Polar Magneto-optical Kerr Effect Research Articles

Enhancement of magneto-optical Kerr effect by surface plasmons in trilayer structure consisting of double-layer dielectrics and ferromagnetic metal.

Giant enhancement of the magneto-optical Kerr effect (MOKE) by surface plasmon polaritons (SPPs) is theoretically shown in a trilayer structure consisting of double-layer dielectrics and a ferromagnetic metal (Al2O3/SiO2/Fe). We calculated the resonant enhancement of the transverse MOKE (TMOKE) and polar MOKE (PMOKE) using the attenuated total reflection (ATR) configuration with the transfer matrix method using a 4 × 4 scattering matrix. At a specific film thickness of the low-index SiO2 layer, where confinement of the SPPs on the Fe surface becomes close to the cutoff condition, the incident light from the Al2O3 couples with the SPPs at the SiO2/Fe boundary most efficiently, resulting in resonant enhancement of the MOKE at an incident angle corresponding to the wave vector of the SPPs. The calculated PMOKE showed orthogonal transformation (90°-rotation) and almost full-orbed deformation (44°-ellipticity) of the polarization, and the TMOKE showed a change in reflectance of about 34 dB upon magnetization reversal.

Nanocrystalline zinc ferrite films studied by magneto-optical spectroscopy

Ferrimagnetic Zn-ferrite (ZnFe2O4) films can be grown with the ferromagnetic resonance linewidth of 40 Oe at 9.5 GHz without going through a high temperature processing. This presents interest for applications. The work deals with laser ablated ZnFe2O4 films deposited at O2 pressure of 0.16 mbar onto fused quartz substrates. The films about 120 nm thick are nanocrystalline and their spontaneous magnetization, 4πMs, depends on the nanograin size, which is controlled by the substrate temperature (Ts). At Ts ≈ 350 °C, where the grain distribution peaks around ∼20–30 nm, the room temperature 4πMs reaches a maximum of ∼2.3 kG. The films were studied by magnetooptical polar Kerr effect (MOKE) spectroscopy at photon energies between 1 and 5 eV. The complementary characteristics were provided by spectral ellipsometry (SE). Both the SE and MOKE spectra confirmed ferrimagnetic ordering. The structural details correspond to those observed in MgFe2O4 and Li0.5Fe2.5O4 spinels. SE experiments confirm the insulator behavior. The films display MOKE amplitudes somewhat reduced with respect to those in Li0.5Fe2.5O4 and MgFe2O4 due to a lower degree of spinel inversion and nanocrystalline structure. The results indicate that the films are free of oxygen vacancies and Fe3+-Fe2+ exchange.

Structure and magnetic properties of tetragonal Heusler D022-Mn3Ge compound epitaxial films with high perpendicular magnetic anisotropy

We investigated the structure and magnetic properties of epitaxial films of D022-Mn3Ge tetragonal Heusler like compounds in detail. Epitaxial films with a stoichiometric composition were grown on Cr-buffered single crystalline (0 0 1) MgO substrates using ultra-high vacuum sputtering at different growth temperatures. X-ray diffraction showed that D022-ordered films were formed at growth temperatures of ⩾200 °C. Epitaxial growth was indicated by cross-sectional high-resolution transmission electron microscopy. Nanobeam diffraction patterns from the D022-Mn3Ge film grown at 400 °C suggests the absence of planar defects such as stacking faults and twins in the film. Out-of-plane magnetic hysteresis curves with perfect squareness were observed for the films grown at ⩾300 °C. These films also showed abrupt magnetization reversal at a coercivity of about 1 T, which is higher than that of other thin film materials with perpendicular magnetic anisotropy such as CoPt and FePt. A huge domain diameter in the D022-Mn3Ge films was indicated by the initial magnetization curves that were measured by the polar magneto-optical Kerr effect.

스핀-궤도 돌림힘의 광학적 측정 및 분석법

Department of Physics and Astronomy, Seoul National University, Seoul 151-747, Korea(Received 15 January 2015, Received in final form 10 February 2015, Accepted 11 February 2015)We demonstrate here an optical measurement technique to quantify the spin-orbit torques. The magnetization dynamics induced bythe spin-orbit torques with a sinusoidal current injection is measured by use of polar magneto-optical Kerr effect. The measured signalis then analyzed based on the Landau-Lifhshitz-Gilbert equation with consideration of the spin-orbit torques. The present measurementtechnique is applied to Pd/Co/Pt films and then, the longitudinal and transverse components of the spin-orbit torques are successfullyquantified. The present optical technique provides an alternative way to quantify the spin-orbit torques.Keywords : spin-orbit torque, optical measurement, magneto-optical Kerr effect

Modifying exchange-spring behavior of CoPt/NiFe bilayer by inserting a Pt or Ru spacer

We herein explore the possibility of obtaining tunable tilted magnetic anisotropy in ordered-CoPt (5 nm)/NiFe(tNiFe) bilayers through modifying their exchange spring behavior by inserting Pt and Ru-spacers. The tuning process of tilt angle magnetization of NiFe-layer was systematically investigated by varying the Pt or Ru thickness (tPt or tRu) from 0 to 8 nm at different thicknesses of NiFe (tNiFe = 1.5, 4.0, and 6.0 nm). Polar magneto-optic Kerr effect (p-MOKE) studies reveal that the bilayers grown in absence of spacers exhibit almost a rectangular hysteresis loop. With the insertion of Pt-spacer, the loop becomes more and more tilted as tPt increases; whereas, in the case of Ru-spacer, the nature of the loops is not simply changing in one direction. The estimated SQR⊥ (= θr/θs) values from the p-MOKE loops are found to monotonically decrease with increasing tPt when tPt ≦ 4 nm. In contrast, in the case of Ru-spacer, an oscillatory behavior for the SQR⊥ values is apparent when tRu ≦ 4 nm. As a result, an oscillatory tilted angle of NiFe spin configuration was obtained in the case of Ru-spacer; while a decoupling effect was prominent for the Pt-spacer. The results of present study reveal that the insertion of Pt and Ru-spacers as an appropriate means for realizing tunable tilted magnetic anisotropy in the CoPt/NiFe exchange springs.

Optical and Magnetic Microstructures in YIG Ferrite Fabricated by Femtosecond Laser

Optical and magnetic microstructures are directly fabricated inside a film of cerium-substituted yttrium iron garnet (Ce:YIG) by using a direct femtosecond laser writing. The laser irradiation induces local changes in both optical and magnetic properties of Ce:YIG. A quantitative phase imaging technique proves that the refractive index is increased by 0.015 ± 0.001, which is about 0.7% of Ce:YIG refrative index 2.2 at 632.8 nm. The magneto-optical polar Kerr effect measurement shows that the magnetization property is changed from hard to soft, decreasing the coercivity. Our proposed technique paves the way toward the development of novel three-dimensional magneto-optical devices for optical communication and photonic applications.

Optical spectroscopy of sputtered nanometer-thick yttrium iron garnet films

Nanometer (nm)-thick yttrium iron garnet (Y3Fe5O12, YIG) films present interest for spintronics. This work employs spectral ellipsometry and magneto-optic Kerr effect (MOKE) spectra to characterize nm-thick YIG films grown on single-crystal Gd3Ga5O12 substrates by magnetron sputtering. The thickness (t) of the films ranges between 10 nm and 40 nm. Independent on t, the polar MOKE hysteresis loops saturate in the field of about 1.8 kOe, consistent with the saturation magnetization in bulk YIG (4πMs ≈ 1.75 kG). The MOKE spectrum measured at photon energies between 1.3 eV and 4.5 eV on the 38-nm-thick film agrees with that measured on single-crystal YIG bulk materials. The MOKE spectrum of the 12-nm-thick film still preserves the structure of the bulk YIG but its amplitude at lower photon energies is modified due to the fact that the radiation penetration depth exceeds 20 nm. The t dependence of the MOKE amplitude is consistent with MOKE calculations. The results indicate that the films are stoichiometric, strain free, without Fe2+, and preserve bulk YIG properties down to t ≈ 10 nm.

Large voltage-induced modification of spin-orbit torques in Pt/Co/GdOx

We report on large modifications of current-induced spin-orbit torques in a gated Pt/Co/Gd-oxide microstrip due to voltage-driven O2− migration. The Slonczewski-like and field-like torques are quantified using a low-frequency harmonic technique based on the polar magneto-optical Kerr effect. Voltage-induced oxidation of Co enhances the Slonczewski-like torque by as much as an order of magnitude and simultaneously reduces the anisotropy energy barrier by a factor of ∼5. Such magneto-ionic tuning of interfacial spin-orbit effects may significantly enhance the efficiency of magnetization switching and provide additional degrees of freedom in spintronic devices.

Magnetic field sensing using evanescent waves in the Kretschmann configuration

To verify the feasibility of magnetic field sensing based on the interaction between evanescent waves and magnetic thin films, we have investigated the polar magneto-optical Kerr effect of Co thin films with various thicknesses in the Kretschmann attenuated total reflection configuration. Because of the influence of the glass magneto-optical Faraday rotation under a polar magnetic field, a multi-step experimental method has been applied to measure the intrinsic magneto-optical Kerr rotations of the thin Co films as a function of the angle of incidence. The significant effects of the angle of incidence, the incident light polarization state and the film thickness on the polar Kerr rotations of magnetic thin films in the Kretschmann configuration have been clearly shown in our experiments. The analysis of the experimental results shows that the interaction between the evanescent wave and the magnetic thin film is the main source of the observed magneto-optical activity. On the basis of these results, we discuss the practicalities of the development of a magnetic field sensor based on an evanescent wave penetrating into the magnetic thin film in the Kretschmann configuration, which is a simple structure that is compatible with both optical fibers and integrated optical waveguides.

Direct imaging of the magnetization reversal in microwires using all-MOKE microscopy.

We report a method of imaging of the magnetization reversal process using analysis of real-time images of magnetic domain structures in cylindrically shaped microwires. This method uses wide-field polarizing optical microscopy and is based on the magneto-optical Kerr effect (MOKE). The aperture diaphragm in MOKE microscope was used to control the incident angles of the light rays that reached the non-planar surface of the microwire and also determined the MOKE geometries. The movement of the non-central position of the hole in this diaphragm leads to a change in the orientation of the plane of incidence of the light along the perpendicular or the parallel direction to the axial direction of the wire. The visualization of the surface magnetic domain structures is obtained using polar and longitudinal MOKE geometries. The hysteresis loops were obtained by plotting the averaged image contrast as a function of the external magnetic field. The separation of the all-magnetization components is performed using different MOKE geometries in a microscope. We demonstrate the use of vector magnetometry to analyze the orientation of the magnetization in a cylindrically shaped microwire under the influence of an external magnetic field.

Angle dependence of the photonic enhancement of the magneto-optical Kerr effect in DMS layers

We investigate theoretically an angle dependence of the enhancement of the polar magneto-optical Kerr effect (MOKE) obtained thanks to a deposition of a paramagnetic Diluted Magnetic Semiconductor (DMS) layer on a one-dimensional photonic crystal layer. Our transfer-matrix-method–based calculations conducted for TE and TM polarizations of the incident light predict up to an order of magnitude stronger MOKE for a (Ga,Fe)N DMS layer when implementing the proposed design. The maximum enhancement for TE and TM polarization occurs for the light incidence at the normal and at the Brewster angle, respectively. This indicates a possibility of tuning of the MOKE enhancement by adjustment of the polarization and of the incidence angle of the light.

Polar magneto-optical effects in magnetoplasmonic thin films illuminated by attenuated total reflection

We investigate the polar magneto-optical Kerr effect as a function of the angle of incidence in Co/Au thin films under attenuated total reflection conditions. An experimental method is proposed to measure the intrinsic polar magneto-optical Kerr rotation of the magnetoplasmonic films while considering the influence of the glass Faraday magneto-optical effect. The experimental results demonstrate that the Kerr rotations of these films are greatly affected by a number of factors including the angle of incidence, the linear polarization of the incident light, and the film thickness. The results suggest that the polar magneto-optical activity is affected by the evanescent fields decaying inside the films; these fields arise not only from attenuated total reflection at the glass/metal interface but also from the surface plasmon polaritons at the metal/air interface. A close relationship between the magneto-optical activity and the electromagnetic field distribution within the magnetoplasmonic films is clearly revealed.

Anisotropic optical properties of Fe/GaAs(001) nanolayers from first principles

We investigate the anisotropy of the optical properties of thin Fe films on GaAs(001) from first-principles calculations. Both intrinsic and magnetization-induced anisotropy are covered by studying the system in the presence of spin-orbit coupling and external magnetic fields. We use the linearized augmented plane wave method, as implemented in the WIEN2k density functional theory code, to show that the $C_{2v}$ symmetric anisotropy of the spin-orbit coupling fields at the Fe/GaAs(001) interface manifests itself in the corresponding anisotropy of the optical conductivity and the polar magneto-optical Kerr effect. While their magnetization-induced anisotropy is negligible, the intrinsic anisotropy of the optical properties is significant and reflects the underlying $C_{2v}$ symmetry of the Fe/GaAs(001) interface. This suggests that the effects of anisotropic spin-orbit coupling fields in experimentally relevant Fe/GaAs(001) slabs can be studied by purely optical means.

Field-dependent magneto-optical Kerr effect spectroscopy applied to the magnetic component diagnosis of a rubrene/Ni system.

Polar magneto-optical Kerr effect (MOKE) spectroscopy in the energy range from 1.75 eV to 5 eV at different magnetic field strength was applied to study Ni nanostructures formed on rubrene nanoislands. The magnetic hysteresis curves measured by MOKE change the shape depending on the photon energy and therefore deviate from those measured by superconducting quantum interference device (SQUID) magnetometry. Similar optical effects were previously observed in inorganic heterostructures. Our observations show that it correlates to the change in lineshape of the MOKE rotation and ellipticity spectra as a function of magnetic field strength. We show that this spectral dependence on magnetic field can be exploited to separate the contributions of two magnetic components to the magneto-optical spectra and hysteresis. The proposed model does not require the a priori knowledge of the (magneto-)optical constants of the heterostructure and its components.

Probing the electrochemical interface with in situ magnetic characterizations: A case study of Co/Au(111) layers

Abstract In this paper, we exploit the high sensitivity of the magnetism of ultrathin Co/Au(111) layer to study the electrochemical interface structure. Using in situ real time magnetic measurements (polar magneto-optical Kerr effect, PMOKE) it is established that the surface chemistry determines the surface magnetization anisotropy energy K S Surf and thereby the direction of magnetization at a given thickness. We next demonstrate that the magnetic properties are strongly modified by the hydrogen evolution reaction (HER) at CO-covered layers and exploit the data to study the reaction mechanism. We also take advantage of the sensitivity of K S Surf to the electric field to discuss the structure of the electrochemical interface at H- and CO-covered cobalt surfaces.

The synthesis of Co1−xDyxFe2O4 nanoparticles and thin films as well as investigating their magnetic and magneto-optical properties

The modification and optimization of the magnetic and magneto-optical properties of cobalt ferrite are of great importance due to their various applications in well-known scientific and industrial categories. In order to observe the effect of adding Dy3+ to the cobalt ferrite composition, Co1−xDyxFe2O4 (0≤x≤0.1) ceramic nanoparticles were synthesized by the co-precipitation chemical method and then their microstructure and magnetism were investigated through x-ray diffractometry, TEM micrography, IR spectroscopy and VSM magnetometry. The polar magneto-optical Kerr effect of the thin film specimens was also studied. The results reveal that the doping of Dy3+ ions could effectively alter the inversion degree of the spinel structure and the following magnetic and magneto-optical features. The ferrite coercivity was enhanced by 150% after adding Dy. The perceptible shifts of peak rotations were observed in the Kerr spectra for the Dy-doped CoFe2O4 films. Furthermore, TC had a descending trend with the addition of Dy from 440°C to 420°C for Co0.95Dy0.05Fe2O4.

Magnetization switching through spin-Hall-effect-induced chiral domain wall propagation

The influence of spin-Hall-effect spin torque (SHE-ST) induced by in-plane charge current was studied in microscale Ta/Co${}_{20}$Fe${}_{60}$B${}_{20}$/TaO${}_{x}$ films with perpendicular magnetization. Simultaneous electrical transport and polar magneto-optical Kerr effect (MOKE) imaging experiments were used to investigate the switching dynamics. A rich set of switching behaviors was observed, which can be well understood by analyzing a switching-phase diagram and polar MOKE images, considering the competition between SHE-ST and the externally applied magnetic field. Furthermore, we found that domain walls with a particular chirality were dominant in our devices, which suggests the presence of the Dzyaloshinskii-Moriya interaction in the present material system.

AlN/Fe/AlN nanostructures for magnetooptic magnetometry

AlN/Fe/AlN/Cu nanostructures with ultrathin Fe grown by sputtering on Si substrates are evaluated as probes for magnetooptical (MO) mapping of weak currents. They are considered for a laser wavelength of λ = 410 nm (3.02 eV) and operate at oblique light incidence angles, φ(0), to enable detection of both in-plane and out-of-plane magnetization. Their performance is evaluated in terms of MO reflected wave electric field amplitudes. The maximal MO amplitudes in AlN/Fe/AlN/Cu are achieved by a proper choice of layer thicknesses. The nanostructures were characterized by MO polar Kerr effect at φ(0) ≈ 5° and longitudinal Kerr effect spectra (φ(0) = 45°) at photon energies between 1 and 5 eV. The nominal profiles were refined using a model-based analysis of the spectra. Closed form analytical expressions are provided, which are useful in the search for maximal MO amplitudes.

Magneto-optical spectroscopy of ferromagnetic shape-memory Ni-Mn-Ga alloy

Magneto-optical properties of single crystal of Ni50.1Mn28.4Ga21.5 magnetic shape memory alloy in martensite and austenite phase were systematically studied. Crystal orientation was approximately along {100} planes of parent cubic austenite. At room temperature, the sample was in modulated 10M martensite phase and transformed to cubic austenite at 323 K. Spectral dependence of polar magneto-optical Kerr effect was obtained by generalized magneto-optical ellipsometry with rotating analyzer in the photon energy range from 1.2 to 4 eV, and from room temperature to temperature above the Curie point. The Kerr rotation spectra exhibit prominent features typical for complexes containing Mn atoms. Significant spectral changes during transformation to austenite can be explained by different optical properties caused by changes in density of states near the Fermi energy.

Polar magneto-optical Kerr effect instrument for 1-dimensional magnetic nanostructures

The magneto-optical Kerr effect (MOKE) is a powerful technique to investigate the magnetization behaviour in magnetic nanostructures. We describe the design of a polar MOKE instrument for investigating the magnetization variation in MOKE signal observed in the exciting regime where the size of the magnetic nanostructures is around 20–350 nm. In particular, when the magnetization of the sample is perpendicular to its plane (i.e., along the axis of a cylindrical nanowire) we use polar MOKE configuration. The sign and magnitude of the small rotation measured in the signal is found proportional to the magnetization and its direction. The MOKE measurements indicate that the hysteresis depends on the shape as well as the density of nanostructures formed. The instrument is sensitive enough to clearly indicate the effect of magneto-static interaction on shape of M-H loop. We have observed the coercive field of ∼269 G for cylindrical nanowire grown in anodic aluminium oxide template and ∼135 G for “pin” shaped nanowire grown in polycarbonate track etched template. The magnetization reversal measurements are intricate in the case of “pin” or “X” shaped nanostructures as seen from the loop. These typical MOKE measurements on the 1-dimensional (1-D) nanostructures explore the effect of magneto-static interaction between the nanostructures.