Polar Magneto-optical Kerr Effect Research Articles

Speed limits of the laser-induced phase transition in FeRh

We use ultrafast x-ray diffraction and the polar time-resolved magneto-optical Kerr effect to study the laser-induced metamagnetic phase transition in two FeRh films with thicknesses below and above the optical penetration depth. In the thin film, we identify an intrinsic timescale for the light-induced nucleation of ferromagnetic (FM) domains in the antiferromagnetic material of 8ps, which is substantially longer than the time it takes for strain waves to traverse the film. For the inhomogeneously excited thicker film, only the optically excited near-surface part transforms within 8ps. For strong excitations, we observe an additional slow rise of the FM phase, which we experimentally relate to a growth of the FM phase into the depth of the layer by comparing the transient magnetization in frontside and backside excitation geometry. In the lower lying parts of the film, which are only excited via near-equilibrium heat transport, the FM phase emerges significantly slower than 8ps after heating above the transition temperature.

Single-Particle Photothermal Circular Dichroism and Photothermal Magnetic Circular Dichroism Microscopy.

Recent advances in single-particle photothermal circular dichroism (PT CD) and photothermal magnetic circular dichroism (PT MCD) microscopy have shown strong promise for diverse applications in chirality and magnetism. Photothermal circular dichroism microscopy measures direct differential absorption of left- and right-circularly polarized light by a chiral nanoobject and thus can measure a pure circular dichroism signal, which is free from the contribution of circular birefringence and linear dichroism. Photothermal magnetic circular dichroism, which is based on the polar magneto-optical Kerr effect, can probe the magnetic properties of a single nanoparticle (of sizes down to 20 nm) optically. Single-particle measurements enable studies of the spatiotemporal heterogeneity of magnetism at the nanoscale. Both PT CD and PT MCD have already found applications in chiral plasmonics and magnetic nanomaterials. Most importantly, the advent of these microscopic techniques opens possibilities for many novel applications in biology and nanomaterial science.

Magnetochiroptical nanocavities in hyperbolic metamaterials enable sensing down to the few-molecule level.

In this work, we combine the concepts of magnetic circular dichroism, nanocavities, and magneto-optical hyperbolic metamaterials (MO-HMMs) to demonstrate an approach for sensing down to a few molecules. Our proposal comprises a multilayer MO-HMM with a square, two-dimensional arrangement of nanocavities. The magnetization of the system is considered in polar configuration, i.e., in the plane of polarization and perpendicular to the plane of the multilayer structure. This allows for magneto-optical chirality to be induced through the polar magneto-optical Kerr effect, which is exhibited by reflected light from the nanostructure. Numerical analyses under the magnetization saturation condition indicate that magnetic circular dichroism peaks can be used instead of reflectance dips to monitor refractive index changes in the analyte region. Significantly, we obtained a relatively high sensitivity value of S = 40 nm/RIU for the case where refractive index changes are limited to the volume inside nanocavities, i.e., in the limit of a few molecules (or ultralow concentrations), while a very large sensitivity of S = 532 nm/RIU is calculated for the analyte region distributed along the entire superstrate layer.

Enhancement of the polar magneto-optical Kerr effect with plasma hybrid microstructure

Enhancement of the polar magneto-optical Kerr effect with plasma hybrid microstructure

Origin of the Magnetic-Optical Properties of Palladium Compounds, an Ab Intio Study

Abstract: The structural, magnetic, and magneto-optical properties of the PdM (M=Fe, Co, Ni) alloy are investigated using first-principles calculations with the exciting code. The study employs the full potential linearized augmented plane wave method (FP-LAPW) based on density functional theory (DFT). The exchange-correlation energy is described in the generalized gradient approximation developed by Perdew-Burke-Ernzerhof (PBE-GGA). The calculated lattice parameters are in good agreement with theoretical and experimental results. The magnetic properties reveal a ferromagnetic stability phase for all compounds. Additionally, the total (TDOS) and partial (PDOS) density of states, along with the band structure, indicate that PdM (M = Fe, Co, Ni) exhibits metallic behavior. The total and local magnetic moments are investigated. We conducted a study on the polar magneto-optical Kerr effect and calculated Kerr spectra. The microscopic origins of the peaks found in Kerr's angle are interpreted as interband transitions, primarily arising from the states (d to d) in the PdNi and (S to d) in the PdFe and PdCo alloys. Keywords: P-MOKE, DFT, GGA-PBE, Intermetallic.

Magneto- and thermo-optic manipulation of imbert-fedorov shift for Laguerre-Gaussian beam in a graphene-VO2 multilayer structure

Imbert-Fedorov (IF) shift of Gaussian beams has been widely studied, while the IF shift of singular beams carrying Orbital Angular Momentum (OAM) also shows unique properties when reflected or refracted at optical interfaces. Here, through theoretical deduction and analysis, the IF shift of Laguerre-Gaussian beams with varying OAM in a graphene-VO2 multilayers structure is explored. The results demonstrate that the IF shift increases with the increase of topological charge l. Under the configuration of transverse magneto-optical Kerr effect (TMOKE), the IF shift increases by a factor of 10 compared to the Gaussian beam when l = 3. Additionally, under the polar magneto-optical Kerr effect (PMOKE) conditions, graphene demonstrates the magneto-optical (MO) effect and cross-polarization effect in the terahertz frequency range, and can effectively control the IF shift through the magnetic field. Both left- and right-handed circular polarization (LHCP and PHCP) exhibit asymmetry and non-reciprocity. Moreover, temperature control is achieved through the unique phase transition characteristics of VO2. These results offer novel insights into precision measurements, sensing techniques and the development of terahertz devices.

High angular sensitivity refractive index sensors based on 1D single-defect magneto-optical photonic crystals

Strong localization and enhancement of fields may arise in the defect layer of an 1D single-defect magneto-optical photonic crystal (1DSDMOPC), which has been applied to design and fabricate several types of optical devices. In this work, we demonstrate numerically that, for the case that a linearly polarized light is normally incident on a selected 1DSDMOPC, rotation angle induced by polar linear magneto-optical Kerr effect may strongly depend on refractive index of the fluid encountered by the transmission, which indicates that 1DSDMOPC is promising to be applied as a refractive index sensor based on Kerr angle detection. Furthermore, it is shown that sensitivity and detection range of the sensor can be adjusted by either modifying refractive index of the dielectric layers or altering number of layers of the 1DSDMOPC. An angular sensitivity in the order of 104 °/RIU is inferred, a value much larger than both the one usually encountered in photonic crystal sensors and the surface plasmon resonance based sensors (the angular sensitivity usually in the order of 101 or 102 °/RIU). Our work may provide a new possible way to design and manufacture high angular sensitivity sensors by using 1DSDMOPCs.

Exchange Bias and Vertical Magnetic Shift on Co−CoO Thin Films

AbstractThis work reports the results of an experimental study on Si(001)/Au(111)/Co/CoO/ZrO2 elaborated by electron beam evaporation. The chemical composition of the oxidized state of cobalt and zirconium cover layers derived from metallic deposition is confirmed by X‐ray photoelectron spectroscopy (XPS). Magnetic properties are probed by magneto‐optical Kerr effect (MOKE). The polar magneto‐optic Kerr effect (PMOKE) measurements reveal that the cooling at 77 K under an applied magnetic field induces an exchange coupling which is manifested by a positive exchange bias (EB) field accompanied by a negative vertical shift of the magnetization. In longitudinal magneto‐optical Kerr effect (LMOKE), the exchange coupling depends on the orientation of the applied measuring magnetic field. It is found that both the exchange field shift rate hshift and the vertical shift rate in magnetization mshift exhibit a peak at 45° between the applied field and the [100] in‐plane direction which may be associated with the presence of uncompensated pinned spins at the Co−CoO interface.

Boron-induced magneto-optical Kerr spectra and dielectric tensors in ferrimagnetic (Mn4N)B antiperovskite thin films

Boron-induced electronic states were investigated via a combination of polar magneto-optical Kerr effect (p-MOKE) spectroscopy and spectroscopic ellipsometry for one of the antiperovskite nitrides, Mn4N. The boron content in the Mn4N film varied from 0 to 4.3 at.%, for which the crystal structure was maintained. The amplitude of p-MOKE spectra and the diagonal and off-diagonal dielectric tensors decreased with increasing boron content, which is in agreement with the magnetic properties such as magnetic anisotropy and saturation magnetization. These results were related to the lattice expansion and displacement of the charge density in the Mn4N by boron doping. However, the peak energy of the Lorentz oscillator in the diagonal elements of dielectric tensors suggests that a dominant inter-band transition was independent of boron content.

Optical Reading of Nanoscale Magnetic Bits in an Integrated Photonic Platform

In this paper, we propose a compact integrated hybrid plasmonic-photonic device for optical reading of nanoscale magnetic bits with perpendicular magnetic anisotropy in a magnetic racetrack on top of a photonic waveguide on the indium phosphide membrane on silicon platform. The hybrid device is constructed by coupling a doublet of V-shaped gold plasmonic nanoantennas on top of the indium phosphide waveguide. By taking advantage of the localized surface plasmons, our hybrid device can enable detection of the magnetization state in magnetic bits beyond the diffraction limit of light and enhance the polar magneto-optical Kerr effect (PMOKE). We further illustrate how combining the hybrid device with a plasmonic polarization rotator provides magneto-optical read-out by transforming the PMOKE-induced polarization change into an intensity variation of the waveguide mode. According to the simulation results based on a three-dimensional finite-difference time-domain method, the hybrid device can detect the magnetization states in targeted bits in a magnetic racetrack medium down to ~ 100x100 nm2, regardless of the magnetization state of the rest of the racetrack with a relative intensity contrast of greater than 0.5% for a ~ 200x100 nm2 magnetic bit. We believe our hybrid device can be an enabling technology that can connect integrated photonics with nanoscale spintronics, paving the way toward ultrafast and energy efficient advanced on-chip applications.

Chiral Interlayer Exchange Coupling for Asymmetric Domain Wall Propagation in Field-Free Magnetization Switching

The discovery of chiral spin texture has unveiled many unusual yet extraordinary physical phenomena, such as the Néel type domain walls and magnetic skyrmions. A recent theoretical study suggests that a chiral exchange interaction is not limited to a single ferromagnetic layer; instead, three-dimensional spin textures can arise from an interlayer Dzyaloshinskii-Moriya interaction. However, the influence of chiral interlayer exchange coupling on the electrical manipulation of magnetization has rarely been addressed. Here, the coexistence of both symmetric and chiral interlayer exchange coupling between two orthogonally magnetized CoFeB layers in PtMn/CoFeB/W/CoFeB/MgO is demonstrated. Images from polar magneto-optical Kerr effect microscopy indicate that the two types of coupling act concurrently to induce asymmetric domain wall propagation, where the velocities of domain walls with opposite chiralities are substantially different. Based on this microscopic mechanism, field-free switching of the perpendicularly magnetized CoFeB is achieved with a wide range of W thicknesses of 0.6-4.5 nm. This work enriches the understanding of interlayer exchange coupling for spintronic applications.

Control of Skyrmion Chirality in Ta/Fe−Co−B/TaOx Trilayers by TaOx Oxidation and Fe−Co−B Thickness

Skyrmions are magnetic bubbles with nontrivial topology envisioned as data bits for ultrafast and power-efficient spintronic memory and logic devices. They may be stabilized in heavy-metal/ferromagnetic/oxide trilayer systems. The skyrmion chirality is then determined by the sign of the interfacial Dzyaloshinskii-Moriya interaction (DMI). Nevertheless, for apparently identical systems, there is some controversy about the DMI sign. Here, we show that the degree of oxidation of the top interface and the thickness of the ferromagnetic layer play a major role. Using Brillouin light-scattering measurements in $\mathrm{Ta}/\mathrm{Fe}$-$\mathrm{Co}$-$\mathrm{B}/{\mathrm{Ta}\mathrm{O}}_{x}$ trilayers, we demonstrate a sign change of the DMI with the degree of oxidation of the $\mathrm{Fe}$-$\mathrm{Co}$-$\mathrm{B}/{\mathrm{Ta}\mathrm{O}}_{x}$ interface. Using polar magneto-optical Kerr effect microscopy, we consistently observe a reversal of the direction of current-induced motion of skyrmions with the oxidation level of ${\mathrm{Ta}\mathrm{O}}_{x}$; this is attributed to their chirality reversal. In addition, a second chirality reversal is observed when changing the $\mathrm{Fe}$-$\mathrm{Co}$-$\mathrm{B}$ thickness, probably due to the proximity of the two $\mathrm{Fe}$-$\mathrm{Co}$-$\mathrm{B}$ interfaces in the ultrathin case. By properly tuning the chirality of the skyrmion, spin-transfer and spin-orbit torques combine constructively to enhance the skyrmion velocity. These observations thus allow us to envision an optimization of the material parameters to produce highly mobile skyrmions. Moreover, this chirality control enables a versatile manipulation of skyrmions and paves the way towards multidirectional devices.

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.

Exploring the structural, electronic, magnetic, and magneto-optical properties of double perovskites Ca2TMIrO6 (TM = Fe, Co) through first principles study

This study is aimed at exploring the electronic, magnetic, and magneto-optical properties of double perovskites Ca2FeIrO6 and Ca2CoIrO6 with monoclinic structure (space group P21/c) in order to examine their potential applications in spintronic and photovoltaic devices. The calculations were done using the full-potential linearized augmented plane wave within the density functional theory. For the electronic exchange-correlation function, we used the generalized gradient approximation (GGA) and GGA+U (Hubbard potential), and spin-orbit coupling (SOC). The study showed that Ca2FeIrO6 and Ca2CoIrO6 exhibit a monoclinic structure (space group P21/c). The structure relaxation shows an antiferromagnetic behavior in both systems with a magnetic moment of about 6.00 μB for Ca2FeIrO6 and 4.00 μB for Ca2CoIrO6 by using GGA+U approximation. The results of GGA and GGA+U predict the half-metallic behavior of Ca2FeIrO6 and Ca2CoIrO6. The magneto-optical polar Kerr effect (MOKE) was examined by studying the variation of Kerr and ellipticity rotation. The Kerr rotation angle is 1.3º at 4.82 eV and -1.21º at 4.3 eV, and the ellipticity angle is -1.21º at 4.3 eV for Ca2FeIrO6. In the case of Ca2CoIrO6, the Kerr rotation angle is -1.04º at 4.05 eV; the significant Kerr rotation in both compounds may suggest the application of these materials in optoelectronics bias. The named compounds have a potential application in the field of spintronics and its devices, such as in optoelectronics technologies.

Tunable polar magneto-optical Kerr rotation of light from a one-dimensional photonic crystal containing a Graphene/InAs slab at THz range

Magneto-optical effects of graphene and InAs layers generated by gyrotropic behaviors under an external magnetic field, make them desirable for magneto-optical applications. In this paper, the polar magneto-optical Kerr effect of reflected light from a one-dimensional photonic crystal containing a magneto-optical slab (a graphene sheet on an InAs layer) has been investigated in the THz frequency region. The intensity reflection coefficient and Kerr rotation angle spectra of the reflected light from the structure have been calculated using the developed 4 × 4 transfer matrix method. By adjusting the thickness of the InAs layer in the structure, a narrow deep mode inside the photonic bandgap appears. The results revealed that the corresponding enhanced Kerr rotation angle occurred at the deep mode frequency. The performance of the structure in the presence and absence of the graphene sheet is investigated considering the FOM behaviors. The tunability and enhancement of the Kerr rotation angle are investigated by varying the external magnetic field strength and the charge career density of graphene. Finally, the polarization state of reflected light at the deep mode frequency is visualized. The results of this work may have potential applications in designing magnetic field sensors and tunable Kerr-rotation-based devices.

The effects of field history on magnetic skyrmion formation in [Pt/Co/Ir]3 multilayers

We report on the formation of Néel-type magnetic bubble skyrmions at room temperature in [Pt/Co/Ir]3 multilayered thin films after an in-plane magnetic field treatment. Polar magneto-optical Kerr Effect (p-MOKE) microscopy images show that the dendritic magnetic configurations observed after AC demagnetization evolve into magnetic bubble skyrmions after the application and subsequent removal of an in-plane magnetic field. Micromagnetic simulations were used to systematically investigate the role of the in-plane magnetic field magnitude, misalignment of the sample, and the Dzyaloshinskii-Moriya interactions (DMI) in generating bubble skyrmions during the field treatment. The simulations show that in-plane fields slightly below the saturation field are the most effective at producing skyrmions, and, furthermore, a small field angle away from the sample plane not only leads to improved skyrmion formation but also provides a means to select the skyrmion polarity where the direction of the out-of-plane component of the field is opposite to the direction of the skyrmion cores. This field treatment scheme leads to a simple and reliable way to create magnetic bubble skyrmions in multilayered thin films with DMI.

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.

Magneto-optical Kerr spectra of gold induced by spin accumulation

We report the magneto optic Kerr effect (MOKE) angle of Au magnetically excited by spin accumulation. We perform time resolved polar MOKE measurements on Au/Co heterostructures. In our experiment, the ultrafast optical excitation of the Co drives spin accumulation into an adjacent Au layer. The spin accumulation, together with spin-orbit coupling, leads to non-zero terms in the off-diagonal conductivity tensor of Au, which we measure by recording the polarization and ellipticity of light reflected from the Au surface for photon energies between 1.3 and 3.1 eV. At energies near the interband transition threshold of Au, the Kerr rotation per A/m exceeds 1 urad. Typical values for Kerr rotation per moment in transition ferromagnetic metals like Ni are < 10 nrad per A/m, while predicted values for heavy metals like Pt or W are < 13 nrad per A/m. The exceptional sensitivity of the optical properties of Au to spin magnetic moments make Au to be an exceptionally sensitive optical magnetometer, with potential applications in the development of optospintronic technologies.

Magnetic domain structure of the ferrimagnetic (001) NiCo2O4 film with perpendicular magnetic anisotropy

A 65-nm thick epitaxial (001) NiCo2O4 film that has metallic and ferrimagnetic properties at room temperature was grown on a (001) MgAl2O4 substrate. Evolutions in magnetic domains of the ferrimagnetic (001) NiCo2O4 film with perpendicular magnetic anisotropy were observed using polar magneto-optical Kerr effect microscopy with changes in magnetic field, temperature, or time. The magnetic domain structure in the magnetic field exhibited noticeable changes around a coercive field; whereas, its change in temperature or time at a fixed magnetic field was indistinct. This suggests that the ferrimagnetic (001) NiCo2O4 film has a relatively stable magnetic domain structure in comparison with Co alloy films with perpendicular magnetic anisotropy and fast domain wall motion. Conclusively, the magnetic domain variation in the NiCo2O4 film signified that the magnetization process at an early stage was mainly caused by domain rotation behavior rather than domain wall motion.

Interface magnetization in Y3Fe5O12 / Nd3Ga5O12 epitaxial heterostructures caused by Fe3+ - Nd3+ exchange coupling

The spectral and magnetic field dependences of the polar magneto-optical Kerr effect (PMOKE) were studied in heterostructures formed by 10 - 40 nm Y3Fe5O12 (YIG) layer grown by laser molecular beam epitaxy on Nd3Ga5O12 (NdGG) and Gd3Ga5O12 (GGG) substrates. The PMOKE spectra can be decomposed into two components, one of which, caused by the Faraday effect in the substrate, is linear in magnetic field H, and the second is saturated at H ∼ 0,04 – 0,056 MA/m. In YIG / NdGG, the spectral dependence of the substrate-caused PMOKE clearly reveals a series of narrow bands corresponding to electronic transitions (absorption bands) in the Nd3+ ion. The saturable component of PMOKE, which reflects the behavior of the magnetization of the YIG film, is similar to that observed in YIG / GGG, but at the same time clearly reveals anomalies at the photon energies, corresponding to electronic transitions in the Nd3+ ion. This indicates the presence of magnetization of Nd3+ ions in the interface region of the YIG / NdGG structure, induced by the exchange interaction Nd3+ - Fe3+. The signs of the PMOKE anomalies and possible influence of NdGG substrate complex refractive index on PMOKE caused by YIG film are discussed.