Magneto-optical Signal Research Articles

Quantitative magnetooptical analysis using indicator films for the detection of magnetic field distributions, temperature, and electrical currents

The accurate characterization of local magnetic fields and temperature is vital for the design of electronic systems. To meet this imperative, we present a novel non-contact approach for simultaneous quantitative magnetic field imaging and temperature sensing using magnetooptics and a bismuth-doped yttrium iron garnet film with out-of-plane anisotropy. For the direct signal quantification, a Stokes polarization camera is employed in a conventional magnetooptical microscope. The magnetization in the garnet is modulated with an external magnetic field to continuously image the Faraday rotation at four distinct points along the saturating magnetization loop. The method enables sensing of the magnetooptical signal in saturation, the magnetooptical susceptibility, the temperature, and self-calibrated driftfree imaging of the out-of-plane magnetic field component. A spatial resolution of magnetic field in the micrometer range with millisecond exposure time is demonstrated. The method is verified by analyzing the stray magnetic field distribution of electrical current in a wire simultaneously to the Joule heating induced by the applied current.

Development of highly oriented crystals in the magnetoplumbite structure of metal (Sr, Cr, Sc) doped M-type BaFe12O19 hexaferrite films with improved optical and magnetic properties

We have deposited magnetoplumbite (P63/mmc) structured Ba-hexaferrite (BaFe12O19) and its metal (T: Sc and Cr) doped (Ba0.7Sr0.3Fe12O19 and BaFe11.5T0.5O19) films on Si substrate using the Pulsed Laser Deposition technique. The as-grown films showed preferential orientation of the crystals along (2 0 3), (2 0 7), (1 1 14), and (4 0 5) directions. The post-heat treatment oriented the crystals only along (2 0 7) direction. The Fe ions varied it charge state from +3 at the surface of the films to a mixed charge state in the interior, which promoted a local clustering of Fe3O4-like phase. Ferrimagnetic properties of the hexaferrite films have been modified due to metal doping. The bi-phased magnetic structure, arising from clustering of the soft ferrimagnetic Fe3O4 phase in the matrix of hard ferrimagnetic hexaferrite films, is suitable for tuning the magnetic coercivity and moment over a wide range of temperatures. The optical band gap in the range of 3.40–3.50 eV and a strong magneto-optical Kerr signal are promising for potential applications of the films in magneto-optical devices working in UV regime of radiation.

Laser engineered architectures for magnetic flux manipulation on superconducting Nb thin films

Custom shaped magnetic flux guiding channels have been fabricated on superconducting Nb thin films by laser nanopatterning of their surface. Preferential pathways are defined by suitable combination of imprinted anisotropic pinning domains through laser-induced periodic surface structures (LIPSS). Generated by the selective energy deposition of femtosecond UV laser pulses, quasi-parallel ripple structures are formed under optimized irradiation conditions. On average, each domain is formed by grooves with a lateral period of 260–270 nm and a depth about 80 nm. By combination of scanning and transmission electron microscopy, magneto-optical imaging, and conductive atomic force microscopy techniques, we conclude that the boundaries of the LIPSS-covered domains play a prominent role in the magnetic flux diversion process within the film. This is confirmed by dedicated modeling of the flux dynamics, combined with the inversion of the magneto-optical signal. The created metasurfaces enable control of the flux penetration process at the microscale.

Real-time dynamics of angular momentum transfer from spin to acoustic chiral phonon in oxide heterostructures.

Chiral phonons have recently been explored as a novel degree of freedom in quantum materials. The angular momentum carried by these quasiparticles is generated by the breaking of chiral degeneracy of phonons, owing to the chiral lattice structure or the rotational motion of ions of the material. In ferromagnets, a mechanism for generating non-equilibrium chiral phonons has been suggested, but their temporal evolution, which obeys Bose-Einstein statistics, remains unclear. Here we report the real-time dynamics of thermalized chiral phonons in an artificial superlattice composed of ferromagnetic metallic SrRuO3 and non-magnetic insulating SrTiO3. Following the photo-induced ultrafast demagnetization in the SrRuO3 layer, we observed the appearance of a magneto-optic signal in the superlattice, which is absent in the SrRuO3 single films. This magneto-optic signal exhibits thermally driven dynamic properties and a clear correlation with the thickness of the non-magnetic SrTiO3 layer, implying that it originates from thermalized chiral phonons. We use numerical calculations considering the magneto-elastic coupling in SrRuO3 to validate our experimental observations and the angular momentum transfer mechanism between the lattice and spin systems in ferromagnetic systems and also to the non-magnetic system.

Magnetic properties and magnetoresistance of hybrid multilayer nanostructures {[(Co40Fe40B20)34(SiO2)66]/[ZnO]}n

The structural, electrical, magnetic, magneto-optical properties and magnetoresistance of {[(Co40Fe40B20)34(SiO2)66]/[ZnO]}n multilayer structures, where n = 50 is the number of bilayers (Co40Fe40B20)34(SiO2)66 nanocomposite and ZnO have been studied. The thicknesses of (Co40Fe40B20)34(SiO2)66 nanocomposite layers as well as ZnO spacers were varied in a wide range. The samples were synthesized by ion-beam sputtering onto glass ceramic substrates. The (Co40Fe40B20)34(SiO2)66 composite have an amorphous structure and the semiconductor ZnO interlayers have a hexagonal crystalline structure with the p63mc symmetry group. The nanocomposite layers containing a ferromagnetic component far from the percolation threshold are in a superparamagnetic state. The presented in the paper data of magnetization, magneto-optical transverse Kerr effect and magnetoresistance indicates that long-range ferromagnetic order does not form down to 77 K both for references ZnO films and studied multilayers with thin and thick ZnO interlayers. An increase in the magneto-optical signal in multilayers compared to references (Co40Fe40B20)34(SiO2)66 composite films has been detected at 1.2 eV. The magnetoresistance of {[(Co40Fe40B20)34(SiO2)66]/[ZnO]}n multilayers with thick (>32 nm) ZnO interlayers is lower than in reference (Co40Fe40B20)34(SiO2)66 nanocomposite, while at thin ZnO interlayers magnetoresistance is significantly higher and reaches 12 % at temperatures of 77 К. Possible mechanisms of ferromagnetic and antiferromagnetic ordering, enhancement of the magneto-optical response and magnetoresistance in {[(Co40Fe40B20)34(SiO2)66]/[ZnO]}n multilayer nanostructures are discussed.

Magneto-optical assessment of Plasmodium parasite growth via hemozoin crystal size

Hemozoin is a natural biomarker formed during the hemoglobin metabolism of Plasmodium parasites, the causative agents of malaria. The rotating-crystal magneto-optical detection (RMOD) has been developed for its rapid and sensitive detection both in cell cultures and patient samples. In the current article we demonstrate that, besides quantifying the overall concentration of hemozoin produced by the parasites, RMOD can also track the size distribution of the hemozoin crystals. We establish the relations between the magneto-optical signal, the mean parasite age and the median crystal size throughout one erythrocytic cycle of Plasmodium falciparum parasites, where the latter two are determined by optical and scanning electron microscopy, respectively. The significant correlation between the magneto-optical signal and the stage distribution of the parasites indicates that the RMOD method can be utilized for species-specific malaria diagnosis and for the quick assessment of drug efficacy.

Hysteresis-free voltage gating of the skyrmion

Magnetic skyrmions, which exhibit Brownian motion in solids, are considered good candidates as information carriers in devices, such as Brownian computers. Voltage control of skyrmions is essential for the ultralow power consumption of such devices. However, the gate operation must be realized with hysteresis-free voltage effects that are independent of ion migration for high-speed devices. In this study, we manipulated the skyrmion diffusion in a Ta|Co-Fe-B|Ta|MgO stacking structure by fabricating a device with a gate introducing an out-of-plane electrical field. Using feedback control, we rectified skyrmion diffusion in one direction, with the number of skyrmions passing through the gate wire from left to right N→ = 28 and from right to left N← = 43. Devices comprising Ta|Co-Fe-B|Pt|MgO junctions were fabricated, and a change in the density of skyrmions was observed upon the application of an out-of-plane electrical field. The creation or annihilation of skyrmions was dependent on the sign of the applied voltage. Furthermore, the skyrmions exhibited no hysteresis during the voltage sweep. Subsequently, the voltage dependence of the hysteresis loops in magneto-optical Kerr signals corresponding to the M–H curve was measured. However, no change was observed, nor was there any change in the saturated magnetization or perpendicular magnetic anisotropy. This result implied the voltage control of the Dzyaloshinskii–Moriya interaction.

Verification of ultrafast spin transfer effects in iron-nickel alloys

The optical intersite spin transfer (OISTR) effect was recently verified in Fe50Ni50 using extreme ultraviolet magneto-optical Kerr measurements. However, one of the main experimental signatures analyzed in this work, namely a magnetic moment increase at a specific energy in Ni, was subsequently found also in pure Ni, where no transfer from one element to another is possible. Hence, it is a much-discussed issue whether OISTR in FeNi alloys is real and whether it can be verified experimentally or not. Here, we present a comparative study of spin transfer in Fe50Ni50, Fe19Ni81 and pure Ni. We conclusively show that an increase in the magneto-optical signal is indeed insufficient to verify OISTR. However, we also show how an extended data analysis overcomes this problem and allows to unambiguously identify spin transfer effects. Concomitantly, our work solves the long-standing riddle about the origin of delayed demagnetization behavior of Ni in FeNi alloys.

A rapid magneto-optical immunoassay method based on liquid optical cavities

The technique investigates the application of the AC magneto-optical dichroism of magnetic fluids in fast and low-cost immunoassays by taking advantage of liquid optical cavities. Magnetic nanoparticles (MNPs) served as biomarkers. Variations in particle size were characterized by monitoring the amplitude of the second harmonic of transmitted light under alternating magnetic field excitation. Differential beams with polarization directions parallel and perpendicular to the magnetic field can improve signal-to-noise ratio. We designed a liquid optical cavity that allowed multiple reflections of light in magnetic fluid to increase the optical path. This cavity enhanced the magneto-optical signal and lowered the detection limits of target molecules. The limit of detecting biotin in the streptavidin–biotin binding system in conjunction with MNPs was 0.027 μg/mL and the least amount of time required for detection was 2.3 s. It is expected to achieve a low-cost, compact, and portable biosensor for rapid detection of trace viruses.

Magnitoopticheskaya kerr-spektroskopiya nanokompozitov

Magnetooptical spectroscopy is an effective method for studying the magnetic microstructure of homogeneous and heterogeneous magnets. This review is devoted to analysis of numerous factors affecting the intensity and spectral dependence of a magnetooptical signal of the equatorial Kerr effect in nanocomposites “ferromagnetic metal–dielectric” in the visible and near infrared spectral regions. Examples of the influence of the metal concentration, nanoparticle size and shape, the substrate, the material of the dielectric, the amorphization of grains, the deposition method, and other factors on the magnetooptical spectrum are considered. The differences in the magnetooptical spectra for the superparamagnetic, superferromagnetic, and ferromagnetic states are demonstrated. It is noted that in the presence of fractions with different field dependences of the magnetization in a nanocomposite, the magnetooptical signal is not proportional to the total magnetization. Examples of enhancement and sign inversion of the magnetooptical signal in nanocomposites are considered. The possibility of the description of magnetooptical spectra using the methods of the effective medium (the Bruggeman method and the Maxwell–Garnett symmetrized approximation) is discussed.

Femtosecond Laser Ablation-Induced Magnetic Phase Transformations in FeRh Thin Films

In this study, we present a novel investigation into the magnetic and morphological properties of equiatomic B2-ordered FeRh thin films irradiated with single high-intensity ultrashort laser pulses. The goal is to elucidate the effect of femtosecond laser ablation on the magnetic properties of FeRh. We employed Scanning Magneto-Optical Kerr Effect (S-MOKE) microscopy to examine the magnetic phase after laser processing, providing high spatial resolution and sensitivity. Our results for the first time demonstrated the appearance of a magneto-optical signal from the bottom of ablation craters, suggesting a transition from antiferromagnetic to ferromagnetic behavior. Fluence-resolved measurements clearly demonstrate that the ablation threshold coincides with the threshold of the antiferromagnet-to-ferromagnet phase transition. The existence of such a magnetic phase transition was independently confirmed by temperature-dependent S-MOKE measurements using a CW laser as a localized heat source. Whereas the initial FeRh film displayed a reversible antiferromagnet-ferromagnet phase transition, the laser-ablated structures exhibited irreversible changes in their magnetic properties. This comprehensive analysis revealed the strong correlation between the femtosecond laser ablation process and the magnetic phase transformation in FeRh thin films.

Enhanced magneto-optical Kerr effect via the synergistic effect of surface plasmon resonance and spin–orbit coupling in Au@Pt nanohybrid layers

Improving the magneto-optical saturation signal of materials is a crucial area of research in the field of magneto-optical effect. Au@Pt nanohybrids (Au@Pt NHs) with surface plasmon resonance and spin–orbit coupling effects were fabricated using a microfluidic technique in this investigation. The Au@Pt NHs were directly dripped onto the CoFeB/W film as a deposition layer, and its dual-mode resonance was applied to increase the longitudinal saturated magneto-optical Kerr angle of the CoFeB/W film by 4.4 times. The innovative concept of using a dual-mode resonance in this paper provides a new idea for the further search of materials that have large magneto-optical coefficients.

Transient magneto-optical spectrum of photoexcited electrons in the van der Waals ferromagnet Cr2Ge2Te6

Femtosecond optical control of magnetic materials shows promise for future ultrafast data storage devices. To date, most studies in this area have relied on quasimonochromatic light in magneto-optical pump-probe experiments, which limited their ability to probe semiconducting and molecule-based materials with structured optical spectra. Here, we demonstrate the possibility of extracting the magneto-optical spectrum of the electrons in the conduction band in the two-dimensional van der Waals ferromagnet ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ (CGT), which is made possible due to broadband probing in the visible spectrum. The magneto-optical signal is a sum of contributions from electrons in the conduction and valence bands, which are of opposite sign for CGT. Depending on the probe wavelength used, this difference could lead to an erroneous interpretation that the magnetization direction is reversed after excitation, which has important consequences for understanding spin toggle switching phenomena.

Magneto-optical spectroscopy of GaSb–MnSb composites

Polycrystalline samples of (GaSb)100 – x(MnSb)x composites with x = 69.2, 52.0 and 31.7 mol %, MnSb, Mn2Sb compounds were synthesized and their magneto-optical properties in the equatorial (transverse) Kerr effect geometry were studied. The difference of magneto-optical spectra for related compounds MnSb and Mn2Sb is shown. The anomalous temperature behavior of the magneto-optical signal in the composites and MnSb with decreasing temperature is associated with a redistribution of the population of d-states with the opposite spin index.

Magneto-Optical Properties of Noble Metal Nanostructures.

The magneto-optical signatures of colloidal noble metal nanostructures, spanning both discrete nanoclusters (<2 nm) and plasmonic nanoparticles (>2 nm), exhibit rich structure-property correlations, impacting applications including photonic integrated circuits, light modulation, applied spectroscopy, and more. For nanoclusters, electron doping and single-atom substitution modify both the intensity of the magneto-optical response and the degree of transient spin polarization. Nanoparticle size and morphology also modulate the magnitude and polarity of plasmon-mediated magneto-optical signals. This intimate interplay between nanostructure and magneto-optical properties becomes especially apparent in magnetic circular dichroism (MCD) and magnetic circular photoluminescence (MCPL) spectroscopic data. Whereas MCD spectroscopy informs on a metal nanostructure's steady-state extinction properties, its MCPL counterpart is sensitive to electronic spin and orbital angular momenta of transiently excited states. This review describes the size- and structure-dependent magneto-optical properties of nanoscale metals, emphasizing the increasingly important role of MCPL in understanding transient spin properties and dynamics.

Modeling of Enhanced Polar Magneto-Optic Kerr Effect by Surface Plasmons in Au Bowtie Arrays

The weak magneto-optical (MO) signal of traditional MO materials is indeed an important issue for their further practical applications. Although many strategies have been proposed to improve the MO effect, hybridization with noble metal nanostructures is a promising route in recent years due to the high localized-surface plasmon resonances (LSPR) effect. A new magneto-optical surface plasmon resonance (MOSPR) structure hybrid with Au bowtie arrays is proposed to increase the measuring range of the polar magneto-optical Kerr effect (PMOKE) and the quality factor through the LSPR effect. It is verified by a numerical simulation of the finite element method (FEM). The optimized parameters were found by modulating the shape and geometric dimensions. Owing to the significant LSPR from the Au bowties, a PMOKE amplification signal spectrum with narrow linewidth, and a high amplitude with high-sensing performance was achieved. Compared with the bare magnetic film alone, by optimizing the relevant parameters of the LSPR structure, the maximum signal increases 3255 times, and the quality factor can be greatly improved, which would provide important guidance and help for the practical application of MO devices.

Giant quadratic magneto-optical response of thin Y3Fe5O12 films for sensitive magnetometry experiments

We report on observation of a magneto-optical effect quadratic in magnetization (Cotton-Mouton effect) in 50 nm thick layer of Yttrium-Iron Garnet (YIG). By a combined theoretical and experimental approach, we managed to quantify both linear and quadratic magneto-optical effects. We show that the quadratic magneto-optical signal in the thin YIG film can exceed the linear magneto-optical response, reaching values of 450 urad that are comparable with Heusler alloys or ferromagnetic semiconductors. Furthermore, we demonstrate that a proper choice of experimental conditions, particularly with respect to the wavelength, is crucial for optimization of the quadratic magneto-optical effect for magnetometry measurement.

Exhaustive characterization of modified Si vacancies in 4H-SiC

Abstract The negatively charged silicon vacancy V S i − $\left({\mathrm{V}}_{\mathrm{S}\mathrm{i}}^{-}\right)$ in silicon carbide is a well-studied point defect for quantum applications. At the same time, a closer inspection of ensemble photoluminescence and electron paramagnetic resonance measurements reveals an abundance of related but so far unidentified signals. In this study, we search for defects in 4H-SiC that explain the above magneto-optical signals in a defect database generated by automatic defect analysis and qualification (ADAQ) workflows. This search reveals only one class of atomic structures that exhibit silicon-vacancy-like properties in the data: a carbon antisite (CSi) within sub-nanometer distances from the silicon vacancy only slightly alters the latter without affecting the charge or spin state. Such a perturbation is energetically bound. We consider the formation of V S i − + C S i ${\mathrm{V}}_{\mathrm{S}\mathrm{i}}^{-}+{\mathrm{C}}_{\mathrm{S}\mathrm{i}}$ up to 2 nm distance and report their zero phonon lines and zero field splitting values. In addition, we perform high-resolution photoluminescence experiments in the silicon vacancy region and find an abundance of lines. Comparing our computational and experimental results, several configurations show great agreement. Our work demonstrates the effectiveness of a database with high-throughput results in the search for defects in quantum applications.

Magnetometry based on the effect of laser-induced plasmas in a sodium-containing environment.

The magneto-optical resonance response of sodium atoms generated by a high-energy solid-state pulse Nd:YAG laser is studied in different external magnetic fields. We investigate the resonance fluorescence signal of sodium atoms in a simulated sea fog environment based on the laser-induced plasma (LIP) effect. By ionizing an NaCl solution spray to generate sodium atoms in an atmospheric environment, we build a Bell-Bloom magneto-optical resonance system under laboratory conditions. With the help of laser-induced breakdown spectroscopy (LIBS) and extinction spectrum, we obtain sodium atoms with a lifetime of 250 µs. A narrowband tunable continuous wave (CW) 589-nm laser tuned at the D2 line with a modulation frequency around the Larmor frequency is used as the pump beam to polarize sodium atoms in the test magnetic field. We find that the magneto-optical resonance signals vary with different external magnetic fields and the positions of the resonance signal are consistent with the theoretical values. An intrinsic magnetometric sensitivity of 620.4 pT in a 1-Hz bandwidth is achieved.

Sensitivity and reproducibility of transverse magneto-optical Kerr effect (T-MOKE) ellipsometry

We report a comprehensive experimental study to analyze the limiting factors and physical mechanisms that determine the achievable performance of transverse magneto-optical Kerr effect (T-MOKE) ellipsometry. Specifically, we explore different approaches to achieve high sensitivity and reduced acquisition times. The best sensitivity is observed for an incident light polarization with balanced s-p components. We also verify experimentally that the method’s theoretical description is accurately describing data for any s-p combination of the incoming light. Furthermore, two alternative measurement strategies are explored by using different measurement sequences for the polarization sensitive optics, which both achieve a very comparable, high quality of results. Signal-to-noise ratios and systematic deviations are measured and analyzed based on a large number of nominally identical measurement repeats, both for entire signal sequences as well as for individual Fourier components of the magneto-optical signal generated by a sinusoidal magnetic field sequence. Hereby, we observe that while higher order Fourier components have a significantly reduced signal amplitude and correspondingly exhibit reduced signal-to-noise and repeatability performance, signal-to-noise ratios always exceed values of 100 even for the lowest signal Fourier component and the lowest signal sample that we investigated, illustrating the extremely precise nature of T-MOKE ellipsometry.