Magneto-optical Kerr Effect Magnetometry Research Articles

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

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

Measurement of Kerr rotation and ellipticity in magnetic thin films by MOKE magnetometry

When polarized light is incident on a magnetic material, the magneto-optical Kerr effect (MOKE) rotates the polarization and induces ellipticity in the reflected light, which allows the magnetization direction to be probed optically. The Kerr rotation and ellipticity determine the magnitude of the effect and are usually measured using dedicated ellipsometers. Here, we demonstrate a simple method for extracting Kerr rotation and ellipticity in magnetic thin films using a conventional MOKE magnetometer consisting of two polarizers and a quarter waveplate. Using this technique, we report the longitudinal Kerr angle of BiYIG, GdCo, and TbCo. We additionally observe a linear decrease in polar complex Kerr angle magnitude in 3 nm GdCo films as the atomic fraction of Gd is increased.

Material informatics for functional magnetic material discovery

Functional magnetic materials are used in a wide range of “green” applications, from wind turbines to magnetic refrigeration. Often the magnetic materials used contain expensive and/or scarce elements, making them unsuitable for long term solutions. Further, traditional material discovery is a slow and costly process, which can take over 10 years. Material informatics is a growing field, which combines informatics, machine learning (ML) and high-throughput experiments to rapidly discover new materials. To prove this concept, we have devised a material informatics workflow and demonstrated the core components of natural language processing (NLP) to extract data from research papers to create a functional magnetic material database, machine learning with semi-heuristic models to predict compositions of soft magnetic materials, and high-throughput experimental evaluation using combinatorial sputtering and high-throughput magneto-optic Kerr effect (MOKE) magnetometry. This material informatics workflow provides a quicker, cheaper route to functional magnetic materials discovery.

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

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

Magnetization reversal in Fe(001) films grown by magnetic field assisted molecular beam epitaxy

We studied the influence of a magnetic field (MF) on epitaxial growth and magnetic properties of Fe(001) films deposited on MgO(001). Thanks to modular sample holders and a specialized manipulator in our multi-chamber ultrahigh vacuum system, the films could be deposited and annealed in an in-plane MF of 100 mT. In situ scanning tunnelling microscopy showed that MF had a strong influence on the film morphology, and, in particular, on the structure of surface steps. The magnetic properties were studied ex situ using magneto-optic Kerr effect (MOKE) magnetometry and microscopy. We showed that the moderate in-plane magnetic field applied during growth has the visible impact on the magnetic properties. The observed angular dependence of the MOKE loops and domain structures were discussed based on a magnetization reversal model. In particular we found that magnetization reversal occurs via 90° domains and the reversal differs for the no-field and in-field grown samples, in correlation with the film morphology.

Single-shot switching in Tb/Co-multilayer based nanoscale magnetic tunnel junctions

Magnetic tunnel junctions (MTJs) are elementary units of magnetic memory devices. For high-speed and low-power data storage and processing applications, fast reversal of the magnetization by an ultrashort laser pulse is extremely important. We demonstrate single-shot switching of Tb/Co-multilayer based nanoscale MTJs by combining the optical writing and the electrical read-out methods. A 90-fs-long laser pulse switches the magnetization of the storage layer (SL). The change in the tunneling magnetoresistance (TMR) between the SL and a reference layer (RL) is probed electrically across the oxide barrier. Single-shot switching is demonstrated by varying the cell diameter from 300 nm to 20 nm. The anisotropy, magnetostatic coupling, and switching probability exhibit cell-size dependence. By suitable association of laser fluence and magnetic field, successive commutation between high-resistance and low-resistance states is achieved. The nature of the magnetization reversal of both SL and RL in a continuous film is probed with a depth-resolved magneto-optical Kerr effect (MOKE) magnetometry. The ultrafast dynamics in the continuous full-MTJ stack is investigated with the time-resolved pump–probe technique. Our experimental findings provide strong support for the growing interest in ultrafast spintronic devices.

Tensile properties of ferromagnetic nanofilms on stretchable substrates: Links between multi-cracking and magnetic properties

The influence of multi-cracking on the magnetic response of more (Co) or less (Ni78Fe22) magnetostrictive ferromagnetic nanofilms on flexible substrates has been studied by combining tensile tests with in situ magneto-optical Kerr effect magnetometry measurements, up to large strain (20%). The results show that the variations of the magnetic features are more significant in the elastic domain (before cracking), while they are slightly affected in the multi-cracking regime, linked to the fact that the stresses evolve relatively little in this regime. This results in a lesser modification of the magnetization curves of Ni78Fe22, which also means weak magnetostatic (dipolar) effects despite a very high crack density. This is very promising for the applicability of magnetic films in highly curved or stretched systems, especially using weakly magnetostrictive materials.

A Brillouin light scattering study of the spin-wave magnetic field dependence in a magnetic hybrid system made of an artificial spin-ice structure and a film underlayer

We present a combined Brillouin light scattering (BLS) and micromagnetic simulation investigation of the magnetic-field-dependent spin-wave spectra in a hybrid structure made of permalloy (NiFe) artificial spin-ice (ASI) systems, composed of stadium-shaped nanoislands, deposited on the top of an unpatterned permalloy film with a nonmagnetic spacer layer. The thermal spin-wave spectra were recorded by BLS as a function of the magnetic field applied along the symmetry direction of the ASI sample. Magneto-optic Kerr effect magnetometry was used to measure the hysteresis loops in the same orientation as the BLS measurements. The frequency and the intensity of several spin-wave modes detected by BLS were measured as a function of the applied magnetic field. Micromagnetic simulations enabled us to identify the modes in terms of their frequency and spatial symmetry and to extract information about the existence and strength of the dynamic coupling, relevant only to a few modes of a given hybrid system. Using this approach, we suggest a way to understand if the dynamic coupling between ASI and film modes is present or not, with interesting implications for the development of future three-dimensional magnonic applications and devices.

Strong interfacial Dzyaloshinskii–Moriya induced in Co due to contact with NiO

The magnetic properties of NiO/Co/Pt as a function of Co layer thickness were investigated by polar magneto-optical Kerr effect (PMOKE) (magnetometry and microscopy) and Brillouin Light Scattering (BLS) spectroscopy. PMOKE measurements revealed strong surface anisotropy (1.8 mJ/m2) favoring perpendicular magnetic anisotropy and asymmetric domain wall propagation explained by anticlockwise chirality. BLS measurements show that this chirality is induced by strong interfacial Dzyaloshinskii–Moriya interaction (+ 2.0 pJ/m). This is one of the highest values reported so far for Co layers surrounded by different layers. The observed chirality is opposite to what has been found in Co/oxide interfaces. These results and data published earlier, indicate that the strength of interfacial Dzyaloshinskii–Moriya interaction increases with the amount of stoichiometric NiO. Therefore, this work shows that NiO is the source of the interfacial Dzyaloshinskii–Moriya interaction.

Enhanced optical mode coherence in exchange coupled soft magnetic multilayers

We report on an all-optical investigation of coupled spin excitation modes in a series of magnetic trilayer structures. Using time-resolved magneto-optic Kerr effect (tr-MOKE) magnetometry, we observe multi-mode coherent spin excitations in Ni81Fe19/Ru/Co49Fe49V2 multilayers even though the tr-MOKE optical detection is sensitive only to the Co49Fe49V2 magnetization dynamics. Frequency shifts of the different modes indicate that the coupling between the Ni81Fe19 and Co49Fe49V2 layers varies from anti-ferromagnetic to ferromagnetic to uncoupled as the Ru spacer layer thickness is increased from 8 Å to 200 Å. The lifetime of the high frequency coherent oscillations in the Co49Fe49V2 layer increases by over 200%–300% even in the case of uncoupled Ni81Fe19 and Co49Fe49V2 layers with a 200 Å thick Ru spacer. The results suggest an additional method to decrease the damping of high-moment alloys in layered magnetic nanostructures.

Comment on "Ultralow magnetostrictive flexible ferromagnetic nanowires" by G. Muscas, P. E. Jönsson, I. G. Serrano, Ö. Vallin, and M. V. Kamalakar, Nanoscale, 2021, 13, 6043-6052.

In a recent paper (G. Muscas et al.),1 the magnetomechanical behavior of cobalt (Co) magnetic nanowire arrays on a polymeric substrate (polyethylene naphthalate (PEN)) under bending was measured by magneto-optical Kerr effect (MOKE) magnetometry in situ. The authors showed that the magnetomechanical effects were very small and assigned this result to a low effective magnetostriction coefficient due to the nanostructuring. In this comment, we show by numerical calculations that it is the ongoing/current stress distribution within the system that generates this effect. Indeed, the nanostructures being very rigid with respect to the compliant substrate, the strains are mainly concentrated in the substrate and less than 3% of the macroscopic stress is transmitted to the nanostructures.

Dzyaloshinskii-Moriya interaction and magnetic anisotropy in Pt/Co/Au trilayers modified by Ga+ ion irradiation

The influence of 30 keV Ga+ ion irradiation in the fluence range 0 ÷ 1·1016 ions/cm2 on magnetic properties of Pt(bottom)/Co(3 nm)/Au(top) trilayers grown by the molecular beam epitaxy was studied. Polar magnetooptical Kerr effect magnetometry and Brillouin light scattering spectrometry were used to measure the magnetic anisotropy and the strength of Dzyaloshinskii-Moriya interaction (DMI). The as-deposited sample exhibits in-plane magnetization with an effective anisotropy field of −0.35 ± 0.02 T. Magnetic anisotropy gradually grows as the ion fluence is increased and out-of-plane magnetic anisotropy appears for the fluence above ∼ 3·1015 ions/cm2. The frequency asymmetry in Stokes and anti-Stokes lines Δf, being a measure of DMI strength, was determined from Brillouin light scattering investigations as a function of applied fluence. The effective DMI constant is equal to −0.48 ± 0.05 mJ/m2 in the as-deposited sample. While increasing fluence the parameter Δf: (i) varies non-monotonically (with two maxima) for F smaller than F≈1015 ions/cm2 and (ii) gradually decreases to zero for higher fluences. Our results can be used for the adjustment of DMI interaction in the manufacturing of magnonic nano-devices.

A UHV MOKE magnetometer complementing XMCD-PEEM at the Elettra Synchrotron

We report on a custom-built UHV-compatible Magneto-Optical Kerr Effect (MOKE) magnetometer for applications in surface and materials sciences, operating in tandem with the PhotoEmission Electron Microscope (PEEM) endstation at the Nanospectroscopy beamline of the Elettra synchrotron. The magnetometer features a liquid-nitrogen-cooled electromagnet that is fully compatible with UHV operation and produces magnetic fields up to about 140 mT at the sample. Longitudinal and polar MOKE measurement geometries are realized. The magneto-optical detection is based on polarization analysis using a photoelastic modulator. The sample manipulation system is fully compatible with that of the PEEM, making it possible to exchange samples with the beamline endstation, where complementary X-ray imaging and spectroscopy techniques are available. The magnetometer performance is illustrated by experiments on cobalt ultra-thin films, demonstrating close to monolayer sensitivity. The advantages of combining in situ growth, X-ray Magnetic Circular Dichroism imaging (XMCD-PEEM) and MOKE magnetometry into a versatile multitechnique facility are highlighted.

Optimization of the high-frequency magnetoimpedance response in melt-extracted Co-rich microwires through novel multiple-step Joule heating

The optimization of the high frequency giant magnetoimpedance (GMI) effect and its magnetic field sensitivity in melt-extracted Co 69.25 Fe 4.25 Si 13 B 12.5 Nb 1 amorphous microwires was systematically studied through a multi-step Joule annealing (MSA) technique. The surface morphology, microstructure, surface magnetic property, and radio frequency GMI response of the Co-rich microwires were explored using techniques like microscopy, magneto-optical Kerr effect (MOKE) magnetometry, and the magnetic field dependence of the wire's radio frequency impedance (GMI). The multi-step Joule annealing protocol begins with an initial dc current amplitude ( i dc ) of 20 mA that was stepped up by 20 mA every 10 min to a maximum amplitude of 300 mA. Radio frequency GMI measurements at 20 MHz demonstrated a remarkable improvement of the GMI ratio and field sensitivity to 760% (1.75 times of that of the as-prepared wire) and 925%/Oe (more than 17.92 times of that of the as-prepared wire), respectively, after the MSA protocol with a maximum current amplitude of 100 mA. Microscopy and MOKE suggest that the MSA technique can enhance the microstructure and surface magnetic domain structure of the Co-rich magnetic microwire, which would give rise to an improved GMI ratio. The high GMI sensitivity at small magnetic fields renders these MSA-treated Co-rich microwires highly promising materials for biomedical devices that sense and monitor small, biological magnetic fields.

Magnetization processes and magnetic domain structures in Ta/CoFeB/MgO stacks

Magnetization processes and magnetic domain structures in Ta/CoFeB/MgO stacks were studied in a series of samples with various CoFeB thicknesses d ranging from 1.24 to 1.60 nm with a step of 0.04 nm, using polar magneto-optical Kerr effect (PMOKE) magnetometry and microscopy. Thickness dependence of the magnetic anisotropy was evaluated and the first and second order anisotropy constants were quantified for each thickness. Accordingly, this dependence was deduced to result in magnetization reorientation from out-of-plane to in-plane through an easy-cone magnetization region (1.39 nm ≤ d ≤ 1.41 nm) as d was increased. PMOKE imaging of the magnetization reversal processes for stacks with out-of-plane easy axis indicated both a significant increase of the density of nucleation centers and a change in domain morphology with increasing d up to the magnetization reorientation thickness. Magnetization reversal dynamics was described by a thermal activation model consistent with a Barkhausen length of about 120 nm. The thinnest films with d = 1.24 and 1.28 nm exhibited straightened narrow stripe domains resulting from magnetic dipolar repulsion. A thorough study of narrow stripe domains was performed via direct and indirect magnetization reversal processes. The application of such structures as spin wave nano-channels could be promising.

Direct imaging of distorted vortex structures and magnetic vortex annihilation processes in ferromagnetic/antiferromagnetic disk structures

Topological objects, such as magnetic vortices in soft-magnetic thin films, have attracted continued attention over the past 20 years for their unique magnetic reversal behavior and compelling potential applications. In exchange-coupled ferromagnetic/antiferromagnetic microdisks, high-resolution magnetic force microscopy imaging reveals a distorted ferromagnetic vortex structure due to randomly distributed (nonplanar) uncompensated spins at the antiferromagnetic/ferromagnetic interface. Further, in-field imaging and magneto-optical Kerr effect magnetometry demonstrate an unexpected increase in the stability of an otherwise somewhat volatile, intermediate vortex-antivortex state in exchange-coupled microdisks.

Magnetic textures in hemispherical thin film caps with in-plane exchange bias

Hemispherical caps of in-plane exchange biased IrMn/CoFe layer systems have been fabricated on top of regularly arranged spherical silica particles by magnetron sputtering, creating magnetic Janus particles. In this thin film layer system cap, the magnetic shape anisotropy of the topographically non-flat hemispheres competes with the unidirectional anisotropy induced by the exchange bias. The magnetic properties of this non-trivial system have been investigated by longitudinal magneto-optical Kerr effect magnetometry, where a characterization method has been developed considering both the curved layer system and the signal contributions of flat parts of the sputtered thin film system. Both remagnetization curves, from Kerr magnetometry and the magnetic force microscopy images, reveal an onion state in the magnetic caps of the ensemble. Additional micromagnetic simulations show a stabilization of the onion state due to the introduced unidirectional anisotropy also in individual hollow hemispheres as compared to the vortex state exhibited by purely ferromagnetic caps.

Magnetic domain structure and magnetization reversal in (Co/Ni) and (Co/Pd) multilayers

The temporal behavior of magnetization reversal mechanism and domain structures of (Co/Ni) and (Co/Pd) multilayers have been studied using magneto-optical Kerr effect microscopy and magnetometry measurements. The domain size and nucleation field of (Co/Ni) multilayers were found to be reduced as the number of bilayers increases. On the other hand, (Co/Pd)×10 multilayer has a much larger domain size, faster magnetization switching, and a higher nucleation field than that in (Co/Ni). From the imaging of magnetic domains, two different types were observed, namely, dendritic domains for (Co/Ni) and blocks type for (Co/Pd). From time dependence of magnetization reversal at a fixed applied magnetic field, it was revealed that the reversal mechanism in (Co/Ni) multilayer occurs by nucleation followed by domain propagation. Whereas, for (Co/Pd) multilayers, few large domains were observed, and they were found to expand at a faster rate until a complete saturation.

Inverted Hysteresis, Magnetic Domains, and Hysterons

In this letter, we use magnetooptical Kerr effect magnetometry on extended samples to measure local hysteresis curves as a function of an externally applied field in a magnetostriction-free amorphous ribbon and an elliptical Permalloy film element. Although these materials are magnetically soft and have almost zero coercive field, we find that the local hysteresis curves have a highly nonlinear hysteretic character, displaying both counterclockwise and clockwise rotation senses when cycling the loops. This unexpected result can be explained by looking at the total hysteresis curve of the materials as a superposition of local curves and can be described mathematically using the formalism of the Preisach model. Within the framework of this model, we derive the conditions under which a superposition of clockwise and counterclockwise hysteresis loops can lead to a hysteresis-less system, and devise a technique to map the hysterons that define the first-order reversal-curve distribution to physical regions in the specimens.

Surface layer morphology of the high fluence Fe implanted polyethylene - Correlation with the magnetic and optical behavior

Fe/Polyethylene nanocomposite was synthesized by ion beam implantation of 56Fe+ into bulk high-density polyethylene. Nanoscale surface morphology along with magnetic and optical behavior was investigated. The aim of the research was to investigate changes of polyethylene's surface layer morphology with the change of Fe implantation fluence in the high fluence range and to find correlations with the magnetic and optical behavior. Four implantation fluences were applied: 5 × 1016, 1 × 1017, 2 × 1017 and 5 × 1017 cm−2, while the implantation energy was 95 keV. Concentration profiles of implanted Fe were analyzed by Rutherford backscattering spectrometry, showing Fe concentration profile maxima closer to the surface with increasing implantation fluence. Cross-sectional transmission electron microscopy showed the formation of metallic nanoparticles with sizes in a range from below 1 nm up to few tens of nanometers, depending on the fluence, and for the highest implantation fluence, a continuous layer was formed. Magneto-optic Kerr effect magnetometry demonstrates weak ferromagnetic behavior for the 2 higher fluences, and superparamagnetic for the 2 lower fluences. The UV-VIS remission function spectra show the peak in the UV region, which we attribute to iron nanoparticles.