Magneto-optical Indicator Film Research Articles

Relative control of domains’ structure in Grain-Oriented electrical steels by Ultra-Short Pulsed laser ablation process

This paper investigates the impact of surface Ultra-Short-Pulsed-Laser ablation process (USPL), mainly the groove depth, shockwave pressure and pattern, on magnetic characteristics of Grain-Oriented Electrical Steels (GOES) by using the Tensor Magnetic Phase Theory (TMPT). A sensitivity analysis to key laser parameters helps specifying the process thanks to a relative control of the surface magnetic structure (Magneto-Optical Indicator Film (MOIF)), dynamics and the behaviors (Single Sheet Tester (SST)) that follow.

Magneto-Optical Indicator Films: Fabrication, Principles of Operation, Calibration, and Applications.

Magneto-optical indicator films (MOIFs) are a very useful tool for direct studies of the spatial distribution of magnetic fields and the magnetization processes in magnetic materials and industrial devices such as magnetic sensors, microelectronic components, micro-electromechanical systems (MEMS), and others. The ease of application and the possibility for direct quantitative measurements in combination with a straightforward calibration approach make them an indispensable tool for a wide spectrum of magnetic measurements. The basic sensor parameters of MOIFs, such as a high spatial resolution down to below 1 μm combined with a large spatial imaging range of up to several cm and a wide dynamic range from 10 μT to over 100 mT, also foster their application in various areas of scientific research and industry. The history of MOIF development totals approximately 30 years, and only recently have the underlying physics been completely described and detailed calibration approaches been developed. The present review first summarizes the history of MOIF development and applications and then presents the recent advances in MOIF measurement techniques, including the theoretical developments and traceable calibration methods. The latter make MOIFs a quantitative tool capable of measuring the complete vectorial value of a stray field. Furthermore, various scientific and industrial application areas of MOIFs are described in detail.

Discriminating the physical impacts of various laser pulses on the magnetic structure of oriented electrical steels

Pulsed laser technologies with Long (LPL Irradiation), Short (SPL Scribing) and Ultra-Short (USPL Ablation) pulse duration can be used on SiFe Grain Oriented Electrical Steels (GOES) to refine the domains and reduce the power loss. The corresponding physics related to the magnetic structure is still a subject of research. This paper identifies separate characteristics responsible for the domains structure and the magnetization properties in a GOES sheet processed with the three laser pulses. Magnetic domains based properties are identified thanks to an average dynamic μ-νc-Λ model, the Tensor Magnetic Phase Theory (TMPT), magnetic measurements/observations with the Single Sheet Tester (SST) and the Magneto-Optical Indicator Film (MOIF) technique.

GOES’s domain structure in the vicinity and on the lines post-laser treatment

Pulsed laser processing for domain’s refinement is a recent technology implemented on an industrial scale. The present study focuses on the impact of different laser pulse durations on the magnetic properties such as total losses and domain structure of 0.28 mm thick grain-oriented electrical steel (GOES). Three pulse durations are considered Long Pulse (LP), Short-Pulse (SP) and Ultra-Short Pulse (USP). The latter has not been yet applied for domain refinement treatments, hence the interest of this research. Iron loss measured with the single sheet tester (SST) demonstrated a maximum loss reduction of 18 % at induction of 0.5 T and a frequency of 50 Hz following USP laser treatment. At a high induction of 1.7 T for the same frequency, both LP and USP pulse lengths achieved a reduction in total losses of 16 %. Moreover, the behaviour of the magnetic structure under laser treatment was detailed. The magnetic images were visualized by magneto-optical indicator film (MOIF). A reduction from 308 µm to 217 µm in the average domain size was observed post laser treatment with USP duration. Furthermore, the domain structure in the vicinity and on the laser line was investigated on a nanoscale using a magnetic force microscope (MFM). All these results confirmed the dependence of the laser-matter interaction on the laser pulse duration.

Sheet thickness dependence of magnetization properties based on domains and walls within the non-linear diffusion-like equation for grain-oriented electrical steels

The definition and development of a dynamic or transient magnetic formulation compatible with the Finite Element Method, able to take the dynamic hysteresis and classical and extra losses into account (not a posteriori but a priori), and using the most adapted state variable that makes the problem well defined and easily convergent requires the knowledge of dynamic behavioral properties of soft magnetic materials whatever the thickness and the working conditions. The dynamic hysteresis of soft magnetic materials corresponds to excess iron losses, due to dynamic magnetization reversal processes within magnetic domains and especially to microscopic eddy currents around the magnetic walls in motion and inside rotating domains. The model properties used are the internal quasi-static permeability μ and the dynamic magnetization property Λ that lumps the magnetization mechanisms (domain walls displacement, bowing, fusion, nucleation and multiplication). The latter are involved in the magnetic field damping due to microscopic eddy currents within the field diffusion that renders the magnetic behavior geometry dependent. This model does not separate the field diffusion process from the magnetization reversal mechanisms. In this paper, the sheet thickness ζ dependence of the dynamic magnetization property Λ(ζ) for a Grain Oriented Electrical Steel (GOES: steel made of 3% Silicon and Iron: SiFe) is analyzed in the Rolling Direction (RD). To this extend, it is proposed to carry out and interpret magnetic measurements on GOES samples with the Epstein frame for four different thicknesses (ζ = 0.23, 0.27, 0.30 and 0.35 mm), but with similar metallurgical and crystallographic properties for the whole specimens. Magnetic properties are first identified at low induction with linear assumptions and at higher induction with non-linearities. It makes it possible to re-compute the dynamic hysteresis loops of the material and to predict the losses whatever the working conditions with frequencies from 50 to 800 Hz. It is found that μ does not depend significantly on the sheet thickness whereas Λ depends on it significantly. Microscopic observation of the magnetic domains width are then performed thanks to the MOIF (Magneto-Optical Indicator Film) technique. It helps us discriminate between three simultaneous origins for the dependence of iron losses to the sheet thickness: a skin-like effect, a domains’ refinement and changes on the walls’ mobility. Results are discussed taking the dipolar magnetic effects, closure domains, the grains and texture and the manufacturing and coating residual stress on the metal surface into account.

Investigation of the Domain Structure of Co-Based Microwires by Magneto-Optical Indicator Films Method

Investigation of the Domain Structure of Co-Based Microwires by Magneto-Optical Indicator Films Method

Radial Domains in DyPr–FeCo–B Microwires

In DyPr–FeCo–B microwires with easy magnetization axis directed along the microwire axis, domains with radial magnetization are detected using magneto-optical indicator films. The width of radial domains decreases as the field increases to 30 mT and increases with the microwire diameter in the range of 60–105 μm. In wires of smaller diameter, the critical field of radial domain formation is lower. The effect of the periodic relief produced by scribing on the magnetization distribution perpendicular to the microwire is detected.

Effect of the in-plane field component on the response of the magneto optical indicator film sensors

In this work we present a study of the effect of the in-plane component of magnetic stray field on the response of Magneto Optical Indicator Film (MOIF) sensors. To take into account this effect, which was neglected in all MOIF measurements done so far, a small bias field with in-plane orientation is applied and the MOIF response thus obtained is compared with the results of numerical modelling. Thus the complete vectorial value of stray field can be determined. Moreover, we show that in a wide range of magnetic fields, up to approximately half the saturation field, the effect of the in-plane field component can be neglected and thus MOIFs are sensitive only to the normal field component, thus making it possible to measure this component directly.

Investigation of the Domain Structure of Co-based Microwires by Magneto-Optical Indicator Films Method

The investigation of domain structure of Co-based microwires with negative magnetostriction was provided by magnetooptical method of indicator films. The thickness of the domain layer and the width of the surface domains were determined. It has been suggested that in thin microwires (with the diameter of about several tens of microns) with negative magnetostriction, the Neel-type of domain walls between the domains of the surface layer are observed. A number of assumptions about the regularities of the surface domain structure formation of microwires with negative magnetostriction have been put forward.

Радиальные домены в микропроводах DyPr-FeCo-B

In DyPr-FeCo-B microwires with an easy magnetization axis directed along the microwire axis, domains with radial magnetization were found by the method of magneto-optical indicator films (MOIF). The width of the radial domains decreases with increasing field up to 30 mT, and increases with increasing diameter of the microwire in the range 60 - 105 μm. In wires of smaller diameter, the critical field for the appearance of radial domains is smaller. The influence of periodic scratches on the distribution of magnetization perpendicular to the microwire is found.

Asymmetric magnetization reversal of the Heusler alloy Co2FeSi as free layer in an CoFeB/MgO/Co2FeSi magnetic tunnel junction

We report the in-plane magnetization reversal behavior of the Co2FeSi Heusler alloy free layer in a bottom-pinned magnetic tunnel junction film with 149% TMR. Using magneto-optic indicator film imaging, we visualize the magnetic domain dynamics of this buried layer integrated within a full magnetic tunnel junction stack. The magnetic domain dynamics reveal anisotropic magnetization reversal within Co2FeSi under applied in-plane magnetic fields. While the reversal behavior under in-plane fields applied perpendicular to the in-plane exchange bias direction indicates smooth, coherent rotation away from the easy axis, asymmetric magnetic reversal behavior is observed along the easy axis. Reversed domains propagate from the interior of the film laterally outward toward the edges as the free layer magnetization switches into parallel alignment with the pinned synthetic antiferromagnetic reference layer (IrMn/CoFe/Ru/CoFeB). On the other hand, domains propagate from the film edges inward for the free layer transition into antiparallel alignment with the reference layer. These results have important implications for Heusler magnetic tunnel junction device performance.

Inhomogeneous field calibration of a magneto-optical indicator film device

A concept for the traceable calibration of magneto-optical indicator film (MOIF) based magnetic field imaging devices is presented and discussed for the example of a commercial MOIF device with a 60 × 45 mm2 sensor. The calibration facilitates a quantitative and fast characterization of magnetic microstructures combining relatively high spatial resolution with large imaging areas. The macroscopic calibration is performed using the homogeneous magnetic stray field of a pre-characterized electromagnet with a large pole shoe diameter of 250 mm. However, this calibration alone cannot yet account for the vectorial and spatially fast decaying stray fields of magnetic microstructures. For that, a forward simulation approach is pursued, based on the temperature-dependent magnetic parameters of the MOIF material as resulting from superconducting quantum interference device magnetometry and ferromagnetic resonance measurements. This is complemented by a transfer function-based approach to correct the impact of the sensor thickness and in-plane stray field components. The validity of the combined calibration and simulation approach is proven by means of a quantitative characterization of a magnetic scale. For the commercial MOIF device a 28.4 µm spatial resolution and 1.18 mT field resolution is achieved. The calibration is validated by a comparison to scanning Hall probe microscopy results. Furthermore, the uncertainty budget is discussed.

Visualization of Magnetic Domains and Magnetization Vectors in Magnetic Shape Memory Alloys Under Magneto-Mechanical Loading

$$\hbox {Ni}_{2}\hbox {MnGa}$$ magnetic shape memory alloys have a microstructure consisting of martensite variants, and magnetic domains exist in each martensite variant. In the absence of a magnetic field, the magnetic domains are equally distributed so that the net magnetization of the material is zero. Application of a magnetic field or mechanical stress can rearrange the martensite variants and magnetic domains. This study focuses on understanding the effects that magnetic field, compressive stress and magneto-mechanical loading have on the magnetic domains, by measuring and quantifying magnetic domain volume fraction and magnetization vector rotation using direct imaging. In particular, a magneto-optical indicator film in conjunction with polarization microscopy was used to visualize the evolution of the magnetic domains and magnetization vector rotation of a $$\hbox {Ni}_{2}\hbox {MnGa}$$ sample for different load cases, namely varying magnetic field at constant strain, and varying compressive stress at a constant magnetic field. Our experiments revealed that the applied magnetic field causes change in domain volume fraction at different rates in each variant, and that domain wall motion is not always fully reversible. Magnetization vector rotation, however, was found to be reversible for all loading cases tested.

Repeatability and stability study of residual magnetic field for domain wall characterization

Residual magnetic field (RMF) for metal material has a relationship with material stress status. Bridging the relationship between macro parameters and material microstructure is an essential step to deal with the characterization of residual stress. This paper applies magneto-optical Kerr microscopy (MOKE) with the help of magneto-optical indicator film (MOIF) to observe and analyze variations of magnetic domain pattern under different stress conditions of high permeability grain oriented electrical steel (HGO steel). Experiment about repeatability is carried under cyclic stress applications and the stability study is proceeded during the relaxation time after release of stress. By using two domain texture features, mean value (MV) of domain wall image intensity linked to domain rotation to characterize RMF amplitude and the angular second moment (ASM) related with domain wall displacement to express the RMF distribution are used for investigation of repeatability and stability. Results show that domain wall displacement is repeatable and stable both under cyclic stress round and during relaxation time after release of stress. It has been found that the situation of domain rotation is varied in different locations, and broad texture area of domain wall images has better repeatability and stability when compared with area with narrow texture of domain wall images. The results provide substantial potential to evaluate the residual stress from the view of characterization of micro-structure.

Correlating non-linear behavior of in-plane magnetic field and local domain wall velocities for quantitative stress evaluation

There is a need in industry to supply safe, effective and reliable technique to characterize the stress of steel components and structures, both at the manufacturing stage and in service. Bridging the correlation between micro and macro magnetic properties and the applied tensile stress is the first conceptual step to come up with a new method of non-destructive material testing. We investigate the stress-associated changes in domain wall dynamics in grain-oriented electrical steel by in-situ magnetic imaging using magneto-optical indicator films. The 180° domain walls velocity distribution is used as a parameter for applied stress determination. Additionally, the in-plane magnetic stray field above the surface of the sample is synchronously measured for stress evaluation. The variations in magnetic stray field outside the sample under different loading are investigated for the analysis of the domain wall dynamics. From this, an interrelation of the domain wall dynamics and magnetic stray fields with varied tensile stress is derived. The results provide substantial microscopic and macroscopic insight for the interplay of domain wall dynamics and stress-induced demagnetizing effect.

Domain Structure and Magnetization Reversal Micromechanisms in Quasi-Two-Dimensional Exchange-Biased Nanomagnetics

Abstract—The domain structure and magnetization reversal mechanisms are studied in quasi-two-dimensional exchange-biased NiFe/FeMn and NiFe/NiO nanomagnetics, using a complex method of magnetooptical indicator films and acoustic emission. The presence of the axial dispersion of unidirectional anisotropy in grains of the antiferromagnetic layer is shown to determine the statistic distribution and chirality of spin springs near the interface. The acoustic emission signals caused by the excitation of elastic Lamb waves upon the magnetization reversal of NiFe/NiO heterostructure are found. The coercive force of these systems is due to irreversible processes to overcome potential barriers that are induced by the formation of spin springs with different chirality, localized in the antiferromagnetic near the ferromagnetic–antiferromagnetic boundary.

Power frequency domain imaging on Goss-textured electrical steel

Abstract A magneto-optical indicator film with perpendicular anisotropy (PMOIF) is used for time-resolved domain imaging on grain-oriented transformer steel at technical frequency field excitation. The high sensitivity of the PMOIF allows for single-shot imaging, even in the presence of insulation coatings. Differences between quasistatic and dynamic processes are elaborated, revealing the mechanisms of domain refinement and showing the role of grain boundaries and mechanical stress on domain formation and flux propagation. It is shown that the PMOIF technique opens a promising window for studying the domain dynamics of transformer steels under practical conditions, which goes beyond established characterization tools like Kerr microscopy.

Доменная структура и микромеханизмы перемагничивания в квазидвумерных обменно-смещeнных наномагнетиках

Abstract —The domain structure and magnetization reversal mechanisms are studied in quasi-two-dimensional exchange-biased NiFe/FeMn and NiFe/NiO nanomagnetics, using a complex method of magnetooptical indicator films and acoustic emission. The presence of the axial dispersion of unidirectional anisotropy in grains of the antiferromagnetic layer is shown to determine the statistic distribution and chirality of spin springs near the interface. The acoustic emission signals caused by the excitation of elastic Lamb waves upon the magnetization reversal of NiFe/NiO heterostructure are found. The coercive force of these systems is due to irreversible processes to overcome potential barriers that are induced by the formation of spin springs with different chirality, localized in the antiferromagnetic near the ferromagnetic–antiferromagnetic boundary.

The change of domain structure of the amorphous microwire of Fe73.5Cu1Nb3Si13.5B9 composition under thermal treatment

The investigation on domain structure evolution under variation of magnetostriction, phase composition and internal stresses was provided. Glass-coated amorphous microwires of Fe73.5Cu1Nb3Si13.5B9 composition with positive magnetostriction were studied. The decrease of internal stresses and controlled crystallization leading to changes in the phase composition and reduction of magnetostriction were observed under different thermal treatment. The domain structure of the as-prepared and annealed microwires was studied by the method of magneto-optical indicator films. The estimation of the domain size was realized by Fourier transform from the domain contrast of magneto-optical images. The regularities of the influence of thermal treatment on the magnetic domain structure of microwires have been estimated.

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.