Magneto-optical Film Research Articles

Spin Hall effect of reflected light in dielectric magneto-optical thin film with a double-negative metamaterial substrate: erratum.

An expression error was produced in the text introducing the dynamic matrix of the magnetic layer in [Opt. Express25(16), 19117 (2017)10.1364/OE.25.019117]. What's more, we used two wrong formula numbers in the analysis section of Eq. (11) and Eq. (13). Here we correct the errors.

Spin Hall effect of reflected light in dielectric magneto-optical thin film with a double-negative metamaterial substrate: erratum.

An expression error was produced in the text introducing the dynamic matrix of the magnetic layer in [Opt. Express25(16), 19117 (2017)10.1364/OE.25.019117]. What's more, we used two wrong formula numbers in the analysis section of Eq. (11) and Eq. (13). Here we correct the errors.

Spin Hall effect of reflected light in dielectric magneto-optical thin film with a double-negative metamaterial substrate

We study spin the Hall effect (SHE) of reflected light in a dielectric magneto-optical thin film of Ce1Y2Fe5O12 (Ce:YIG) with a double-negative (DNG) metamaterial substrate. The spin-dependent splitting expressions of left- and the right-handed circularly polarized (LHCP and RHCP) components in longitudinal, polar and transverse magneto-optical Kerr effect (MOKE) configurations are obtained. Meanwhile we first obtain the analytical expressions of the SHE shift of reflected light for three MOKE configurations by proper approximation. Owing to the enhancement of the MOKE by DNG metamaterial, the external magnetic field shows a large enhancement and modulation to spin-dependent splitting of reflected light. Based on simulation results, the influences of magnetic field direction and substrate material on the transverse centroid shifts of the reflected left- and right-handed circularly polarized light perpendicular to incident plane are analyzed. We find the maximum spin-dependent splitting between LHCP and RHCP components achieves about 9.2 μm and the maximum value of the magneto-optical spin Hall effect (MOSHE) shift reaches 9 μm in polar MOKE configuration. In order to make our results convincing we use a realizable DNG metamaterial with silver nanostructures as substrate to verify our conclusion. The DNG metamaterial provides a flexible method to manipulate and enhance SHE of light.

Breakdown of antiferromagnet order in polycrystalline NiFe/NiO bilayers probed with acoustic emission

Magnetization reversal of polycrystalline NiFe/NiO bilayers was investigated using magneto-optical indicator film imaging and acoustic emission techniques. Sporadic acoustic signals were detected in a constant magnetic field after the magnetization reversal. It is suggested that they are related to elastic waves excited by sharp shocks in the NiO layer with strong magnetostriction. Their probability depends on the history and number of repetitions of the field cycling, thus testifying the thermal-activation nature of the long-time relaxation of an antiferromagnetic order. These results provide evidence of spontaneous thermally activated switching of the antiferromagnetic order in NiO grains during magnetization reversal in ferromagnet/antiferromagnet (FM/AFM) heterostructures. The respective deformation modes are discussed in terms of the thermal fluctuation aftereffect in the Fulcomer and Charap model which predicts that irreversible breakdown of the original spin orientation can take place in some antiferromagnetic grains with disordered anisotropy axes during magnetization reversal of exchange-coupled FM/AFM structures. The spin reorientation in the saturated state may induce abrupt distortion of isolated metastable grains because of the NiO magnetostriction, leading to excitation of shock waves and formation of plate (or Lamb) waves.

Generation of an ultralong pure longitudinal magnetization needle with high axial homogeneity using an azimuthally polarized beam modulated by pure multi-zone plate phase filter

Based on the vector diffraction theory and the inverse Faraday effect in the magneto-optic film, light-induced magnetization distributions, for a high numerical aperture focusing configuration with an azimuthally polarized beam modulated by an optimized pure multi-zone plate phase filter, are investigated. By making use of the compeletely destructive interference of its inter circle with the π phase shift between adjacent sub-annuli, and the capability to extend the constructive interference in the propagating direction through its narrow outer annulus modulated by three misplaced helical phases, an ultralong () magnetization needle with both transverse super-resolution () and uniform axial field strength is achieved in the focal region. The perfect magnetization needle and the accessible method give a guide for ultrahigh density magnetic storage, fabricating magnetic lattices for spin wave operation, as well as atomic trapping.

Tight butt joint weld detection based on optical flow and particle filtering of magneto-optical imaging

It is a challenge to detect the weld position during tight butt joint laser welding in that the tight butt joint is non-grooved and invisible. This paper proposes a novel method for tight butt joint weld detection based on magneto optical imaging. Two pieces of weldment were magnetized by an electromagnet so that they could show magnetic N and S polarity respectively. When a polarized light was projected on a magneto-optical film, it would deflect due to magneto-optical effect. In accordance with magneto field distribution, an image formed on the visual sensor. A transition zone of magnetic field distribution which corresponded to the butt joint could be shown in a magneto optical image of weldment. Variation features of magnetic field distribution were obtained by using image sequence optical flow method, and a particle filter was integrated to make an accurate prediction on weld position. Weld position was obtained by calculating the maximum value of optical flow intensity in the vertical direction, and a particle filter was used to realize the accurate prediction on weld position. Experimental results showed that the proposed method was effective in detection of weld and realizing weld seam tracking.

Characterization of applied tensile stress using domain wall dynamic behavior of grain-oriented electrical steel

Stress measurement that provides early indication of stress status has become increasingly demanding in the field of Non-destructive testing and evaluation (NDT&E). Bridging the correlation between micro magnetic properties and the applied tensile stress is the first conceptual step to come up with a new method of non-destructive testing. This study investigates the characterization of applied tensile stress with in-situ magnetic domain imaging and their dynamic behaviors by using magneto-optical Kerr effect (MOKE) microscopy assisted with magneto-optical indicator film (MOIF). Threshold magnetic field (TMF) feature to reflect 180° domain wall (DW) characteristics behaviors in different grains is proposed for stress detection. It is verified that TMF is a threshold feature with better sensitivity and brings linear correlation for stress characterization in comparison to classical coercive field, remanent magnetization, hysteresis loss and permeability parameters. The results indicate that 180° DWs dynamic in the inner grain is highly correlated with stress. The DW dynamics of turn over (TO) tests for different grains is studied to illustrate the repeatability of TMF. Experimental tests of high permeability grain oriented (HGO) electrical steels under stress loading have been conducted to verify this study.

Magneto-optical imaging and analysis of magnetic field micro-distributions with the aid of biased indicator films

Magneto-optical imaging films (MOIFs) were used for the study of magnetic stray field distributions above topographically and thermomagnetically patterned permanent magnet films and other miniature elements that could be used in magnetic micro-systems. Calibrated external bias fields were applied to the samples in situ to simplify the graduation procedure, to expand the measurement range and to minimize the measurement errors. Both uniaxial and planar MOIF were used to make quantitative measurements. A scheme for the fast reconstruction of magnetic serial sections of non-uniform field profiles is proposed. The usefulness of biased MOIF imaging is illustrated by a number of specific examples, such as mechanical hole-drilling adapted to the case of magnetic analysis, dynamic range expansion beyond the limits set by MOIF saturation, rendering surface magnetic flux variations at the microscale and in the presence of strong background.

Influence of internal geometry on magnetization reversal in asymmetric permalloy rings

We report the magnetization reversal behavior of microstructured Ni80Fe20 rings using magneto-optic indicator film imaging and magnetometry. While the reversal behavior of rings with a symmetric (circular) interior hole agrees with micromagnetic simulations of an onion → vortex → onion transition, we experimentally demonstrate that rings possessing an elliptical hole with an aspect ratio of 2 exhibit complex reversal behavior comprising incoherent domain propagation in the rings. Magneto optic images reveal metastable magnetic configurations that illustrate this incoherent behavior. These results have important implications for understanding the reversal behavior of asymmetric ferromagnetic rings.

Enhancements of Faraday rotation through ferromagnetic domain matching in arrayed magneto-optic films

In linear arrays of optically probed bismuth-doped iron garnet films, the Faraday rotation signal produced by an external magnetic field is a complex function of film shapes, stacking geometries, optical path length, and insertion losses. Studies of arrayed responsivities under various configurations reveal a proximity coupling effect between ferromagnetic domains of adjacent films, which enhances Faraday rotation by minimizing demagnetization effects at the interfaces. The effect is activated by matching ferromagnetic domain patterns in adjacent films and has resulted in over a twofold improvement in detection sensitivities when the arrays were configured as polarimetric magneto-optic sensors.

Design of a compact waveguide optical isolator based on multimode interferometers using magneto-optical oxide thin films grown on silicon-on-insulator substrates

We report the design of a waveguide optical isolator based on multimode interferometer (MMI) structure using silicon on insulator (SOI) and deposited magneto-optical (MO) thin films. The optical isolator is based on a vertical 1 × 2 SOI MMI utilizing the nonreciprocal phase shift (NRPS) difference of different TM modes of the MO garnet thin film/SOI waveguide. By constructing a silicon/MO thin film/silicon structure, we demonstrate that the NRPS of the fundamental and first order TM modes can show opposite signs for certain device dimensions, therefore significantly reduce the device length. For a 310.42 μm long device, 20 dB isolation bandwidth larger than 1.6 nm with total insertion loss of 0.817 dB is achieved at 1550 nm wavelength. The fabrication tolerances and materials losses are also discussed to satisfy the state-of-the-art fabrication technology and material properties.

Reverse Magnetomechanical Effect Measurement in Magnetooptical Films at Nonmagnetic Substrate Bending Deformation

On the example of yttrium–iron garnet films on a nonmagnetic substrate, the results of measurements of the magnetization relation to the crystal compression (stretching) stress due to the bending deformation of a substrate are presented. The measurements were performed by passing radiation in a waveguide structure formed by the magnetooptical crystal film on a gadolinium–gallium garnet substrate using the Faradey effect. Parallel magnetization sensitivity (1.5–1.7) $\times \,\, 10^{\mathrm {-5}}$ Am $^{\mathrm {-1}}$ Pa $^{\mathrm {-1~}}$ to stress is obtained, with stress at range ±6 MPa. Research was made at a bias magnetic field of 350 and 600 A/m.

Changes in magnetic domain structure during twin boundary motion in single crystal Ni-Mn-Ga exhibiting magnetic shape memory effect

The complexity of Ni-Mn-Ga single crystal originates from the interplay between ferromagnetic domain structure and ferroelastic twinned microstructure. Magnetic domain structure in the vicinity of single twin boundary was studied using magneto-optical indicator film and magnetic force microscopy technique. The single twin boundary of Type I was formed mechanically and an initial magnetization state in both variants were restored by local application of magnetic field (≈40 kA/m). The differently oriented variants exhibited either stripe or labyrinth magnetic domain pattern in agreement with the uniaxial magnetocrystalline anisotropy of the martensite. The twin boundary was then moved by compressive or tensile stress. The passage of the boundary resulted in the formation of granular or rake domains, respectively. Additionally, the specific magnetic domains pattern projected by twin boundary gradually vanished during twin boundary motion.

Epitaxial Bi–Gd–Sc iron-garnet films for magnetophotonic applications

Novel magneto-optical epitaxial bismuth-substituted iron garnet films of composition (BiGd)3(FeSc)5O12 with large lattice parameter of 12.570 Å have been grown and investigated. Their composition was uniquely chosen to get necessary values of the magnetic and magneto-optical parameters crucial for magneto-optical applications. In particular, the fabricated films have the suppressed cubic magnetic anisotropy (down to 1.2 × 103 erg/cm3) and the large magnetic susceptibility (∼50), due to introducing Sc3+ ions; the diminished saturation magnetization (4πMS ∼ 750 Gs), caused by Gd3+ ions, and relatively large specific Faraday rotation (1.75°/μm, at 655 nm), due to Bi3+ ions. Therefore, the grown magnetic films have record values of the product of the magnetic susceptibility by the Faraday rotation that makes them extremely promising for applications, including magnetometers, magneto-optical visualizers, light modulators and nonreciprocal elements of the integrated optics.

Determination of magneto-optical constants by measuring Kerr rotation angles of magnetic films with different sample structures

A new data analysis method is proposed to determine magneto-optical (MO) constants by measuring Kerr rotation angles on samples with different multilayer film structures. The MO film in the multilayer samples is protected by a SiO2 film with two different thicknesses. An existing algorithm is used to calculate the Kerr rotation angle based on the MO theory developed in the early 1990s. A new error function is defined to evaluate the distance from the calculated Kerr angles to the measured ones. The proposed method is developed based on the Staged Continuous Tabu Search (SCTS) algorithm to retrieve the MO constant by searching the minimum of the error function. The uniqueness and accuracy of the MO constant determined by this method are discussed, and its relative error introduced by the measurement error of Kerr rotation angles is also analyzed. Without measuring the ellipticity, the proposed method is relatively simple and can be applied to determine the MO constants for the samples with two or three layers of films.

One-way transmission through merging of magnetic defect state and optical Tamm states

Abstracts To achieve a new type of one-way transmission, we design a semi-infinite one-dimensional photonic crystal (PC) coated with a magneto-optical (MO) material film. In the surface of this structure there are merging states of MO defect state and optical Tamm state (OTS). We deduce the dispersion equation of the merging states basing on the Bloch theorem of periodic structures and transfer matrix method. There are two nonreciprocal dispersion curves of the merging states for k x and − k x , respectively. The positions of the merging states can be adjusted through changing the thickness of the MO film. With the use of the attenuated total reflection (ATR) prism coupling, we consider a one-way transmission based on a finite one-dimensional PC structure. The simulations by using a finite element method demonstrate the one-way transmission results.

The effects of magnetic and mechanical microstructures on the twinning stress in Ni-Mn-Ga

The ferromagnetic 10M Ni-Mn-Ga alloy exhibits complex magnetic and mechanical microstructures, which are expected to form barriers for motion of macro twin boundaries. Here, the contributions of both microstructures to the magnitude of the twinning stress property are investigated experimentally. A series of uniaxial loading-unloading curves are taken under different orientation angles of a constant magnetic field. The different 180° magnetic domains microstructures that are formed across the twin boundary in each case are visualised using a magneto optical film. Analysis of the different loading curves and the corresponding magnetic microstructures show that the latter does not contribute to the barriers for twin boundary motion. In accordance, the internal resisting stress for twin boundary motion under any magnetic field can be taken as the twinning stress measured in the absence of an external field. In addition, a statistical analysis of the fine features in the loading profiles reveals that the barrier for twinning is associated with a μm sized characteristic length scale. This length scale corresponds to the typical thickness of micro-twinning laminates that constitute a mechanical microstructure. These findings indicate that the magnitude of the twinning stress in 10M Ni-Mn-Ga is determined by the characteristic fine twinned mechanical microstructure of this alloy.

Intensity distribution in magneto-optical images of singularities of a nonuniform magnetic field

It is demonstrated that singularities (with different topological indices) of the projection of a nonuniform magnetic field onto the plane of the magneto-optical indicator film are imaged as singularities of the magneto-optical image related to variations in the orientation of angular dependences of image intensity on the circles with the given radius centered at singularities upon rotations of the magnetic system that generates the nonuniform field.

Liquid-Phase Epitaxial Growth of 3 in Diameter Bismuth-Doped Thulium Iron Garnet Films for Magneto-Optical Applications

We report here the successful growth of a 3 in diameter magneto-optical (MO) garnet film having the composition (BiTm)3Fe5O12 by use of the liquid-phase epitaxy (LPE) method. In these experiments, Bi2O3 has been used as the fluxing agent thus omitting the use of PbO, which is highly toxic and often detrimental to the quality of film growth. Films with low defect concentration, mirror-like optical surface, high Faraday rotation angle (FRA), and large thicknesses have been obtained by optimizing the flux ratio and LPE process parameters. The thicknesses of the films have been demonstrated to reach 50–60 $\mu \text{m}$ for 3 in GGG substrates. The lattice parameter mismatch between Bi-doped TmIG (Bi:TmIG) film and the GGG substrate reaches a minimum resulting in optimal magnetic and MO properties at a growth rate of 0.87 $\mu \text{m}$ /min. The largest FRA of 0.54°/ $\mu \text{m}$ corresponds to an external magnetic field of 25 Oe. The saturation magnetization ( $4\pi M_{s}$ ), 48 emu/cm3, and coercivity ( $H_{c}$ ), 2.5 Oe, of Bi:TmIG film reach respective maximum and minimum values for the growth rate 0.87 $\mu \text{m}$ /min.

A magneto-optical microscope for quantitative measurement of magnetic microstructures.

An optical system is presented to quantitatively map the stray magnetic fields of microscale magnetic structures, with field resolution down to 50 μT and spatial resolution down to 4 μm. The system uses a magneto-optical indicator film (MOIF) in conjunction with an upright reflective polarizing light microscope to generate optical images of the magnetic field perpendicular to the image plane. A novel single light path construction and discrete multi-image polarimetry processing method are used to extract quantitative areal field measurements from the optical images. The integrated system including the equipment, image analysis software, and experimental methods are described. MOIFs with three different magnetic field ranges are calibrated, and the entire system is validated by measurement of the field patterns from two calibration samples.