A method for large area magnetic domain direct imaging

Abstract

The application relates to a method for directly imaging a large-area magnetic domain, and belongs to the technical field of magnetic domain observation. The method comprises the following steps: arranging a plurality of permanent magnets on the two sides of an oriented silicon steel sheet, arranging the plurality of permanent magnets along a straight line at a first set interval, and the distance between each permanent magnet and the boundary of the oriented silicon steel sheet is a second set interval; arranging a coil at the center position of the oriented silicon steel sheet, so that the magnetic field of the coil and the magnetic field of the permanent magnet are superposed to form an enhanced magnetic field; under the action of the enhanced magnetic field, a magnetic fluid with a set gradient particle size is uniformly coated on the surface of the oriented silicon steel sheet at one time, so that direct imaging of the magnetic domain is realized. Through magnetic field-magnetic fluid collaborative regulation, the problem of unclear and incomplete large-area magnetic domain imaging in the prior art is solved, high-definition and high-integrity imaging of a super-large-area magnetic domain is realized, and a new technical means is provided for magnetic material research.

Description

Technical Field

[0001] This application relates to the field of magnetic domain observation technology, and in particular to a method for direct imaging of large-area magnetic domains. Background Technology

[0002] Magnetic domains are regions within magnetic materials that possess specific magnetization directions, and their morphology, distribution, and evolution significantly influence the magnetic properties of the material. Studying magnetic domain structure not only reveals the magnetic properties of materials but also provides crucial information for material optimization and application. Currently, commonly used magnetic domain imaging techniques include Kerr microscopy, scanning electron microscopy, and powder mapping. While each of these techniques has its advantages, they generally have limitations. Kerr microscopy, although offering high spatial resolution, struggles to cover large areas, making it suitable for studying microscopic magnetic domains but unsuitable for large-area imaging. Scanning electron microscopy also boasts high resolution and imaging accuracy but similarly struggles to visualize large-area magnetic domain morphology. Powder mapping is a classic magnetic domain imaging method, simple to operate and low-cost, but traditional powder mapping has a limited imaging area. As the observation area increases, it faces challenges such as inhomogeneous magnetic fields and the inability to attract magnetic particles, making it difficult to meet the needs of ultra-large-area magnetic domain imaging.

[0003] Therefore, there is an urgent need for a method that can achieve ultra-large area, high-resolution magnetic domain imaging. Summary of the Invention

[0004] This application provides a method for direct imaging of large-area magnetic domains to solve the following technical problem: how to achieve the integrity and clarity of large-area magnetic domain imaging.

[0005] In a first aspect, embodiments of this application provide a method for direct imaging of large-area magnetic domains, the method comprising:

[0006] Multiple permanent magnets are respectively arranged on both sides of the oriented silicon steel sheet. The multiple permanent magnets are arranged in a straight line with a first predetermined spacing, and the distance between each permanent magnet and the boundary of the oriented silicon steel sheet is a second predetermined spacing.

[0007] A coil is placed at the center of the oriented silicon steel sheet so that the magnetic field of the coil is superimposed on the magnetic field of the permanent magnet to form an enhanced magnetic field;

[0008] Under the action of the enhanced magnetic field, a magnetic fluid with a set gradient particle size is uniformly coated onto the surface of the oriented silicon steel sheet in one go to achieve direct imaging of magnetic domains.

[0009] The first and second predetermined spacings are both 4cm to 6cm. The intensity of the enhanced magnetic field is 0.3mT to 0.4mT, with the magnetic field direction being north pole upwards. The predetermined gradient particle sizes D1 to D4 are selected from the intervals 15μm≤D1≤20μm, 25μm≤D2≤30μm, 35μm≤D3≤40μm, and 60μm≤D4≤80μm, respectively. The coating amount for a single application is 0.4mL / cm³. 2 ~0.6mL / cm 2 ;

[0010] The coil has at least one of the following properties: it is rectangular in shape, with dimensions of 70mm×250mm to 90mm×300mm, 8 to 12 turns, and a current of 0.2A to 1A.

[0011] Optionally, the first preset spacing is 5cm; and / or,

[0012] The second set spacing is 5cm.

[0013] Optionally, the material of the permanent magnet is selected from at least one of the following: NdFeB, SmCo, AlNiCo, or ferrite;

[0014] in:

[0015] When the material of the permanent magnet is NdFeB, the magnetic energy product of the permanent magnet is 35MGOe~50MGOe, the remanent magnetic induction Br is 1.2T~1.4T, and the coercivity Hc is 900kA / m~1200kA / m;

[0016] When the material of the permanent magnet is SmCo, the magnetic energy product of the permanent magnet is 20MGOe~30MGOe, the remanent magnetic induction Br is 1.0T~1.2T, and the coercivity Hc is 700kA / m~1000kA / m;

[0017] When the material of the permanent magnet is AlNiCo, the magnetic energy product of the permanent magnet is 5MGOe~10MGOe, the remanent magnetic induction Br is 0.8T~1.0T, and the coercivity Hc is 300kA / m~500kA / m;

[0018] When the permanent magnet is made of ferrite, the magnetic energy product of the permanent magnet is 3MGOe to 6MGOe, the remanent magnetic induction Br is 0.4T to 0.6T, and the coercivity Hc is 200kA / m to 300kA / m.

[0019] Optionally, the N pole of the permanent magnet is connected to the oriented silicon steel sheet, the thickness of which is 0.2 mm.

[0020] 0.35mm, the arrangement direction of the permanent magnets is parallel to the rolling direction of the oriented silicon steel sheet.

[0021] Optionally, the set gradient particle size includes 75μm, 37μm, 25μm and 19μm.

[0022] Optionally, the magnetic fluid is obtained by surface modification with magnetic particles.

[0023] Optionally, the magnetic fluid is applied using a casting method.

[0024] Optionally, during the application of the magnetic fluid, the temperature of the oriented silicon steel sheet is controlled between 20°C and 30°C.

[0025] Optionally, the magnetic fluid with a set gradient particle size is applied to the surface of the oriented silicon steel sheet in a single application, so as to...

[0026] To achieve magnetic domain imaging, the following are included:

[0027] A magnetic fluid with a set gradient particle size is applied to the surface of the oriented silicon steel sheet in one go to achieve magnetic domain imaging;

[0028] in,

[0029] The method for preparing the magnetic fluid includes:

[0030] Magnetic particles with the set gradient particle size are obtained;

[0031] The magnetic particles, dispersion, and activator are mixed for passivation treatment to obtain a magnetic fluid.

[0032] Optionally, the method further includes:

[0033] The oriented silicon steel sheet is pretreated; wherein the pretreatment includes cutting, surface degreasing and drying in sequence.

[0034] The technical solutions provided in this application have the following advantages compared with the prior art:

[0035] The method for direct imaging of large-area magnetic domains provided in this application includes: arranging multiple permanent magnets on both sides of an oriented silicon steel sheet, wherein the multiple permanent magnets are arranged in a straight line at a first predetermined spacing, and the distance between each permanent magnet and the boundary of the oriented silicon steel sheet is a second predetermined spacing; arranging a coil at the center of the oriented silicon steel sheet, such that the magnetic field of the coil is superimposed with the magnetic field of the permanent magnets to form an enhanced magnetic field; and uniformly coating a magnetic fluid with a predetermined gradient particle size in one step under the action of the enhanced magnetic field. Direct imaging of magnetic domains is achieved on the surface of the oriented silicon steel sheet; wherein the first and second predetermined spacings are both 4cm to 6cm, the intensity of the enhanced magnetic field is 0.3mT to 0.4mT, the magnetic field direction is N pole upward, the predetermined gradient particle sizes D1 to D4 are selected from the intervals 15μm≤D1≤20μm, 25μm≤D2≤30μm, 35μm≤D3≤40μm, and 60μm≤D4≤80μm, respectively, and the coating amount in a single application is 0.4mL / cm. 2 ~0.6mL / cm 2 The coil includes at least one of the following properties: rectangular shape, dimensions from 70mm×250mm to 90mm×300mm, 8 to 12 turns, and a current of 0.2A to 1A. A large-area magnetic field is provided by synchronously enhancing the magnetic field of the coil center with permanent magnets arranged in a straight line. This synchronous enhancement mechanism eliminates the attenuation gradient of a single permanent magnet's magnetic field, making the magnetic field strength from the center to the edge of the silicon steel sheet more uniform. This ensures uniform and controllable adsorption behavior of the magnetic fluid over a large area, facilitating uniform adsorption of the magnetic fluid onto the sample surface and forming a clear magnetic domain image. The magnetic fluid has a set gradient particle size; large-diameter particles provide high-contrast signals and enhance the overall image visibility; small-diameter particles fill the microscopic magnetic field gradient region, improving resolution and supplementing magnetic domain details, thereby further improving the uniformity of the large-area magnetic field and enhancing the clarity and integrity of the magnetic domain image. The magnetic fluid is applied to the surface of the oriented silicon steel sheet in a single application, ensuring the integrity of the large-area magnetic domain imaging. Therefore, by constructing a uniform large-area magnetic field through the regular linear arrangement of permanent magnets and the magnetic field enhancement of the central coil, combined with the multi-scale adsorption capacity of gradient-size magnetofluids and a one-time uniform coating technique, this method, based on the synergistic control of magnetic field and magnetofluid, solves the problems of non-uniform magnetic field and incomplete particle coverage in traditional magnetic domain imaging, ultimately achieving a large-area magnetic domain structure (≥600cm). 2 High-definition, high-completeness direct imaging. Attached Figure Description

[0036] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0037] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0038] Figure 1 A flowchart illustrating a method for direct imaging of large-area magnetic domains provided in this application embodiment;

[0039] Figure 2 A schematic diagram of a magnetic domain imaging sample stage for a method of direct imaging of large-area magnetic domains provided in this application embodiment; wherein, 1-bottom acrylic plate, 2-silicon steel sheet, 3-upper acrylic plate;

[0040] Figure 3 An exploded view of the magnetic domain imaging sample stage for a method of direct imaging of large-area magnetic domains provided in this application embodiment; wherein, 11-bottom acrylic plate (with groove for placing coil), 12-coil, 2-silicon steel sheet, 31-upper acrylic plate (with observation window and permanent magnet fixing position window), 32-permanent magnet, 33-rubber frame;

[0041] Figure 4 The diagram shows the positions of the permanent magnet and coil provided in the embodiments of this application. The dimensions in the diagram are as follows: a=400mm, b=400mm, c=210mm, d=310mm, e=95mm, f=45mm, g=50mm, h=100mm, i=50mm.

[0042] Figure 5 The magnetic induction intensity distribution on the surface of the silicon steel sheet before (a) and after (b) coil compensation, provided for embodiments of this application;

[0043] Figure 6 An observation result of magnetic domain imaging for a method of direct imaging of large-area magnetic domains provided in Embodiment 1 of this application;

[0044] Figure 7 The image shows the observation results of large-area central magnetic domain imaging (400-mesh magnetofluid under permanent magnet magnetic field) provided in Comparative Example 1 of this application.

[0045] Figure 8 This is an observation result of magnetic domain imaging for a method of direct imaging of large-area magnetic domains provided in Embodiment 2 of this application. Detailed Implementation

[0046] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0047] Various embodiments of this application may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a hard limitation on the scope of this application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Furthermore, whenever a numerical range is referred to herein, it means including any referenced number (fraction or integer) within the range referred to.

[0048] In this application, unless otherwise stated, directional terms such as "upper" and "lower" specifically refer to the drawing directions in the accompanying drawings. Furthermore, in the description of this application, terms such as "comprising" and "including" mean "including but not limited to." In this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. In this document, "and / or" describes the relationship between related objects, indicating that three relationships can exist; for example, A and / or B can represent: A alone, A and B simultaneously, or B alone. A and B can be singular or plural. In this document, "at least one" means one or more, and "more than one" means two or more. "At least one," "at least one of the following," or similar expressions refer to any combination of these items, including any combination of single or plural items. For example, "at least one of a, b, or c" or "at least one of a, b, and c" can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be a single or multiple.

[0049] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0050] Grain-oriented silicon steel: Grain-oriented silicon steel is a special type of silicon alloy steel, typically produced and used in sheet or strip form. Its grains achieve a high degree of orientation (typically {110}) through a specific annealing process during rolling. <001> (Goss texture), which gives it excellent magnetic properties in the rolling direction.

[0051] Magnetic domains in grain-oriented silicon steel: Magnetic domains are tiny regions that spontaneously form within ferromagnetic materials (such as grain-oriented silicon steel). The atomic magnetic moments within each domain are aligned, but the directions of magnetic moments may differ between different domains. In grain-oriented silicon steel, 180° strip-shaped domains are predominant. By optimizing the magnetic domains (such as refining the domains or controlling the movement of the domain walls), their magnetic properties can be significantly improved.

[0052] Powder mapping: Powder mapping is a method that reveals the magnetic domain structure by applying fine magnetic powder to the surface of a magnetized sample. After magnetizing a silicon steel plate with an external magnetic field, the fine magnetic particles are affected by the local magnetic field on the sample surface and align along the domain walls, thus revealing the distribution and structure of the magnetic domains of the sample to the naked eye or under a low-magnification microscope.

[0053] Firstly, embodiments of this application provide a method for direct imaging of large-area magnetic domains. Figure 1 A flowchart illustrating a method for direct imaging of large-area magnetic domains provided in this application embodiment; please refer to... Figure 1 The method includes:

[0054] S1. Multiple permanent magnets are respectively arranged on both sides of the oriented silicon steel sheet. The multiple permanent magnets are arranged in a straight line with a first predetermined spacing, and the distance between each permanent magnet and the boundary of the oriented silicon steel sheet is a second predetermined spacing.

[0055] S2. A coil is placed at the center of the oriented silicon steel sheet so that the magnetic field of the coil is superimposed with the magnetic field of the permanent magnet to form an enhanced magnetic field;

[0056] S3. Under the action of the enhanced magnetic field, a magnetic fluid with a set gradient particle size is uniformly coated onto the surface of the oriented silicon steel sheet in one go to achieve direct imaging of magnetic domains.

[0057] The first and second predetermined spacings are both 4cm to 6cm. The intensity of the enhanced magnetic field is 0.3mT to 0.4mT, with the magnetic field direction being north pole upwards. The predetermined gradient particle sizes D1 to D4 are selected from the intervals 15μm≤D1≤20μm, 25μm≤D2≤30μm, 35μm≤D3≤40μm, and 60μm≤D4≤80μm, respectively. The coating amount for a single application is 0.4mL / cm³. 2 ~0.6mL / cm 2 ;

[0058] The coil has at least one of the following properties: it is rectangular in shape, with dimensions of 70mm×250mm to 90mm×300mm, 8 to 12 turns, and a current of 0.2A to 1A.

[0059] Multiple permanent magnets are arranged in a straight line along two sides of an oriented silicon steel sheet with a first predetermined spacing, creating a stable and uniform basic magnetic field environment. This initial spacing effectively reduces magnetic field superposition interference between adjacent permanent magnets, avoids local magnetic field anomalies caused by magnetic field interactions, and ensures that the magnetic field generated by each permanent magnet can act independently on the silicon steel sheet, thus forming a relatively uniform initial magnetic field distribution on the surface of the silicon steel sheet. A second predetermined spacing is maintained between each permanent magnet and the boundary of the oriented silicon steel sheet. When a permanent magnet approaches the boundary of the silicon steel sheet, magnetic field distortion occurs at the boundary due to the difference in permeability between the air, the permanent magnet, and the silicon steel sheet, resulting in an uneven magnetic field distribution in the edge region. Maintaining the second predetermined spacing ensures a stable and uniform magnetic field within the effective observation area of ​​the silicon steel sheet, providing reliable magnetic field conditions for subsequent magnetic domain observation and eliminating interference from edge magnetic field anomalies on experimental results.

[0060] The coil is positioned at the center of the grain-oriented silicon steel sheet to ensure that the magnetic field generated by the coil can uniformly cover the entire area of ​​the silicon steel sheet. The centrally symmetrical layout allows for a more balanced distribution of the coil's magnetic field on the silicon steel sheet, avoiding situations where the local magnetic field is too strong or too weak, and ensuring that the magnetic field effect on all parts of the silicon steel sheet is consistent.

[0061] The synergistic superposition of the coil and permanent magnet forms an enhanced magnetic field. The permanent magnet provides the basic bias magnetic field, while the coil acts as a supplementary magnetic field through electromagnetic induction. When the coil is energized, its generated magnetic field superimposes with the inherent magnetic field of the permanent magnet within the silicon steel sheet region. The two fields are aligned and mutually reinforce each other, resulting in a stronger enhanced magnetic field. Therefore, by aligning the permanent magnets in a straight line and synchronously enhancing the magnetic field with the coil's center, magnetic field uniformity can be ensured, facilitating the uniform adsorption of magnetofluid onto the sample surface and forming a clear magnetic domain image. The strength of this enhanced magnetic field is precisely controlled by adjusting the coil's energizing current.

[0062] A rectangular coil ranging from 70mm × 250mm to 90mm × 300mm is used, with its long side aligned with the rolling direction of the silicon steel sheet. This design allows the magnetic field generated by the coil to more effectively cover the main observation area of ​​the silicon steel sheet. An 8-12 turn coil, coupled with a current of 0.2A to 1A, generates a magnetic field of a certain strength, forming a magnetic field of approximately 0.2mT using electromagnetic induction, with the north pole pointing upwards. Therefore, the shape, size, number of turns, and current of the coil synergistically compensate for the central magnetic field. For example, the coil current can be 0.2A, 0.3A, 0.4A, 0.5A, 0.7A, 0.8A, 0.9A, or 1A.

[0063] By aligning permanent magnets in a straight line and synchronously enhancing the magnetic field with the center of the coil, the average magnitude of the overall magnetic field is achieved.

[0064] With a magnetic field strength of 0.3 mT to 0.4 mT and a north-pole-upward orientation, the magnetic fluid is uniformly adsorbed onto the sample surface, forming a high-contrast, clear magnetic domain image, while ensuring that the magnetic domains are not altered by the external magnetic field. This 0.3 mT to 0.4 mT intensity range effectively drives the magnetic domain movement of the silicon steel sheet, while avoiding excessive magnetic field strength that could lead to domain saturation. This ensures the sensitivity and dynamic range of the observation, improves the magnetic field strength in the center of the observation field, enhances the problem of magnetic field inhomogeneity, and avoids the heating problem when the coil acts alone. Therefore, a rectangular coil (70 mm × 250 mm to 90 mm × 300 mm) is located at the center of the silicon steel sheet. By passing an electric current (0.2 to 1 A), a supplementary magnetic field is generated, which is superimposed with the basic magnetic field of the permanent magnet to form an enhanced uniform magnetic field (0.3 to 0.4 mT, north-pole-upward). This synchronous magnetic field enhancement mechanism eliminates the attenuation gradient of the magnetic field of a single permanent magnet, making the magnetic field strength from the center to the edge of the silicon steel sheet more consistent, ensuring uniform and controllable adsorption behavior of the magnetic fluid over a large area. For example, the strength of the enhanced magnetic field can be 0.3mT, 0.32mT, 0.34mT, 0.36mT, 0.38mT, 0.4mT, etc.

[0065] A single coating application avoids the uneven coating thickness or localized drying problems that can occur with multiple applications, ensuring the overall coating thickness of the silicon steel is consistent.

[0066] The magnetohydrodynamic fluid on the surface of the sheet simultaneously responds to the magnetic field, thereby achieving complete imaging of large-area magnetic domains. The coating amount in a single application can be 0.4 ml / cm². 2 ~0.6ml / cm 2 This method can form a uniformly thick magnetofluidic film on the surface of silicon steel sheets, ensuring that the magnetic particles fully respond to the magnetic field while preventing particle settling due to gravity from affecting imaging results. For example, the coating amount in a single application can be 0.4 ml / cm². 2 0.45ml / cm 2 0.5ml / cm 2 0.55ml / cm 2 0.6ml / cm 2 wait.

[0067] By superimposing the magnetic fields of permanent magnets and coils, the stray magnetic field strength at the domain walls can be enhanced, making particle aggregation more significant and improving imaging contrast. The synergistic effect of multi-scale particles can capture fine domain wall details and enhance the boundary visibility of low-contrast areas, thus improving the clarity of magnetic domain imaging.

[0068] Ideally, when the magnetic field strength variance is zero, the adsorption of magnetic particles is most uniform, with magnetic particles simultaneously adsorbed onto the domain walls, resulting in consistent response times (magnetic domain imaging time). However, in large-area applications, the magnetic field cannot be completely uniform; at the outermost edges, a small area still exhibits a relatively larger magnetic field compared to the center. In this embodiment, a magnetic fluid with a set gradient particle size is designed. Larger particles have stronger coercivity, requiring a larger magnetic field to adsorb onto the sample, making them easier to adsorb at the edges and more stable after the magnetic field is removed. Smaller particles have weaker coercivity, adsorbing onto the sample even under a smaller magnetic field, and preferentially adsorbing in the center of the field of view. Therefore, using a magnetic fluid with a gradient particle size can further offset the adsorption unevenness caused by small, non-uniform magnetic fields. Using only large particles results in a weaker magnetic field in the center, making adsorption difficult; using only small particles leads to particle detachment after adsorption and removal of the magnetic field, reducing imaging quality. Therefore, using a combination of large and small particles is the best choice. The above premise is that the large-area magnetic field must be uniform. Magnetofluids further counteract the problems of adsorption response time and adsorption uniformity caused by the inhomogeneity of the small magnetic field. Inconsistent adsorption response time of magnetic particles will cause mutual disturbance between magnetic particles (their movements will be inconsistent, which will lead to agglomeration), affecting the final adsorption.

[0069] A single coating of the magnetic fluid is essential for forming a magnetic domain image. If the coating is applied in stages, the first layer of magnetic fluid will form an image, but when the later layer comes into contact with the first layer, disturbances will occur, causing magnetic particles to detach from the domain walls, especially small particles (with low coercivity). This will result in an unclear magnetic domain image.

[0070] In some embodiments, the first predetermined spacing is 5 cm; and / or,

[0071] The second set spacing is 5cm.

[0072] Multiple permanent magnets are arranged in a straight line on both sides of the grain-oriented silicon steel sheet at a predetermined spacing of 4cm to 6cm, which can avoid adjacent magnets

[0073] While the magnetic fields of the permanent magnets are excessively superimposed, a stable and moderately strong basic magnetic field is formed. This regular arrangement ensures a uniform lateral distribution of the magnetic field on the silicon steel sheet surface, avoiding differences in magnetohydrodynamic adsorption caused by uneven local magnetic field strength. Each permanent magnet maintains a second predetermined distance of 4cm to 6cm between its boundary and the oriented silicon steel sheet to prevent edge magnetic field distortion (near the magnet) and avoid affecting the observation of edge magnetic domains, thereby achieving effective magnetic field action across the entire sheet. For example, the first and second predetermined distances can both be 4cm, 5cm, 6cm, etc.

[0074] In some embodiments, the material of the permanent magnet is selected from at least one of the following: NdFeB, SmCo, AlNiCo, or ferrite; wherein:

[0075] When the material of the permanent magnet is NdFeB, the magnetic energy product of the permanent magnet is 35MGOe~50MGOe, the remanent magnetic induction Br is 1.2T~1.4T, and the coercivity Hc is 900kA / m~1200kA / m;

[0076] When the material of the permanent magnet is SmCo, the magnetic energy product of the permanent magnet is 20MGOe~30MGOe, the remanent magnetic induction Br is 1.0T~1.2T, and the coercivity Hc is 700kA / m~1000kA / m;

[0077] When the material of the permanent magnet is AlNiCo, the magnetic energy product of the permanent magnet is 5MGOe~10MGOe, the remanent magnetic induction Br is 0.8T~1.0T, and the coercivity Hc is 300kA / m~500kA / m;

[0078] When the permanent magnet is made of ferrite, the magnetic energy product of the permanent magnet is 3MGOe to 6MGOe, the remanent magnetic induction Br is 0.4T to 0.6T, and the coercivity Hc is 200kA / m to 300kA / m.

[0079] The advantages of the aforementioned permanent magnet materials are: the permanent magnet materials provide high coercivity, which can keep the magnetic domain structure of the sample under observation stable and avoid the influence of external temperature fields; and the materials themselves have strong chemical stability and constant magnetic field strength.

[0080] In some embodiments, the N pole of the permanent magnet is connected to the oriented silicon steel sheet, the thickness of the oriented silicon steel sheet is 0.2 mm to 0.35 mm, and the arrangement direction of the permanent magnet is parallel to the rolling direction of the oriented silicon steel sheet.

[0081] Connecting the N pole of the permanent magnet to the oriented silicon steel sheet creates a controllable magnetic field environment, unifies the magnetic field direction, optimizes magnetic circuit coupling, and provides a reference condition for subsequent magnetic field superposition. When the coil is energized, the upward magnetic field direction of the N pole is consistent with the magnetic field direction of the permanent magnet. This co-directional superposition method maximizes the strength of the synthesized magnetic field.

[0082] For example, S1 includes: permanent magnets arranged in a straight line with a spacing of 5cm, 5cm away from the boundary of the observation area (oriented silicon steel sheet), to prevent edge magnetic field distortion (near the magnet position) and avoid affecting the observation of edge magnetic domains, and specifying that the N pole of the permanent magnet is used to connect to the silicon steel sample, and the coil is fixed at the center of the entire observation area, using a rectangular coil with a size of 80×280mm and 10 turns, connected to a DC power supply box with a current of 0.2~1A, using electromagnetic induction to form a magnetic field of about 0.2mT, with the magnetic field direction being N pole upward.

[0083] In addition, apply vacuum grease or glycerin to the edge of the area to be observed, and then place a rubber frame of appropriate size on it (to prevent the magnetic fluid from overflowing). The purpose of this step is to constrain the range of the magnetic fluid and prevent the magnetic fluid from overflowing due to uneven (excessive) magnetic field at the edge, which would disturb the magnetic domain pattern.

[0084] In some embodiments, the set gradient particle size includes 75 μm, 37 μm, 25 μm, and 19 μm.

[0085] Gradient particle sizes were selected from the intervals 15μm≤D1≤20μm, 25μm≤D2≤30μm, 35μm≤D3≤40μm, and 60μm≤D4≤80μm. Smaller and moderately sized particles (15μm≤D1≤20μm, 25μm≤D2≤30μm, 35μm≤D3≤40μm) have higher resolution for fine magnetic domain structures (such as domain walls). That is, small-sized particles have weaker coercivity and can be adsorbed onto the sample under a small magnetic field, and can preferentially adsorb in the center of the field of view. On the other hand, larger-sized particles (60μm≤D4≤80μm) can better aggregate in low-contrast areas (such as weak magnetic field areas), enhancing the visibility of magnetic domain boundaries. That is, larger-sized particles have stronger coercivity, require a larger magnetic field to adsorb onto the sample, are easy to adsorb at the edge position, and are more stable after the magnetic field is removed. Therefore, the combination of gradient particle sizes allows the magnetic fluid to cover a large-area macroscopic magnetic field while also capturing local microscopic magnetic domain features, avoiding the problems of insufficient adsorption efficiency due to excessively large particle sizes or insufficient adsorption efficiency due to excessively small particle sizes. For example, the gradient particle sizes can include 75 μm, 37 μm, 25 μm, and 19 μm. Furthermore, the mass ratio of the magnetic fluid with different particle sizes can be: D1 particle size magnetic fluid : D2 particle size magnetic fluid : D3 particle size magnetic fluid : D4 particle size magnetic fluid = 1:1:1:1.

[0086] In some embodiments, the magnetic fluid is obtained by surface modification with magnetic particles.

[0087] Magnetic particles can be coated with a layer on their surface by being modified with surfactants, which effectively inhibits particle aggregation and ensures stable dispersion of the colloid, thus facilitating the display of clear magnetic domain images by magnetohydrodynamics.

[0088] In some embodiments, the magnetic fluid is applied using a casting method.

[0089] The casting method is beneficial for the uniform adsorption of magnetic particles of different magnetic sizes, avoiding the disturbance of magnetic domain patterns caused by multiple coatings.

[0090] In some embodiments, during the application of the magnetic fluid, the temperature of the oriented silicon steel sheet is controlled between 20°C and 30°C.

[0091] The positive effects of controlling the temperature of oriented silicon steel sheets between 20℃ and 30℃ include: avoiding a decrease in the saturation magnetization of magnetic particles due to high temperatures, maintaining the stability of the colloid, and preventing particle agglomeration that reduces observation clarity. For example, the temperature of the oriented silicon steel sheets can be 20℃, 22℃, 24℃, 26℃, 28℃, 30℃, etc.

[0092] In some embodiments, the step of applying a magnetic fluid with a predetermined gradient particle size to the surface of the oriented silicon steel sheet in a single application to achieve magnetic domain imaging includes:

[0093] A magnetic fluid with a predetermined gradient particle size is applied to the surface of the oriented silicon steel sheet in a single application to achieve magnetic domain imaging; wherein,

[0094] The method for preparing the magnetic fluid includes:

[0095] Magnetic particles with the set gradient particle size are obtained;

[0096] The magnetic particles, dispersion, and activator are mixed for passivation treatment to obtain a magnetic fluid.

[0097] Passivation treatment can effectively improve the dispersion stability of magnetic fluids. During the preparation process, the activator (sodium benzenesulfonate) forms a protective film on the surface of the magnetic particles to prevent particle aggregation; the dispersion (ultrapure water) provides a uniform medium environment, allowing the particles to move freely and respond to the magnetic field.

[0098] The specific preparation method of the magnetic fluid includes: ① Weighing 0.4g of commercially available analytical grade iron oxide particles, containing 0.1g of 200-mesh (approximately 75μm) particles, 0.1g of 400-mesh (approximately 37μm) particles, 0.1g of 600-mesh (approximately 25μm) particles, and 0.1g of 800-mesh (approximately 19μm) particles, and dissolving them in a beaker containing 150ml of ultrapure water. ② Heating to 60±1℃ at a heating rate of 2℃ / min and maintaining this temperature for 15±0.5 minutes. During this process, maintaining a stirring speed of 800 rpm. ③ Adding 10ml of sodium benzenesulfonate activator with a concentration of 0.1±0.005mol / L dropwise, adjusting the stirring speed to 1000 rpm, and continuing to maintain the temperature for 30 minutes. ④ After preparation, place the reacted suspension above a NdFeB permanent magnet with a strength of 0.5T and let it stand for 15±1 minutes to allow the magnetic particles to settle completely. Discard the supernatant and add an equal volume (150ml) of ultrapure water. Repeat the magnetic separation operation three times. ⑤ Redisperse the purified magnetic particles in 150ml of ultrapure water and stir at 1000±50 rpm for 15±0.5 minutes at 60±1℃. Add 10ml of sodium benzenesulfonate aqueous solution with a concentration of 0.1±0.005mol / L dropwise again at a rate of 0.5ml / min and continue the reaction for 30±0.5 minutes.

[0099] In some embodiments, the method further includes:

[0100] The oriented silicon steel sheet is pretreated; wherein the pretreatment includes cutting, surface degreasing and drying in sequence.

[0101] Oriented silicon steel sheets undergo pretreatment to eliminate interference factors introduced by the material itself or the processing, optimize the surface and internal properties of the sample, and ensure the accuracy and reliability of subsequent magnetic domain observations and magnetic field experiments. Pretreatment of the oriented silicon steel sheets includes: ① Cutting samples by mechanical cutting or wire cutting; samples can be square, round, or other shapes. ② Surface degreasing treatment using ultrasonic cleaning (40kHz, 300W), followed by sequential passing through dilute sodium hydroxide solution, acetone, and ethanol for 15 minutes each, and then air-drying or oven-drying. ③ Fixing the sample on a horizontal sample stage. Figure 2 A schematic diagram of a magnetic domain imaging sample stage for a method of direct imaging of large-area magnetic domains provided in this application embodiment; please refer to... Figure 2 1. Bottom acrylic sheet, 2. Silicon steel sheet, 3. Top acrylic sheet. Figure 3 An exploded view of the magnetic domain imaging sample stage for a method of direct imaging of large-area magnetic domains provided in this application embodiment; please refer to... Figure 3 This indicates that the sample stage adopts a modular design, which allows for the rapid replacement of silicon steel sheets of different thicknesses or sizes according to experimental needs, significantly improving the versatility and scalability of the device, as well as the repeatability of the test results.

[0102] The method for direct imaging of large-area magnetic domains provided in this application has the following advantages:

[0103] 1. The permanent magnets are arranged in a straight line and the magnetic field is synchronously enhanced with the center of the coil, resulting in a large and uniform vertical magnetic field.

[0104] 2. The use of gradient particle size magnetic fluid improves the uniformity of magnetic domain adsorption and solves the problem of inconsistent magnetic domain image response time. This allows magnetic particles in the magnetic fluid to be stably arranged on a large sample surface, thereby obtaining a precise and clear magnetic domain distribution pattern; gradient particle size magnetic fluid displays high-resolution (50 micrometer) magnetic domains.

[0105] 3. It can prepare magnetic fluids with excellent dispersibility. Large-diameter magnetic particles exhibit strong coercivity and a long response time when adsorbed onto magnetic domain walls, thus easily adsorbing at the outermost, stronger magnetic fields to exhibit domain walls. Small-diameter magnetic particles have weak coercivity but strong saturation magnetochemicals, so they can be adsorbed at the center with a slightly weaker magnetic field, also exhibiting domain walls. Therefore, using gradient-size magnetic fluids further compensates for the potential problem of uneven magnetic particle adsorption.

[0106] 4. An enhanced magnetic field strength of 0.3mT to 0.4mT is sufficient to drive the directional movement of magnetohydrodynamic particles of different sizes. Large-diameter particles are rapidly positioned under a strong magnetic field, while small-diameter particles are stably adsorbed in weak gradient regions. Together, they construct a magnetic domain image over a large area. The size of the rectangular coil matches the large-area specifications of the silicon steel sheet, ensuring that the magnetic field coverage is consistent with the imaging area and avoiding insufficient magnetic field at the edges due to an undersized coil.

[0107] The present application is further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the application. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national standards. If there is no corresponding national standard, then generally accepted international standards, conventional conditions, or conditions recommended by the manufacturer are followed.

[0108] Example 1

[0109] A method for direct imaging of large-area magnetic domains includes:

[0110] Multiple permanent magnets are respectively arranged on both sides of the oriented silicon steel sheet. The multiple permanent magnets are arranged in a straight line with a first predetermined spacing, and the distance between each permanent magnet and the boundary of the oriented silicon steel sheet is a second predetermined spacing.

[0111] A coil is placed at the center of the grain-oriented silicon steel sheet so that the magnetic field of the coil is superimposed on the magnetic field of the permanent magnet to form an enhanced magnetic field;

[0112] Under the influence of an enhanced magnetic field, a magnetic fluid with a predetermined particle size gradient is uniformly coated onto the surface of an oriented silicon steel sheet in a single application to achieve direct imaging of magnetic domains; among which,

[0113] The first and second preset spacings are both 5cm, and the large-area magnetic domain morphology (30×20cm) of a 0.2mm thick 1.88T type oriented silicon steel sample is shown. Pretreatment of the oriented silicon steel sheet: ① Samples are cut into square shapes using mechanical or wire cutting methods. ② Surface degreasing treatment is performed using ultrasonic cleaning (40kHz, 300W power), followed by sequential passing through dilute sodium hydroxide solution, acetone, and ethanol for 15 minutes each, and then air-drying or oven-drying. ③ The sample is fixed on a horizontal sample stage.

[0114] Among them: when the material of the permanent magnet is NdFeB, the magnetic energy product of the permanent magnet is 40MGOe, the remanent magnetic induction Br is 1.2T, and the coercivity Hc is 1000kA / m;

[0115] The coil is a rectangular coil, measuring 80×280mm, with 10 turns. It is connected to a DC power supply box with a current of 0.2A. It uses electromagnetic induction to generate a magnetic field of approximately 0.2mT, with the N pole pointing upwards.

[0116] The strength of the enhanced magnetic field is 0.4 mT, and the direction of the magnetic field is from the N pole upward;

[0117] Under the influence of an enhanced magnetic field, a magnetic fluid with a predetermined gradient particle size is coated onto the surface of an oriented silicon steel sheet in a single application to achieve magnetic domain imaging. The predetermined gradient particle sizes include 75 μm, 37 μm, 25 μm, and 19 μm, and the coating amount is 0.5 mL / cm². 2 During the application of the magnetic fluid, the temperature of the oriented silicon steel sheet is controlled at 25℃.

[0118] After the magnetic fluid coating is left to stand for 5 minutes, images of the magnetic domains are taken.

[0119] Post-processing: The acquired images need to undergo noise reduction to enhance contrast and detail.

[0120] Example 2

[0121] A method for direct imaging of large-area magnetic domains includes:

[0122] Multiple permanent magnets are respectively arranged on both sides of the oriented silicon steel sheet. The multiple permanent magnets are arranged in a straight line with a first predetermined spacing, and the distance between each permanent magnet and the boundary of the oriented silicon steel sheet is a second predetermined spacing.

[0123] A coil is placed at the center of the grain-oriented silicon steel sheet so that the magnetic field of the coil is superimposed on the magnetic field of the permanent magnet to form an enhanced magnetic field;

[0124] Under the influence of an enhanced magnetic field, a magnetic fluid with a predetermined particle size gradient is uniformly coated onto the surface of an oriented silicon steel sheet in a single application to achieve direct imaging of magnetic domains; among which,

[0125] The first and second preset spacings are both 5cm, and the large-area magnetic domain morphology (30×20cm) of a 0.2mm thick 1.88T type oriented silicon steel sample is shown. Pretreatment of the oriented silicon steel sheet: ① Samples are cut into square shapes using mechanical or wire cutting methods. ② Surface degreasing treatment is performed using ultrasonic cleaning (40kHz, 300W power), followed by sequential passing through dilute sodium hydroxide solution, acetone, and ethanol for 15 minutes each, and then air-drying or oven-drying. ③ The sample is fixed on a horizontal sample stage.

[0126] Among them: when the material of the permanent magnet is ferrite, the magnetic energy product of the permanent magnet is 3.5MGOe, the remanent magnetic induction Br is 0.4T, and the coercivity Hc is 200kA / m;

[0127] The coil is a rectangular coil, measuring 80×280mm, with 10 turns. It is connected to a DC power supply box with a current of 0.2A. It uses electromagnetic induction to generate a magnetic field of approximately 0.2mT, with the N pole pointing upwards.

[0128] The strength of the enhanced magnetic field is 0.3 mT, and the direction of the magnetic field is from the N pole upward;

[0129] Under the influence of an enhanced magnetic field, a magnetic fluid with a predetermined gradient particle size is coated onto the surface of an oriented silicon steel sheet in a single application to achieve magnetic domain imaging. The predetermined gradient particle sizes include 75 μm, 37 μm, 25 μm, and 19 μm, and the coating amount is 0.5 mL / cm². 2 During the application of the magnetic fluid, the temperature of the oriented silicon steel sheet is controlled at 25℃.

[0130] After the magnetic fluid coating is left to stand for 5 minutes, images of the magnetic domains are taken.

[0131] Post-processing: The acquired images need to undergo noise reduction to enhance contrast and detail.

[0132] Comparative Example 1

[0133] Under the magnetic field of a permanent magnet, a 400-mesh (37 μm) magnetic fluid (prepared using the magnetic fluid preparation method of the present application embodiment) is used to achieve direct imaging of magnetic domains.

[0134] Test graph analysis: Figure 4 This is a schematic diagram showing the positions of the permanent magnet and coil provided in an embodiment of this application; please refer to [link / reference]. Figure 4 This indicates that the sample stage adopts a permanent magnet-electromagnetic coil composite magnetic field source design, which is beneficial for adjusting the required magnetic field for samples of different sizes.

[0135] Figure 5 The magnetic induction intensity distribution diagrams on the surface of the silicon steel sheet before (a) and after (b) coil compensation are provided for embodiments of this application; please refer to Figure 5 The diagram shows the magnetic field strength distribution before and after coil compensation as measured in the experiment. After coil compensation, the average magnetic field strength increased from 0.33 mT to 0.45 mT, and the magnetic field strength variance decreased from 0.0194 to 0.0088. The decrease in variance indicates that the magnetic field uniformity has been greatly improved. Figure 6 This is an observation result image of magnetic domain imaging for a method of direct imaging of large-area magnetic domains provided in Embodiment 1 of this application; please refer to... Figure 6 ,show Figure 6 The magnetic domain image is clearly displayed, showing a 180° domain structure, and grain boundaries can also be observed. Figure 7 The image shows the observation results of large-area central magnetic domain imaging (400-mesh magnetofluid under permanent magnet magnetic field) provided in Comparative Example 1 of this application. Figure 8 This image shows the observation results of magnetic domain imaging for a method of direct imaging of large-area magnetic domains provided in Embodiment 2 of this application. Please refer to... Figures 7-8 It can be seen Figure 7 The boundaries of magnetic domains are blurred and unclear; in some images, the morphology of magnetic domains cannot even be determined. Figure 8 The magnetic domains are clearly visible, with 180° domains, allowing for clear observation of domain orientation and measurement of domain width. Therefore, it enables fine observation of large-area magnetic domain morphology, increasing the observed domain area by 600 times compared to other methods, while also improving image response time and image accuracy.

[0136] One or more technical solutions in the embodiments of this application have at least the following technical effects or advantages:

[0137] 1. Ultra-large area imaging: Enables direct observation of large-area magnetic domains (≥600cm²) in a single imaging process. 2 The distribution and morphology of oriented silicon steel samples can be determined without the need for image stitching technology;

[0138] 2. High imaging uniformity and precision: It can effectively improve the uniformity of magnetic particle adsorption. ① Prerequisite: Ensure high uniformity of the magnetic field over a large area; ② Use of gradient particle size magnetofluid ensures uniform adsorption. Both of these factors further improve the imaging response time and magnetic domain precision, providing an efficient and reliable solution for magnetic domain research on large-size silicon steel materials.

[0139] 3. Improved imaging efficiency: Traditional magnetic domain observation methods require multiple image stitching steps to achieve large-area magnetic domain observation. This invention allows for a single coating of the magnetic fluid, enabling the simultaneous capture of the required magnetic domain area in a single operation. This makes the operation more convenient and suitable for large-scale experiments and industrial testing.

[0140] 4. Strong applicability: It can be interconnected and complementary with industrial magnetic property testing and sample defect investigation. For example, the magnetic properties of grain-oriented silicon steel are an average result of macroscopic grain orientation distribution. By testing the morphology of large-area magnetic domains, defects in the magnetic domains can be used to identify poorly oriented grains, thereby finding the reasons for the quality of magnetic properties.

[0141] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method of large area magnetic domain direct imaging, characterized by, The method comprises: a plurality of permanent magnets are arranged on both sides of the oriented silicon steel sheet, the plurality of permanent magnets are arranged in a straight line with a first set interval, and the distance between each permanent magnet and the boundary of the oriented silicon steel sheet is a second set interval; a coil is arranged at the center position of the oriented silicon steel sheet, so that the magnetic field of the coil and the magnetic field of the permanent magnet are superimposed to form an enhanced magnetic field; under the action of the enhanced magnetic field, a magnetic fluid with a set gradient particle size is uniformly coated on the surface of the oriented silicon steel sheet at one time to realize direct imaging of magnetic domains; The first set distance and the second set distance are both 4cm-6cm, the strength of the enhanced magnetic field is 0.3mT-0.4mT, the magnetic field direction is N pole upward, the set gradient particle sizes D1-D4 are respectively selected from intervals 15um≤D1≤20um, 25um≤D2≤30um, 35um≤D3≤40um and 60um≤D4≤80um, and the coating amount of the one-time uniform coating is 0.4mL / cm-0.6mL / cm. 2 2 ;​ The coil has at least one of the following properties: rectangular shape, size of 70mm×250mm to 90mm×300mm, number of turns of 8 to 12, and current of 0.2A to 1A.

2. The method of claim 1, wherein, The first set interval is 5cm; and / or, The second set interval is 5cm.

3. The method of claim 1, wherein, The material of the permanent magnet is selected from at least one of the following: NdFeB, SmCo, AlNiCo or ferrite; wherein: When the material of the permanent magnet is NdFeB, the magnetic energy product of the permanent magnet is 35MGOe-50MGOe, the residual magnetic induction Br is 1.2T-1.4T, and the coercive force Hc is 900kA / m-1200kA / m; When the material of the permanent magnet is SmCo, the magnetic energy product of the permanent magnet is 20MGOe-30MGOe, the residual magnetic induction Br is 1.0T-1.2T, and the coercive force Hc is 700kA / m-1000kA / m; When the material of the permanent magnet is AlNiCo, the magnetic energy product of the permanent magnet is 5MGOe-10MGOe, the residual magnetic induction Br is 0.8T-1.0T, and the coercive force Hc is 300kA / m-500kA / m; When the material of the permanent magnet is ferrite, the magnetic energy product of the permanent magnet is 3MGOe-6MGOe, the residual magnetic induction Br is 0.4T-0.6T, and the coercive force Hc is 200kA / m-300kA / m.

4. The method of claim 1, wherein, The N-pole of the permanent magnet is connected to the oriented silicon steel sheet, the thickness of the oriented silicon steel sheet is 0.2mm-0.35mm, and the arrangement direction of the permanent magnet is parallel to the rolling direction of the oriented silicon steel sheet.

5. The method of claim 1, wherein, The set gradient particle size includes 75μm, 37μm, 25μm and 19μm.

6. The method of claim 1, wherein, The magnetic fluid is obtained by surface modification of magnetic particles.

7. The method of claim 1, wherein, The coating of the magnetic fluid adopts a flow casting method.

8. The method of claim 1, wherein, During the coating of the magnetic fluid, the temperature of the oriented silicon steel sheet is controlled at 20℃-30℃.

9. The method of claim 1, wherein, The method further comprises: coating the magnetic fluid with a set gradient particle size on the surface of the oriented silicon steel sheet at one time to realize imaging of magnetic domains; wherein, The preparation method of the magnetic fluid comprises: obtaining magnetic particles with the set gradient particle size; mixing the magnetic particles, a dispersion liquid and an activator to perform passivation treatment to obtain the magnetic fluid. The method further comprises:

10. The method of claim 1, wherein, ​ The oriented silicon steel sheet is pretreated; wherein the pretreatment comprises cutting, surface degreasing treatment and drying in sequence. The oriented silicon steel sheet is pretreated; wherein the pretreatment comprises cutting, surface degreasing treatment and drying in sequence.

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