Kerr Intensity Research Articles

Generation and control of circular Airy solitons in fractional nonlinear optical systems under different modes

In this paper, the dynamics of the circular Airy beam (CAB) in the spatial fractional nonlinear Schrödinger equation (FNLSE) optical system are investigated. The propagation characteristics of CABs modulated by the quadratic phase modulation (QPM) in a Kerr (cubic) nonlinear medium under power function diffractive modulation modes and parabolic potentials are numerically simulated by using a step-by-step Fourier method. Specifically, the threshold for CABs to form solitons in the Kerr medium is controlled by the Lévy index and the QPM coefficient. Secondly, the parabolic potential has the ability to stabilize the FNLSE optical system, making it easier for the formation of CAB solitons. The addition of QPM allows the refocusing of the split beam caused by the Lévy index, and it can change the position and intensity of solitons. Finally, we also study the transmission evolution of QPM-modulated CABs in the Kerr medium under the power function diffraction modulation mode. We can obtain different types of solitons by varying the power function modulation coefficients. A dark soliton with high stability is formed, and we can control its size. Results show that it is possible to optimize the parameter settings (parabolic potential coefficients, power function modulation coefficients, QPM coefficients, Lévy indices, and nonlinear Kerr intensity coefficients) to obtain different types of solitons as well as to modulate the soliton transport. It provides more degrees of freedom for the study of CAB soliton propagation in the Kerr media, which is of great significance and application in fields of nonlinear optical transport, particle manipulation, and optical metrology.

Growth of ultra-thin Cobalt on fullerene (C60) thin-film: in-situ investigation under UHV conditions

With the advent of organic spintronics, material such as fullerene (C60) is drawing immense attention due to the long spin relaxation time. C60 based spin valve structures are always challenging due to the complicated structure at the interfaces of metal (cathode) and C60. In the present work, the growth of e-beam evaporated Co films on the C60 layer has been studied in-situ, starting from a fraction of nanometer to tens of nanometers, using reflection high energy electron diffraction (RHEED), electron transport (ET) and magneto-optic Kerr effect (MOKE) measurements. While RHEED and ET provided information about structure and film morphology, in-situ MOKE provided information about magnetic properties, thus making it possible to correlate the evolution of magnetic properties with that of structure and morphology of the Co thin film. The film grows via island growth, where small isolated islands grow larger to connect with other islands and eventually combine into a continuous film at around 75 Å thickness. Combined analysis of Kerr intensity and resistivity variation with Co thickness revealed the origin of the 23 Å thick magnetic dead layer at the interface, which is attributed to the formation of small clusters with paramagnetic or superparamagnetic properties.

A Review of Many-Body Interactions in Linear and Nonlinear Plasmonic Nanohybrids

In this review article, we discuss the many-body interactions in plasmonic nanohybrids made of an ensemble of quantum emitters and metallic nanoparticles. A theory of the linear and nonlinear optical emission intensity was developed by using the many-body quantum mechanical density matrix method. The ensemble of quantum emitters and metallic nanoparticles interact with each other via the dipole-dipole interaction. Surfaces plasmon polaritons are located near to the surface of the metallic nanoparticles. We showed that the nonlinear Kerr intensity enhances due to the weak dipole-dipole coupling limits. On the other hand, in the strong dipole-dipole coupling limit, the single peak in the Kerr intensity splits into two peaks. The splitting of the Kerr spectrum is due to the creation of dressed states in the plasmonic nanohybrids within the strong dipole-dipole interaction. Further, we found that the Kerr nonlinearity is also enhanced due to the interaction between the surface plasmon polaritons and excitons of the quantum emitters. Next, we predicted the spontaneous decay rates are enhanced due to the dipole-dipole coupling. The enhancement of the Kerr intensity due to the surface plasmon polaritons can be used to fabricate nanosensors. The splitting of one peak (ON) two peaks (OFF) can be used to fabricate the nanoswitches for nanotechnology and nanomedical applications.

Thickness dependent magnetic properties of ferromagnetic films (Fe, Co) interfaced with Ta

In the present work, magnetic properties of ferromagnetic films viz. Co and Fe interfaced with Ta have been studied using magneto-optical Kerr effect (MOKE), X-ray reflectivity (XRR) and soft X-ray absorption spectroscopy (SXAS) on wedge shaped samples. Thickness of ferromagnetic film at different points of wedge sample as determined from XRR were employed to study the evolution of magnetic properties with increasing film thickness using MOKE measurements. Co film exhibits a uniaxial magnetic anisotropy on Ta which varies as a function of increasing film thickness, while no thickness dependent magnetic anisotropy has been observed for Fe film grown on Ta. The variation of coercivity with increasing film thickness is in agreement with the observed variation of magnetic anisotropy. SXAS measurements qualitatively support alloying / intermixing at interface. Thicknesses of interfacial magnetic dead layer as seen from variation of Kerr intensity with film thickness is relatively higher for Co as compared to Fe, while interfaced with Ta.

Magnetization and Giant Magnetoimpedance Effect of Co-Rich Microwires under Different Driven Currents

Co68.25Fe4.5Si12.25B15amorphous microwires with a diameter of 34 μm were prepared via the melt extraction method. The dependency of AC driving currentIacand frequency on giant magnetoimpedance (GMI) effect and magnetization were investigated using a 4294A impedance analyzer and the transverse Kerr effect. The GMI effect was analyzed whenIacchanged from 6 mA to 20 mA at a frequency ranging from 0.1 MHz to 15 MHz. The influence of AC current dependent on the frequency is correlated with the magnetization mechanism. The maximum transverse Kerr intensity (MTKI) decreased with the increase inIacunder direct magnetic field when the frequency was below megahertz. However, MTKI values were similar with the increase ofIacwhen it was over 2 MHz. Meanwhile, the GMI effect was optimized by selecting an adequate value of AC driving currentIp, at which the circular permeability was higher when the frequency was not over 2 MHz. Results showed that the influence ofIacon magnetoimpedance became weak with strong skin effect and slightly stronger GMI effect driven by a higherIacwhen the frequency was between 2 MHz and 15 MHz. The skin effect turned out to be the key factor to the GMI effect; thus, there were no obvious differences in magnetization and GMI effect with AC driving current changing when the frequency was as high as 15 MHz.

Layered structure and related magnetic properties for annealed Fe/Ir(111) ultrathin films

After annealing treatments for fcc-Fe/Ir(111) below 600 K, the surface layers remain pseudomorphic. The Ir(111) substrate plays an important role on the expanded Fe lattice. At temperatures between 750 and 800 K, the surface composition shows a stable state and a c(2 × 4) structure is observed. We discover a layered structure composed of some Fe atoms on the top of a Fe0.5Ir0.5 interfacial alloy supported on the Ir(111) substrate. The competition between the negative formation heat of Fe0.5Ir0.5 and surface free energy of Fe causes the formation of layered structure. The existence of ferromagnetic dead layer coincides with the formation of fcc-Fe for ultrathin Fe on Fe0.5Ir0.5/Ir(111). For Fe films thicker than three monolayers, the linear increase of the Kerr intensity versus the Fe coverage is related to the growing of bcc-Fe on the surface where the Fe layer is incoherent to the underlying Fe0.5Ir0.5/Ir(111). These results emphasize the importance of the substrate induced strain and layered structure of Fe/Fe0.5Ir0.5/Ir(111) on the magnetic properties and provide valuable information for future applications.

Pinning of magnetic moments at the interfacial region of ultrathin CoO/Co bilayers grown on Ge(1 0 0)

For CoO overlayers prepared by evaporating Co atoms in an oxygen atmosphere, both the oxidation of the Co atoms at the interface and the segregation of oxygen atoms into the Co surface occur. Parts of Co atoms at the interface become nonferromagnetic and this causes the reduction of Kerr intensity at 300K. After field cooling treatments, further reduction of the Kerr intensity is detected. The change of the Kerr intensity is an indicator of the pinned magnetic moments. In the case of forming thicker interfacial region, more pinned magnetic moments are observed and result in the larger exchange bias field for CoO on thicker Co/Ge(100). This tunable pinning provides a practical way of increasing exchange bias field by controlling the thickness of the interfacial region for ultrathin CoO/Co bilayers on semiconductor substrates.

Critical hydrogenation effect on magnetic coercivity of perpendicularly magnetized Co/Pd multilayer nanostructures

Low dimensional materials of perpendicularly magnetized [Co(0.5 nm)/Pd(3 nm)] multilayer, including continuous thin film, nanodots and nanodot-chains were prepared for the investigation of reversible hydrogenation effect on the magnetic and optical properties. For the continuous film, after hydrogenation the magnetic coercivity (Hc) was enhanced by 47% and the Kerr intensity was significantly reduced to 10% of the pristine value. In nanodots, hydrogenation led to 25% reduction of Hc and Kerr intensity as well. For nanodot-chains, the shape of magnetic hysteresis loop was modulated by hydrogenation. The hydrogenation and desorption completed within few seconds in nanodots.

Structure Related Magnetic Dead Layer for Ultrathin Fe/Ir(111) Films

Magnetic properties and surface structure of ultrathin Fe/Ir(111) films have been investigated using the surface magneto-optic Kerr effect and low-energy electron diffraction. Layer-by-layer growth of Fe/Ir(111) is observed for the first three monolayers at room temperature. For Fe thinner than three monolayers, pseudomorphic growth of Fe films is observed. The layer distance is close to that of fcc(111) Fe. For Fe thicker than three monolayers, the surface structure can be identified to be related to the bcc(110) arrangement of Fe atoms in Kurdjumov-Sachs orientation. As the Fe thickness increases, the linear increase of the Kerr intensity is observed. The Kerr intensity comes from the bcc-Fe and a thin magnetic dead layer is observed at the interface.

Compositions and Magnetic Properties of CoO/Co/Ge(111) Films

On the top of Co/Ge(111) films, cobalt oxides are prepared by evaporating Co atoms in an oxygen atmosphere. Depth profiling measurements show a layered structure of a pure Co layer covered by a CoO overlayer. As the CoO thickness increases, the Auger intensity ratio of O KL2L2 and Co L3M45M45 Auger signals increases until reaching a saturated value which shows a CoO layer with a concentration ratio of Co and O close to 1:1. After introduction of CoO overlayers on Co/Ge(111), hysteresis occurs in the longitudinal configuration and CoO/Co/Ge(111) exhibits in-plane anisotropy. A slight reduction of the Kerr intensity occurs due to the oxidation of cobalt at the CoO/Co interface while an enhanced coercive force is observed owing to the imperfection introduced by oxygen to impede the magnetization reversal. Under conditions of cooling in a magnetic field, exchange bias field increases as the sample temperature decreases resulting from the formation of an antiferromagnetic/ferromagnetic interface.

Ag-related alloy formation and magnetic phases for Ag/Co/Ir(111) ultrathin films

The Kerr intensity versus the Ag thickness for Ag grown on the top of Co/Ir(111) exhibits an oscillating behavior with a period around one monolayer which should be due to the morphological change related electronic structure differences of the Ag layer. From systematical investigations of Ag/Co/Ir(111) films with the Co layer thinner than 4monolayers at temperatures below 900K, a magnetic phase diagram has been established. As the annealing temperature increases for Ag/Co/Ir(111) films, enhancements of the coercive force occur in both the polar and longitudinal configurations due to the intermixing of Ag and Co at the interface and the formation of Co–Ir alloy. The disappearance of ferromagnetism is mainly attributed to the reduced atomic percent of cobalt in Co–Ir alloy, the lowered Curie temperature by a reduction of the thickness of magnetic layers, and the intermixing of Ag and Co at the Ag/Co interface.

Comparative studies of magnetic properties of Co films on annealed and unannealed rubrene/Si(100)

Abstract Structures and magnetic properties of Co films on rubrene/Si(1 0 0) have been investigated by employing atomic force microscopy, X-ray diffraction and magneto-optic Kerr effect techniques. For Co/rubrene/Si(1 0 0) films, separated Co clusters are found as the Co thickness increases. For 20 nm Co overlayer, the Co clusters are uniform with an average diameter around 90 nm. After annealing treatments at 550 K for rubrene/Si(1 0 0), rubrene molecules nucleate to form clusters and pinholes develop in the rubrene layer. For Co/annealed rubrene/Si(1 0 0) films, the formation of a Co/Si(1 0 0) interface is important for the occurrence of the pyramid-like nanostructure with an hcp stacking of the Co layer. Comparative studies of thickness dependencies of the magnetic properties for x nm Co on rubrene/Si(1 0 0) prepared at 300 and 550 K have been performed. With increasing the cobalt thickness, the Kerr intensities approach a saturation value for thicker Co due to an attenuation of the laser intensity in a condensed material. For Co deposited on annealed rubrene/Si(1 0 0), the enhancement of the coercive force is attributed to the imperfection introduced by rough interface to impede the magnetization reversal. As the Co thickness increases from 10 to 40 nm on rubrene/Si(1 0 0), the squareness increases from 0.9 to near unity. For Co deposited on annealed rubrene/Si(1 0 0), the squareness of magnetization curve is close to unity for all the Co thicknesses. The formation of Co islands in triangular shapes plays an important role on the enhancement of the squareness of magnetization curve for Co overlayers.

Frequency dependence of magnetization and giant magneto impedance effect of amorphous wires

The frequency dependence of magnetization process and giant magneto impedance (GMI) effect of Co-rich melt-extracted amorphous wires was studied by Kerr effect and impedance analyzer, respectively. It is demonstrated that the transverse Kerr intensity and the corresponding GMI response increase with increasing frequency, which contributes to the upgraded skin effect. However, the skin depth has a slothful trend with frequency when it is up to the megahertz range, which gives rise to the transformation of magnetization. The process is much more sensitive to the direct current magnetic field and the sensitive change of the circular permeability, and GMI response is observed as its consequence. This proves that the evolution of circumferential magnetization and the corresponding permeability with the direct current magnetic field is the essence of GMI response, and a much more sensitive magnetization promises a better GMI response.

Effect of Fe layer thickness and Fe/Co intermixing on the magnetic properties of Sm–Co/Fe bilayer exchange-spring magnets

Sm–Co(20 nm)/Fe(tFe) bilayers are fabricated with different Fe layer thicknesses (tFe = 0, 2, 4, 6 and 8 nm) in order to systematically investigate the intermixing effect between Fe and Sm–Co layers and its influence on the magnetic properties. X-ray diffraction analysis indicates that the deposition of soft layer (Fe) promotes crystallization of hard layer (Sm–Co) at a low annealing temperature (400 °C). The annealed films consist of hexagonal Sm2Co7, Sm2Co17 and SmCo5 phases in the hard layer and bcc-structured Fe(Co) in the soft layer. Rutherford backscattering is employed to investigate the atomic composition of the individual layers and thereby the extent of diffusion of Co atoms into the Fe layer. The estimated Co content is higher for lower tFe. The magnetic properties of the bilayers very close to the interface are analysed by the magneto-optical Kerr effect. The calculated Kerr intensities (which represent the magnetization process) are significantly affected by the extent of diffused Co in the Fe layer. Superconducting quantum interference device magnetic measurements demonstrate smooth and single-phase magnetic behaviour for tFe up to 6 nm and a good combination of high coercivity (6.5 kOe) and high magnetic remanence (834 emu cm−3) is obtained for tFe = 4 nm.

Hydrogen absorption induced reversible effect on magneto-optical property of Pd/Fe, Pd/Co and Pd/Ni bilayers

We report on the reversible change of the magneto-optical Kerr effect (MOKE) in Pd covered magnetic thin films by controlling H2 absorption. After H2 exposure, the extinction angle of MOKE was shifted and correspondingly the Kerr intensity was significantly increased. When properly choosing the analyzer angle, we obtained large enhancement of 40%, 35% and 50–70% for Pd/Fe, Pd/Co and Pd/Ni bilayers, respectively. The time-dependence of this H2 absorption effect on MOKE was investigated. The reversibility of MOKE enhancement was demonstrated in Pd/Ni bilayer by multiple cycles (at least 4 times) of H2 absorption and desorption. These observations reveal the universal responses when combining the hydrogenated Pd layer with a magnetic thin film, and can be applied in various magnetic materials.

Hydrogenation-induced change of magneto optical Kerr effect in Pd/Fe bilayers

The hydrogenation induced change of magneto-optical Kerr effect (MOKE) was studied in n ML Pd/30 ML Fe bilayers on Al2O3(0001). With the increasing of Pd thickness from 3 ML to 60 ML, the MOKE extinction angle was gradually shifted by 0.6° and the enhancement of Kerr intensity reached 35%–40% after exposure to 1 atm hydrogen. The reversibility of this significant change was demonstrated by cyclic desorption and reabsorption of hydrogen. This study reveals the sensitive MO response in the combination of a magnetic Fe thin film with a highly hydrogenated Pd capping layer.

Thickness dependent reactivity and coercivity for ultrathin Co/Si(111) films

For Co/Si(111) films thinner than 15 ML, the thickness dependent reactivity and magnetic properties have been systematically studied. As the Co coverage increases, Co adatoms on the Si(111) surface show enhanced chemical reactivity for oxidation due to the change of the chemical state. After the saturation oxygen exposure, oxygen atoms interact with a thick Co layer to form a rougher interface. Complex adsorption kinetics of oxygen in the Co layer is observed. From the depth-profiling measurements for Co layers close to the Co–Si interface, the sputtering rate is enhanced due to that the solid surfaces of Si and Co–Si compounds are resistive against oxidation. The descending of the Kerr intensity by saturation oxygen exposure shows the limited diffusion length of oxygen atoms into the films. The inertness of the Co–Si interface, the reduction of pure cobalt and imperfection introduced by oxygen influence the coercivity of O/Co/Si(111).

Structures and magnetic properties for electrodeposited Co ultrathin films on copper

The formation of Co films on polycrystalline copper in diluted sulphuric acid was investigated by employing cyclic voltammetry (CV), atomic force microscopy, and in-situ magneto-optic Kerr effect (MOKE) techniques. By comparing CV measurements in the pure supporting electrolyte (11 mM K 2SO 4/1 mM H 2SO 4) and the cobalt sulphate solution (10 mM K 2SO 4/1 mM H 2SO 4/1 mM CoSO 4), peaks from voltammetric cycling for copper dissolution, readsorption of dissolved copper ions, cobalt bulk dissolution and oxidation of hydrogen could be resolved. As the electroplating time increases, the size of the Co clusters increases and the deposition of Co corresponds to island growth. The first hysteresis loop occurs at a Co thickness of 0.33 nm in the longitudinal configuration. For films thinner than 7 nm, the Kerr intensity increases linearly because the Curie temperature of the film is well above 300 K.

Magnetic properties of ultrathin Si/Co/Ir(111) films

A magnetic phase diagram for Si/Co/Ir(111) films has been established by the surfacemagneto-optic Kerr effect technique. The Si coverage, where ferromagnetism vanishes,increases from one monolayer to more than three monolayers as the coverageof the Co layer increases from two to five monolayers. As silicon is depositedon the surface of Co/Ir(111) films, Si atoms interact with the Co layer to formSi–Co compounds. The rough Si/Co interface impedes the motion of domain wallsduring the magnetization reversal process, thus increasing the coercive force. Theformation of nonmagnetic Co–Si compounds reduces the effective magnetic layers,leading to a decrease in Kerr intensity and superparamagnetic behavior of the films.

Ag-induced spin-reorientation transition of ultrathin Fe films on Pt(111)

The spin-reorientation transition (SRT) was observed when Ag overlayers deposited on Fe∕Pt(111) by magneto-optical Kerr effect measurements. The easy axis of the magnetization changes completely from the in-plane to out-of-plane direction after the coverage of Ag is 1 ML. The polar Kerr intensity and its coercivity as a function of Ag coverage are investigated during SRT. The easy axis of magnetization can be reversed back to the in-plane direction after the Ag overlayers are removed by sputtering. The chemical compositions of the interfaces are measured by Auger electron spectroscopy. The mechanism of SRT induced by Ag is discussed.