magnetization after a single 60 fs laser pulse excitation followed by a recovery. Full demagnetization occurs within 1 ps after laser excitation. The magnetization dynamics reveals a small helicity dependence caused by magnetic circular dichroism. When an external magnetic field is applied, a heat-assisted magnetization reversal occurs on a nanosecond time scale.

Correlation between static and dynamic magnetic properties of highly perpendicular magnetizedCo49Pt51thin films

The static and dynamic magnetic behavior of 5-nm-thick $\mathrm{C}{\mathrm{o}}_{49}\mathrm{P}{\mathrm{t}}_{51}$ films with strong perpendicular magnetic anisotropy (PMA) grown at different deposition temperatures $({T}_{\mathrm{d},\mathrm{CoPt}})$ was investigated using complementary techniques such as vibrating sample magnetometry (VSM), magneto-optical Kerr effect (MOKE) imaging, and Brillouin light scattering (BLS). Our previous study on these films demonstrated the evolution of phase transformation from an $A3$-disordered (hexagonal) to an $L{1}_{1}$-ordered (rhombohedral) structure against increasing ${T}_{\mathrm{d},\mathrm{CoPt}}$ from room temperature (RT) to 350\ifmmode^\circ\else\textdegree\fi{}C. Along these lines, the changes in the domain configuration, magnetization reversal, and spin wave behavior of the 5-nm-thick $\mathrm{CoPt}$ films due to varying ${T}_{\mathrm{d},\mathrm{CoPt}}$ are emphasized in this study. The VSM out-of-plane hysteresis loops confirmed the existence of strong PMA for all the $\mathrm{CoPt}$ films, irrespective of ${T}_{\mathrm{d},\mathrm{CoPt}}$. MOKE studies revealed that the films deposited at RT and at $150\phantom{\rule{0.16em}{0ex}}{}^{\ensuremath{\circ}}\mathrm{C}$ containing hard and soft magnetic areas, while the films grown at higher ${T}_{\mathrm{d},\mathrm{CoPt}}$, 250 and $350\phantom{\rule{0.16em}{0ex}}{}^{\ensuremath{\circ}}\mathrm{C}$, are more uniform and homogeneous. The MOKE findings are validated by the BLS spectra in terms of high and low frequency lines corresponding to the hard and soft magnetic areas, respectively. A suitable model is hypothesized to interpret the frequency variation of BLS modes corresponding to the easy saturated regions of the $\mathrm{CoPt}$ films. By this means, a good correlation between both static and dynamic behavior of the 5-nm-thick $\mathrm{CoPt}$ films has been established in this study.

Two-dimensional control of field-driven magnetic bubble movement using Dzyaloshinskii–Moriya interactions

The field-induced asymmetric growth of magnetic bubble domains in Pt/Co/Pt out-of-plane magnetized films with Dzyaloshinskii–Moriya interactions (DMI) is used to control the lateral displacement of bubbles. We demonstrate experimentally that we can laterally translate bubbles away from their nucleation site by applying a series of alternating 3-dimensional field pulses with a controlled relative sign between the out-of-plane and in-plane components. Using magneto optical Kerr effect imaging, the domain wall velocity as a function of applied field strength was measured from which the magnitude of the DMI field was estimated.

Magnetization switching through spin-Hall-effect-induced chiral domain wall propagation

The influence of spin-Hall-effect spin torque (SHE-ST) induced by in-plane charge current was studied in microscale Ta/Co${}_{20}$Fe${}_{60}$B${}_{20}$/TaO${}_{x}$ films with perpendicular magnetization. Simultaneous electrical transport and polar magneto-optical Kerr effect (MOKE) imaging experiments were used to investigate the switching dynamics. A rich set of switching behaviors was observed, which can be well understood by analyzing a switching-phase diagram and polar MOKE images, considering the competition between SHE-ST and the externally applied magnetic field. Furthermore, we found that domain walls with a particular chirality were dominant in our devices, which suggests the presence of the Dzyaloshinskii-Moriya interaction in the present material system.