Polar Magneto-optical Kerr Effect Research Articles

Vector-Resolved Magnetooptic Kerr Effect Measurements of Spin–Orbit Torque

We demonstrate simultaneous detection of current-driven dampinglike and fieldlike spin–orbit torques in heavy metal/ferromagnetic metal bilayers by measuring all three magnetization components $m_{x},m_{y}$ , and $m_{z} $ using a vector-resolved magnetooptic Kerr effect (MOKE) technique based on quadrant detection. We investigate the magnitude and direction of spin–orbit torques in a series of platinum/permalloy samples, finding good agreement with results obtained via polar and quadratic MOKE measurements without quadrant detection.

Magneto-optical Kerr spectroscopy in ferromagnetic semiconductors: determination of the intrinsic complex magneto-optical Voigt constant

Thin films of ferromagnetic semiconductors of the (Ga, Mn)(As, P) family are investigated by means of magneto-optical Kerr spectroscopy in order to determine the spectral dependence of the complex magneto-optical Voigt constant an intrinsic parameter responsible for Kerr rotation and ellipticity. We obtain the spectral dependence of these two conjugate quantities: the Kerr rotation and ellipticity angles. Contrary to strongly absorbing metallic ferromagnetic layers, it is expected that they vary non-linearly with the layer thickness at a fixed wavelength, as a consequence of the weak absorption and the non-negligible optical phase shift in weakly absorbing semiconductor ferromagnets. We show that, from the Kerr rotation and ellipticity angles and the complex refractive index –obtained by ellipsometric measurements—we are able to determine the spectral dependence of the complex intrinsic Voigt constant, thus avoiding the need for samples with different thicknesses. Incidentally, the use of phosphorus substituted (Ga,Mn)As provides an out-of-plane magnetic easy axis, the appropriate configuration for polar magneto-optical Kerr effect, without the need for a strong external magnetic field.

Time resolved imaging of the non-linear bullet mode within an injection-locked nano-contact spin Hall nano-oscillator

Time-resolved scanning Kerr microscopy (TRSKM) has been used to image precessional magnetization dynamics excited by a DC current within a nano-contact (NC) spin Hall nano-oscillator (SHNO). Injection of a radio frequency (RF) current was used to phase lock the SHNO to TRSKM. The out of plane magnetization was detected by means of the polar magneto optical Kerr effect (MOKE). However, longitudinal MOKE images were dominated by an artifact arising from the edges of the Au NCs. Time resolved imaging revealed the simultaneous excitation of a non-linear “bullet” mode at the centre of the device, once the DC current exceeded a threshold value, and ferromagnetic resonance (FMR) induced by the RF current. However, the FMR response observed for sub-critical DC current values exhibits an amplitude minimum at the centre, which is attributed to spreading of the RF spin current due to the reactance of the device structure. This FMR response can be subtracted to yield images of the bullet mode. As the DC current is increased above threshold, the bullet mode appears to increase in size, suggesting increased translational motion. The reduced spatial overlap of the bullet and FMR modes, and this putative translational motion, may impede the injection locking and contribute to the reduced locking range observed within NC-SHNO devices. This illustrates a more general need to control the geometry of an injection-locked oscillator so that the autonomous dynamics of the oscillator exhibit strong spatial overlap with those resulting from the injected signal.

Micro magnetic dendritic domain structure in the demagnetized state of perpendicular magnetic ultrathin film: The effect of the width of the distribution of the energy barriers

Abstract We have used polar Kerr microscopy to study the magnetic domain structure in an ultrathin Au/Co(1 nm)/Au layer with perpendicular magnetic anisotropy. We present our demagnetization method of the sample. In the demagnetized state, we observed a dendritic domain structure. We theoretically calculated the lateral domain size in the demagnetized state by using two different models and we saw that the prefactor x = 0.389 corresponds to the domain structure observed. We used polar magneto-optical Kerr effect magnetometry (PMOKE) to study the magnetization reversal and to extract an activation energy E a in zero field and the width of the distribution of the energy barriers σ w . We correlated the dendritic domain structure observed with the large value of σ w .

Roles of Joule heating and spin-orbit torques in the direct current induced magnetization reversal

Current-induced magnetization reversal via spin-orbit torques (SOTs) has been intensively studied in heavy-metal/ferromagnetic-metal/oxide heterostructures due to its promising application in low-energy consumption logic and memory devices. Here, we systematically study the function of Joule heating and SOTs in the current-induced magnetization reversal using Pt/Co/SmOx and Pt/Co/AlOx structures with different perpendicular magnetic anisotropies (PMAs). The SOT-induced effective fields, anisotropy field, switching field and switching current density (Jc) are characterized using electric transport measurements based on the anomalous Hall effect and polar magneto-optical Kerr effect (MOKE). The results show that the current-generated Joule heating plays an assisted role in the reversal process by reducing switching field and enhancing SOT efficiency. The out-of-plane component of the damping-like-SOT effective field is responsible for the magnetization reversal. The obtained Jc for Pt/Co/SmOx and Pt/Co/AlOx structures with similar spin Hall angles and different PMAs remains roughly constant, revealing that the coherent switching model cannot fully explain the current-induced magnetization reversal. In contrast, by observing the domain wall nucleation and expansion using MOKE and comparing the damping-like-SOT effective field and switching field, we conclude that the current-induced magnetization reversal is dominated by the depinning model and Jc also immensely relies on the depinning field.

Magnetic domain wall creep and depinning: A scalar field model approach.

Magnetic domain wall motion is at the heart of new magnetoelectronic technologies and hence the need for a deeper understanding of domain wall dynamics in magnetic systems. In this context, numerical simulations using simple models can capture the main ingredients responsible for the complex observed domain wall behavior. We present a scalar field model for the magnetization dynamics of quasi-two-dimensional systems with a perpendicular easy axis of magnetization which allows a direct comparison with typical experimental protocols, used in polar magneto-optical Kerr effect microscopy experiments. We show that the thermally activated creep and depinning regimes of domain wall motion can be reached and the effect of different quenched disorder implementations can be assessed with the model. In particular, we show that the depinning field increases with the mean grain size of a Voronoi tessellation model for the disorder.

First-order reversal curve investigated magnetization switching in Pd/Co/Pd wedge film**Project supported by the National Basic Research Program of China (Grant Nos. 2015CB921403 and 2016YFA0300701) and the National Natural Science Foundation of China (Grant Nos. 51427801, 11374350, and 51671212).

The magnetization switching plays an essential role in spintronic devices. In this study, a Pd(3 nm)/Co(0.14–1.68 nm)/Pd(5 nm) wedge film is deposited on an MgO (111) substrate by molecular beam epitaxy. We investigate the polar magneto-optical Kerr effect (MOKE) and carry out the first-order reversal curve (FORC) measurements. For the wedge system, it is observed that the Co thickness could drive the spin reorientation transition (SRT) from out-of-plane to in-plane. Meanwhile, we find the different types of magnetization switchings in the continuous SRT process, which can originate from the formation of different magnetic compositions. Our work provides the possibility of tuning the interfacial effect, and paves the way to analyzing magnetization switching.

Growth and Structural, Magnetic, and Magnetooptical Properties of ZnO Films Doped with a Fe57 3d Impurity

ZnO films obtained by high-frequency magnetron sputtering and doped with a Fe57 metallic 3d impurity by the diffusion method are studied. The type of local environment of Fe57 impurity atoms on varying the deposition parameters of ZnO films is determined by Mossbauer spectroscopy. It is established that the ground state of Fe57 impurity atoms corresponds to metallic iron in the magnetically ordered state and there is a small fraction of Fe57 atoms with a local environment corresponding to the complex oxide Fe3O4, having the magnetically ordered state; there is also a fraction of iron atoms in the paramagnetic state. The magnetic and magnetooptical parameters of the films were measured using magnetooptic Kerr effect. The spectral dependences of the polar magnetooptic Kerr effect in ZnO(Fe57) films are measured in a photon energy range of 1.5–4.5 eV and simulated by the effective-medium method. It is established that ZnO(Fe57) possess an easy-plane magnetic anisotropy with a magnetization lying in the film plane.

Laser MBE-grown CoFeB epitaxial layers on MgO: Surface morphology, crystal structure, and magnetic properties

Epitaxial layers of CoFeB were grown on MgO by means of laser molecular beam epitaxy using $\mathrm{C}{\mathrm{o}}_{40}\mathrm{F}{\mathrm{e}}_{40}{\mathrm{B}}_{20}$ target. The growth was combined with in situ structural characterization by three-dimensional reciprocal space mapping obtained from reflection high energy electron diffraction (RHEED) data. High-temperature single stage growth regime was adopted to fabricate CoFeB layers. As confirmed by the atomic force microscopy, the surface of CoFeB layers consists of closely spaced nanometer sized islands with dimensions dependent on the growth temperature. As shown by RHEED and XRD analysis, the CoFeB layers grown at high-temperature on MgO(001) possess body centered cubic (bcc) crystal structure with the lattice constant $a=2.87\phantom{\rule{0.16em}{0ex}}\AA{}$ close to that of the $\mathrm{C}{\mathrm{o}}_{75}\mathrm{F}{\mathrm{e}}_{25}$ alloy. It was further shown that following the same high-temperature growth technique the MgO/CoFeB/MgO(001) heterostructures can be fabricated with top and bottom MgO layers of the same crystallographic orientation. The CoFeB layers were also grown on the GaN(0001) substrates using MgO(111) as a buffer layer. In this case, the CoFeB layers crystallize in bcc crystal structure with the (111) axis perpendicular to the substrate surface. The magnetic properties of the CoFeB/MgO (001) heterostructures have been investigated by measuring magnetization curves with a vibrating sample magnetometer as well as by performing magneto-optical Kerr effect (MOKE) and ferromagnetic resonance (FMR) studies. FMR spectra were obtained for the variety of the magnetic field directions and typically consisted of a single relatively narrow resonance line. The magnetization orientations and the resonance conditions were calculated in the framework of a standard magnetic energy minimization procedure involving a single ${K}_{1\mathrm{c}}$ cubic term for the magnetocrystalline anisotropy. This allows a fairly accurate description of the angular dependences of the resonance fields---both in-plane and out-of-plane. It was shown that CoFeB layers exhibit in-plane fourth-order magnetic anisotropy. A two-step magnetization reversal model has been adopted for the CoFeB layers based on the VSM measurement analysis. Magnetization reversal studies performed by polar MOKE indicate that the magnetization lies in-plane in absence of magnetic field. Observed magnetic field dependences of reflected light ellipticity in geometry of longitudinal Kerr effect give convincing evidence for contribution of quadratic in magnetization terms in the dielectric tensor and clearly show the in-plane magnetization rotation.

Magneto-optical properties of BaTiO3/La0.76Sr0.24MnO3/BaTiO3 heterostructures

The magnetic properties of epitaxial BaTiO3/La0.76Sr0.24MnO3/BaTiO3 (BTO/LSMO/BTO) heterostructures have been studied using magneto-optic Kerr effect (MOKE) technique. Both longitudinal and polar MOKE were probed as a function of magnetic field and temperature (in the range between 80 and 320 K) for epitaxial films of BTO/LSMO/BTO and LSMO grown on TiO2-terminated SrTiO3 (001) substrates by pulsed laser deposition technique. The LSMO film without the BTO layers exhibits nearly square field-dependent MOKE hysteresis loops with low saturation fields below a bulk-like Curie temperature (TC) of ∼ 350K. In contrast, the film with the BTO layers exhibits a significantly suppressed TC of 155 K, accompanied by significantly enhanced coercive fields and perpendicular magnetic anisotropy.

Demonstration of four-state memory structure with perpendicular magnetic anisotropy by spin-orbit torque

Current, instead of magnetic field, induced magnetization switching is very important for future spintronics in information storage or/and information processing. As one of the effective current-induced magnetization methods, spin-orbit torque (SOT) has aroused considerable interest because it has low-power consumption and can improve the device endurance. Normal metal (NM)/ferromagnetic metal (FM) are the common materials used for SOTs, where the NM denotes the materials with strong spin-orbit coupling such as Pt, Ta, W, etc. Owing to the spin Hall effect, the in-plane current in NM layer can be converted into a vertical spin current that exerts torques on the adjacent FM layers. Spin current can also come from the NM/FM interface charge-spin conversion due to interfacial asymmetry, exerting torques on the adjacent FM layers. Materials with in-plane and perpendicular magnetic anisotropy are used to study the SOT-induced magnetization switching. Compared with the memories using the in-plane ferromagnetic films, the magnetic memories using NM/FM multilayers with perpendicular magnetic anisotropy can have much high integration density. Currently the used information storage was based on the two-state memory cell. Owing to more than two states contained in one memory cell, multiple states memory manipulated by electric current could further benefit the higher-density memory. In this paper, a four-state memory unit is demonstrated by the influence of TaOx buffer layer on the magnetic anisotropy of Pt/Co/Pt multilayers. The memory unit consists of two regions. One is directly deposited on thermal oxide Si substrate[Pt(3 nm)/Co(0.47 nm)/Pt(1.5 nm)] and the other has a buffer layer of TaOx[TaOx(0.3 nm)/Pt(3 nm)/Co(0.47 nm)/Pt(1.5 nm)], thus leading to the difference in magnetic property between these two regions. According to the Z axis magnetic hysteresis loops of two regions, measured by polar magneto-optical Kerr effect, the coercivity of the region with TaOx is obtained to be 23 Oe and that without TaOx is 11 Oe. At the junction between two regions, the magnetic hysteresis loop shows the superposition of hysteresis loops of two regions, resulting in switching four times as the magnetic field changes. Under a fixed magnetic field along the current direction, the magnetization orientation of region with TaOx and that of region without TaOx are switched by spin-orbit torques with threshold currents of 5 mA and 1.5 mA respectively. The switching direction can be changed as the in-plane magnetic field changes to the opposite direction, which is one of the typical features of SOTs-induced magnetization switching. At the junction between two regions, through applying different-form current pulses to one conductive channel of the device, the magnetic state of the memory cell can be switched between four clear states. This kind of structure provides a new idea to design SOT multi-state memory devices.

Рост, структурные, магнитные и магнитооптические свойства пленок ZnO, легированных 3d-примесью Fe-=SUP=-57-=/SUP=-

AbstractZnO films obtained by high-frequency magnetron sputtering and doped with a Fe^57 metallic 3 d impurity by the diffusion method are studied. The type of local environment of Fe^57 impurity atoms on varying the deposition parameters of ZnO films is determined by Mössbauer spectroscopy. It is established that the ground state of Fe^57 impurity atoms corresponds to metallic iron in the magnetically ordered state and there is a small fraction of Fe^57 atoms with a local environment corresponding to the complex oxide Fe_3O_4, having the magnetically ordered state; there is also a fraction of iron atoms in the paramagnetic state. The magnetic and magnetooptical parameters of the films were measured using magnetooptic Kerr effect. The spectral dependences of the polar magnetooptic Kerr effect in ZnO(Fe^57) films are measured in a photon energy range of 1.5–4.5 eV and simulated by the effective-medium method. It is established that ZnO(Fe^57) possess an easy-plane magnetic anisotropy with a magnetization lying in the film plane.

Overview and advances in skyrmionics

Microelectronic technologies have been developing rapidly in the past half-century following the famous Moore's Law. However, this tendency is beginning to break down due to the thermal effects induced by the leakage current and data traffic. Spintronics sheds light on eliminating this bottleneck by using the spin degree of electron, which attracts great attention from both the academia and industry. The magnetic skyrmion is a particle-like spin texture with topological protection, envisioned as an energy efficient spintronic information carrier due to its nanoscale size, ultra-low driven energy, and high thermal stability. Recent research progress shows that the nucleation, transportation, and detection of skyrmion in room temperature, which affirm its potential application in electronics, lead to a new research field called skyrmionics. In this review article, we first introduce the fundamental concepts and recent progress of magnetic skyrmions, from both the theoretical and experimental point of view. Different types of magnetic skyrmions have different properties due to their physical dynamics. We only focus on the skyrmions stabilized by Dzyaloshinskii-Moriya interaction (DMI) in the ultra-thin film structures as their small size, high mobility and room temperature stability can provide the perspectives for electronic devices. The skyrmions have already been extensively investigated from both the theoretical and experimental aspects in recent years. Micromagnetic simulation is the main approach to theoretically studying the dynamics of skyrmions and their applications. Most of the innovative skyrmionic devices have first been demonstrated by this method. Experimentally, skyrmions can be measured by various methods, such as the neutron scattering, Lorentz transmission electron microscopy, scanning X-ray transmission microscopy, polar magneto-optical Kerr effect microscope, etc. In the third part of this paper, we present four basic functions of skyrmionic devices ranging from nucleation, motion, detection, to manipulation. The nucleation of skyrmions, corresponding to the information writing in skyrmionic devices, has been widely investigated. A skyrmion can be nucleated by conversion from domain wall pairs, local spin injection, local heating, and spin waves. Then, we focus on the current induced skyrmion motion and compare the two different torques:the spin transfer torque and the spin orbit torque. To read the data, it is necessary to detect skyrmions electrically. One way is to measure the topological Hall effect in a Hall bar. More commonly, skyrmions can be detected through magnetoresistance effects, i.e., giant magnetoresistance/anisotropic magnetoresistance, tunnel magnetore sistance, and non-collinear magnetoresistance, in a junction geometry. For manipulation, it is mainly demonstrated by the voltage controlled magnetic anisotropy (VCMA). Finally we discuss several representative skyrmionic nano-devices in memory, logic, and neuromorphic applications. The magnetic tunnel junction and the racetrack are two common designs for skyrmionic memory devices. The former can store multiple values in one bit, and the latter can realize fast and efficient data transmission. To control the skyrmionic data in these memories, the VCMA effect is one of the promising approaches, which is used in several designs. For the skyrmionic logic devices, they can be divided into two main types:the transistor and the logic gate. However, until now, these ideas are only demonstrated in simulation, and more efforts in experiment are needed. Besides, novel devices such as artificial synapses and neurons can be realized more naturally by skyrmion due to its particle-like property. In summary, skyrmionics is promising in several aspects, including performance improvement, emerging function and architecture design, and bio-inspired computing. Remarkable progress has been made in the past few years, however the device integration, the materials, and the data transmission still restrict its application. We hope this overview article may present a clear picture about skyrmionics and receive more attention, thus promoting its fast research and development in the future.

Magneto-optical Kerr spectroscopy of noble metals

Magneto-optical (MO) response of the noble metals Cu, Ag, and Au in the joint experimental and ab initio theoretical study is reported. The magneto-optical polar Kerr effect (MOKE) spectra of the noble-metal films were measured with the high sensitivity in the applied magnetic field of 1.5 T over the photon energy range 0.74--5.8 eV. Complete set of the optical conductivity tensor elements was determined precisely from the MOKE and the optical spectra measured at the same energy points. The importance of the off-diagonal intraband Drude-type transitions is demonstrated explicitly for each noble metal and found to be a substantial contribution to the observed spectra. It is shown that the first-principles calculations using the spin-polarized fully relativistic Dirac linear-muffin-tin-orbital method with the inclusion of correlation effects by GGA+$U$ approach reproduce well the experimental spectra and allow to explain the microscopic origin of the noble metals' magneto-optical response in terms of interband transitions. Although the energy band structures of Cu, Ag, and Au are very similar, there are some distinctive differences in bandwidths and the energy positions of the bands (especially in $X$ and $L$ symmetry points), mainly due to different spin-orbit splitting and differences in the spatial extent of $3d, 4d$, and $5d$ valence wave functions of noble metals. It was found that the small differences in the band positions lead to significant differences in the MO properties of three noble metals. Although the spin-orbit interaction in Au is about six times larger than in Cu, and approximately two times larger than in Ag, the absolute value of Kerr rotation in Au is of the same magnitude as in Cu and one order of magnitude smaller as compared to Ag. The sharp Kerr effect spectral peak in Ag is not due to the electronic interband transitions, but rather to the plasma-edge splitting. The band-by-band decomposition of the Cu, Ag, and Au MO spectra is presented and the interband transitions responsible for the prominent structures in the spectra are identified. It has been found that main magneto-optical activity of noble metals in external magnetic field originates from interband transitions at well-defined small-volume regions of Brillouin zone located near the ``neck'' and ``belly'' of the Fermi surface.

Excess velocity of magnetic domain walls close to the depinning field

Magnetic field driven domain wall velocities in [Co/Ni] based multilayers thin films have been measured using polar magneto-optic Kerr effect microscopy. The low field results are shown to be consistent with the universal creep regime of domain wall motion, characterized by a stretched exponential growth of the velocity with the inverse of the applied field. Approaching the depinning field from below results in an unexpected excess velocity with respect to the creep law. We analyze these results using scaling theory to show that this speeding up of domain wall motion can be interpreted as due to the increase of the size of the deterministic relaxation close to the depinning transition. We propose a phenomenological model which allows to accurately fit the observed excess velocity and to obtain characteristic values for the depinning field $H_d$, the depinning temperature $T_d$, and the characteristic velocity scale $v_0$ for each sample.

Influence of Nanovoids Depth on the Surface Magneto-Optical Kerr Effect Modulations in Magnetic Photonic Crystal Slab

Magnetic photonic crystal slab with nanovoids buried in Ni films were fabricated using nanosphere lithography combined with electrochemical deposition. One featured structure was dominated the spectra of polar surface magneto-optical Kerr effect (MOKE) of samples with different voids depth. For the shallow ones, surface lattice resonance (SLR) was revealed through angle resolved spectra analysis, where the location of degeneracy points corresponded to that of the featured structure in MOKE signal. For the deeper ones, localized plasmons and degeneracy points were all demonstrated in angle resolved reflection spectra, where localized plasmons dominated the resonant MOKE signal, and the degeneracy points blue-shifted compared with that of shallow ones. Changing the lattice of samples, similar phenomena were presented. The physical mechanism of SLR could be attributed to the diffraction of surface plasmons by the reciprocity vector, and the localized plasmons were attributed to the Mie scattering. The featured structure in polar MOKE spectra demonstrated high sensitivity, which might pave the way to on-chip MO devices.

A high-resolution combined scanning laser and widefield polarizing microscope for imaging at temperatures from 4 K to 300 K.

Polarized light microscopy, as a contrast-enhancing technique for optically anisotropic materials, is a method well suited for the investigation of a wide variety of effects in solid-state physics, as, for example, birefringence in crystals or the magneto-optical Kerr effect (MOKE). We present a microscopy setup that combines a widefield microscope and a confocal scanning laser microscope with polarization-sensitive detectors. By using a high numerical aperture objective, a spatial resolution of about 240 nm at a wavelength of 405 nm is achieved. The sample is mounted on a 4He continuous flow cryostat providing a temperature range between 4 K and 300 K, and electromagnets are used to apply magnetic fields of up to 800 mT with variable in-plane orientation and 20 mT with out-of-plane orientation. Typical applications of the polarizing microscope are the imaging of the in-plane and out-of-plane magnetization via the longitudinal and polar MOKE, imaging of magnetic flux structures in superconductors covered with a magneto-optical indicator film via the Faraday effect, or imaging of structural features, such as twin-walls in tetragonal SrTiO3. The scanning laser microscope furthermore offers the possibility to gain local information on electric transport properties of a sample by detecting the beam-induced voltage change across a current-biased sample. This combination of magnetic, structural, and electric imaging capabilities makes the microscope a viable tool for research in the fields of oxide electronics, spintronics, magnetism, and superconductivity.

Effects of oxide replacement with fluoride at the CoFeB interface on interface magnetic anisotropy and its voltage control

In this paper we report results of improving Co60Fe20B20 interface perpendicular magnetic anisotropy (PMA) by replacing neighbor oxide layer with fluoride one. We expected that fluorine as element with higher than oxide electronegativity could more effectively attract electrons from out-of-plane d orbitals of ferromagnetic, increasing role of in-plane orbitals. By this we wanted to increase PMA and its response to applied voltage bias. Polar magneto-optic Kerr effect measurement show decreasing of out-of-plane magnetic field needed to change magnetization to perpendicular in stacks with oxygen replaced by fluorine as well as increasing of coefficient of response to applied voltage α from < 10 fJ/Vm for CoFeB/Al2O3 interface to 20 fJ/Vm for CoFeB/AlF3/Al2O3 and 22 fJ/Vm for CoFeB/MgF2 stacks. Direct chemical interaction of Co with F was confirmed by x-ray photoelectron spectroscopy (XPS) measurement of Co2p core level region. Moreover angular-resolved XPS showed that F tends to stay at CoFeB interface rather than diffuse out of it.

Resonant magneto–optical Kerr effect induced by hybrid plasma modes in ferromagnetic nanovoids**Project supported by the Natural Science Foundation of Shandong Province, China (Grant No. ZR2015AM024) and the Doctoral Research Started Funding of Qufu Normal University, China (Grant No. BSQD20130152).

With nanovoids buried in Co films, resonant structures were observed in spectra of polar magneto–optical Kerr effect (MOKE), where both a narrow bandwidth and high intensity were acquired. Through changing the thickness of Co films and the lattice of voids, different optical modes were introduced. For a very shallow array of voids, the resonant MOKE was induced by Ag plasma edge resonance, for deeper ones, hybrid plasma modes, such as void plasmons in the voids, surface lattice plasmons between the voids, and the co-action of them, etc. resulted in resonant MOKE. We found that resonant MOKE resulted from the void plasmons resonance which possesses the narrowest bandwidth for the lowest absorption of voids. The simulated electromagnetic field (EF) distribution consolidated different effects of these three optical modes on resonant MOKE modulation. Such resonant polar MOKE possesses high sensitivity, which might pave the way to on-chip MO devices.

Magnetization switching behavior of exchange-coupled bilayer nanodots characterized by magneto-optical Kerr effect

We investigated the magnetization switching behavior of exchange-coupled bilayer nanodots consisting of a soft magnetic Ni81Fe19 (permalloy; Py) layer and a perpendicularly magnetized L10-FePt layer by measuring the polar magneto-optical Kerr effect (MOKE). A MOKE system having a 2-μm-diameter laser spot was used, which detected the signal from around 20 nanodots of 200 nm in diameter. The measured polar MOKE loops exhibited a two-step behavior attributable to each magnetization reversal process of the Py and L10-FePt layers, allowing us to evaluate the switching field (Hsw) of L10-FePt from the MOKE loop. Under the application of both a static magnetic field and an rf magnetic field (hrf), the nanodots showed a clear Hsw reduction with characteristic hrf frequency dependence. This Hsw reduction is well interpreted as a behavior of spin wave-assisted magnetization switching. We also discuss the influence of the Py layer thickness on spin wave-assisted Hsw reduction.