KR20130043410A - Magnetic particle and the fabrication method thereof - Google Patents

Abstract

PURPOSE: A magnetic particle and a manufacturing method thereof are provided to reduce yellowing by preventing damage to a silver screen when the magnetic particle forms a protecting layer for manufacturing ink. CONSTITUTION: A shell is formed at the outside of a magnetic core(100). A protecting layer(300) is formed at the outside of the shell. The shell includes a metallic material or a dielectric material. The shell is formed in a single layer or in multiple layers. The shell includes a first shell(210) formed with a dielectric material and a second shell(220) formed with a metallic material.

Description

Magnetic Particles and the Fabrication Method Thereof

The present invention relates to a magnetic particle and a method of manufacturing the same, and more particularly, to a magnetic particle having improved physical properties by forming a protective film on the outer surface of a shell layer formed on the outer surface of a magnetic core, and a method of manufacturing the same.

The main application fields of magnetic powder are information such as permanent magnet whose main purpose is to provide magnetic flux, various magnetic sensors that infer the change of external physical quantity by reading the characteristic change of magnetic body according to the change of external magnetic field, and hard disk And a storage device for storing the same, and in the field of anti-counterfeiting, it is generally included in a product for reading a change in characteristics of a magnetic material. However, since the general magnetic powder has a color of dark gray or dark brown, its application is limited in applications requiring bright colors, and there is a disadvantage in that the anti-counterfeiting effect is insufficient only by magnetic properties. Therefore, in order to solve this problem, bright magnetic materials using core-shell technology have been developed. Through this, it is used for various purposes such as color inks, general paints, automotive powder pigments, cosmetic pigments, catalyst paints, and anti-counterfeiting inks by giving the magnetic particles the characteristics of vivid color and high brightness.

Therefore, there has been a market demand for a magnetic material having a variety of specialized colors used for such various uses, and various studies are being conducted on techniques for concealing dark colors of the magnetic material.

As a conventional technique of concealing the dark color of the magnetic material, a white powder and a manufacturing method thereof (Korean Patent Laid-Open Publication No. 10-2006-0028393) provide a method of manufacturing a bright magnetic body using a titanium oxide film and a silver film.

However, in the case of manufacturing as described above, a problem occurs that scratches occur in the silver film in the ink-forming step, and the lightness is significantly reduced. This scratch phenomenon is due to the ductility that is inherent to the silver film. In order to be used in color inks, general paints, automotive powder pigments, cosmetic pigments, catalyst paints, and anti-counterfeiting inks, the pale magnetic material must be subjected to a fleshy or dispersion process. Therefore, it is impossible to apply the product because the light color deterioration inevitably occurs with the existing structure. 1 is a photograph showing damage of a silver film generated during an ink process of a conventional pale magnetic material, FIG. 1 (a) shows a pale color magnetic material before an ink process, and FIG. 1 (b) shows a process of hardening during an ink process. The photograph shows that the silver film was damaged by friction with the filler and pressure between the rolls. As described above, when the silver film of the pale color magnetic material is damaged, there is a problem that the reflectance is lowered and the light color effect is reduced.

The present invention has been made to solve the above problems, to prevent damage to the silver film during the ink process. That is, in order to prevent damage due to friction with fillers generated during the inkling process and damage caused by pressure between the rolls, a protective layer is formed on the outermost layer of the pale magnetic material to reduce the pale color effect. This is to improve. In addition, as the thickness of the protective layer increases, the light color decreases greatly, and therefore, an optimal material and thickness for securing scratch resistance, chemical resistance, and light resistance are set.

Magnetic particles according to the invention is characterized in that it comprises a magnetic core, a shell formed on the outer shell of the magnetic core, and a protective film formed on the shell outer shell. The shell comprises a metal or dielectric material, and is characterized in that it is single layer or multilayer, and the shell may comprise a first shell of dielectric material and a second shell of metal. In addition, the dielectric material is characterized in that at least one selected from the group consisting of titanium dioxide, silicon dioxide, alumina, calcium carbonate, zirconium oxide, magnesium fluoride, zinc oxide and zinc sulfide, the metal is copper, nickel, gold, platinum At least one selected from the group consisting of silver, aluminum, and chromium, wherein the protective film is at least one selected from the group consisting of titanium dioxide, silicon dioxide, alumina, calcium carbonate, zirconium oxide, magnesium fluoride, zinc oxide, and zinc sulfide. It features. The thickness of the silicon dioxide of the protective film material is 5 ~ 160 nm, the thickness of the titanium dioxide is characterized in that 5 ~ 80 nm.

Method for producing a magnetic particle according to the present invention, forming a first shell layer comprising a dielectric material on a magnetic core, forming a second shell layer comprising a metal on the first shell layer, and the second shell layer Forming a protective film on the outer shell is characterized in that it comprises. In forming the first shell layer, the dielectric material is at least one selected from the group consisting of titanium dioxide, silicon dioxide, alumina, calcium carbonate, zirconium oxide, magnesium fluoride, zinc oxide, and zinc sulfide, and the second shell layer is formed. The metal is characterized in that at least one selected from copper, nickel, gold, platinum, silver, aluminum and chromium, the protective film is titanium dioxide, silicon dioxide, alumina, calcium carbonate, zirconium oxide, magnesium fluoride, oxide At least one selected from the group consisting of zinc and zinc sulfide. The thickness of the silicon dioxide of the protective film material is 5 ~ 160 nm, the thickness of the titanium dioxide is characterized in that 5 ~ 80 nm.

Magnetic particles according to the present invention may be contained in the security ink of the oil price certificate.

The magnetic particles according to the present invention form a protective film and prevent damage of the silver film in the process of inkization, thereby preventing the lowering of the brightness of the bile magnetic material. That is, the protective film improves abrasion resistance and chemical resistance of the magnetic particles, thereby bringing about a reduction in the reflectance, and also improves light resistance, thereby alleviating yellowing.

1 is a photograph showing the damage of the silver film generated during the ink process of the existing pale color magnetic material,
2 is a cross-sectional view showing an example of the magnetic particles according to the present invention,
3 is a cross-sectional view showing another example of the magnetic particles according to the present invention;
4 is a cross-sectional view showing another example of the magnetic particles according to the present invention;
5 is a sectional view showing another example of the magnetic particles according to the present invention;
6 is a cross-sectional view showing another example of the magnetic particles according to the present invention;
7 is a view showing another manufacturing method of magnetic particles according to the present invention.
8 is a photograph showing that the protective layer formed on the magnetic particles according to the present invention,
9 is a graph showing reflectances before and after ink softening;
10 is a graph showing reflectances before and after chemical resistance experiments.

Hereinafter, the magnetic particles and the manufacturing method of the present invention will be described in detail with reference to the accompanying drawings. The drawings introduced below are provided by way of example so that the spirit of the invention to those skilled in the art can fully convey. Therefore, the present invention is not limited to the following drawings, but may be embodied in other forms, and the following drawings may be exaggerated in order to clarify the spirit of the present invention. Also, throughout the specification, like reference numerals designate like elements.

Hereinafter, the technical and scientific terms used herein will be understood by those skilled in the art without departing from the scope of the present invention. Descriptions of known functions and configurations that may be unnecessarily blurred are omitted.

In the accompanying drawings, the magnetic particles are shown as being spherical, but the shape of the magnetic particles is not limited to the spherical shape, of course, also includes a plate-shaped case.

FIG. 2 illustrates an example of magnetic particles according to the present invention. As shown in FIG. 2, the magnetic particles according to the present invention include a magnetic core 100 and a shell 200 formed on an outer surface of the magnetic core. In addition, the shell 200 may include a passivation layer 300 formed on an outer surface thereof.

The magnetic core 100 imparts magnetic properties to the magnetic particles according to the present invention.

Meanwhile, the shell 200 includes a metal or a dielectric, which may be a single layer or a multilayer and may be made of the same or different materials.

As an example, FIG. 3 illustrates a case where the shell is made of a metal material, and FIG. 4 illustrates a case where the shell is made of a dielectric material and a metal material.

The shell 220 of metal material in FIG. 3 is a single layer (a, 220 in FIG. 3), two layers (b, 221, 222 in FIG. 3) or three layers (c, 221, 222 and 223 in FIG. 3). ) Or more.

 Based on FIG. 3, when the shell 200 is a shell 220 made of a metal material, the magnetic particles I according to the present invention will be described in detail.

As shown in FIG. 3, the magnetic particles I include a shell 220 made of a metal material; Magnetic core 100; And a protective film 300 formed on an outer surface of the sphere of the shell 220 made of a metal material.

In the magnetic particles (I) according to the present invention, the magnetic core 100 is positioned to impart magnetic properties to the magnetic particles (I).

In detail, the magnetic core 100 of the magnetic particles (I) is a ferromagnetic particle, the magnetic core 100 is iron; nickel; cobalt; Iron oxide; Nickel oxide; Cobalt oxide; And at least one selected from the group consisting of a multicomponent material comprising at least two elements selected from iron, nickel, and cobalt.

The magnetic core 100 may adjust its size in consideration of the field in which the magnetic particles (I) are utilized. When the magnetic ink (I) is contained in the security ink, the size of the magnetic core 100 is preferably 0.5 to 50㎛.

The shell 220 made of the metal material increases the brightness of the magnetic particles (I). The metal material included in the shell 220 of the metal material is preferably at least one selected from copper, nickel, gold, platinum, silver, aluminum, and chromium, and at least one from copper, nickel, silver, and chromium. More preferably, it is the selected material.

In this case, the shell 220 made of the metal material may be a single film composed of a single metal sphere, or may be a multilayer film in which films of different metals are stacked.

In detail, when the structure of the shell 220 made of the metal material is a single film, the single film is preferably one or more materials selected from copper, nickel, silver, and chromium, and the shell 210 made of the metal material. The thickness of is preferably 10 to 500nm in terms of increasing the brightness of the magnetic particles (I).

In detail, the spherical film of the shell 220 made of the metal material includes a laminated film in which two or more metal films, which are different metals, are stacked, and as shown in FIGS. 3 (b) to 3 (c), the first metal The laminated film in which the film 221 and the second metal film 222 are laminated, or as shown in FIG. 3C, the first metal film 221, the second metal film 222, and the third metal film 223 includes a laminated film.

The metal of the first metal film 221 to the third metal film 223 is preferably at least one material selected from copper, nickel, gold, platinum, silver, aluminum and chromium.

The passivation layer 300 protects the shell 220 made of a metal material to prevent a decrease in reflectance. In particular, by forming the protective film 300, to prevent damage to the shell 220 due to friction between the filler and the pressure between the rolls in the grinding process during the ink process, the wear resistance, chemical resistance, light resistance of the magnetic particles (I) To improve. Protective As the material, one or more selected from the group consisting of titanium dioxide, silicon dioxide, alumina, calcium carbonate, zirconium oxide, magnesium fluoride, zinc oxide and zinc sulfide may be used. When silicon dioxide is used as a material of a protective film, it is preferable that the thickness is 5-160 nm, and it is more preferable that thickness is 10-50 nm. In addition, when titanium dioxide is used as a material of a protective film, it is preferable that the thickness is 5-80 nm, and it is more preferable that thickness is 10-30 nm. desirable. And the reflectance to fall is thicker than the thickness range of the protective film, the protective film thinner than a thickness in the range of the scratch resistance, chemical resistance and light resistance away, can drop the reduction preventing effect of reflectance by the protective film.

Based on FIG. 4, when the shell 200 is a shell 210 made of a dielectric material and a shell 220 made of a metal material, the magnetic particles II according to the present invention will be described in detail.

As shown in Figure 4, the magnetic particles (II) is a magnetic core (100); A shell 210 made of a dielectric material formed on the outer surface of the magnetic core 100; A shell 220 made of a metal material formed on an outer surface of the shell 210 made of the dielectric material 210; And a protective film 300 formed on the outer surface of the shell 220 made of a metal material.

In the magnetic particles (II) according to the embodiment of the present invention, the magnetic core 100 is positioned at the center to impart magnetic properties to the magnetic particles (II).

Specifically, the magnetic core 100 of the magnetic particles (II) is a ferromagnetic particle, similar to the magnetic core 100 of the magnetic particles (I), the magnetic core 100 is iron; nickel; cobalt; Iron oxide; Nickel oxide; Cobalt oxide; And at least one selected from the group consisting of a multicomponent material comprising at least two elements selected from iron, nickel, and cobalt.

The magnetic core 100 is adjusted in consideration of the field in which the magnetic particles (II) are utilized. When the magnetic ink (II) is contained in the security ink, the size of the magnetic core 100 is preferably 0.5 to 50㎛.

The second shell 220 made of a metal material is formed on the outer surface of the shell 210 made of a dielectric material (hereinafter referred to as a first shell), and the protective film 300 is formed on the outer surface of the second shell 220 made of the metal material. It is formed to prevent a decrease in reflectance of the magnetic particles (II).

The second shell 220 made of a metal material is preferably a material selected from at least one selected from copper, nickel, gold, platinum, silver, aluminum, and chromium, similarly to the shell made of the metal material of the magnetic particles (I). More preferably at least one selected from copper, silver and chromium.

The thickness of the second shell 220 is preferably 40 to 150nm. The second shell 220 increases the brightness of the magnetic particles (II).

In this case, as shown in FIG. 5, the second shell 220 made of the metal material may be a single film made of a single metal film, and a multilayer film in which films 221 to 222 of different metals are stacked. Can be.

In detail, the second shell 220 may include a laminated film in which two or more metal films, which are different metals, are stacked. 5 illustrates an example in which the second shell 220 is formed by stacking two layers of metal films (the first metal shell film 221 and the second metal shell film 222), but the present invention. It is not limited to the number of this laminated metal film. At this time, as shown in FIG. 5, the thickness of the laminated film in which the first metal shell film 221 and the second metal shell film 222 are stacked (that is, the total thickness of the metal shell) is preferably 40 to 150 nm, similar to the metal shell of the single film. .

The protective film 300 formed on the outer shell of the second shell 220 prevents damage to the shell 220 due to friction with the filler during the inkling process and pressure between the rolls, thereby preventing the magnetic particles II. To improve wear resistance, chemical resistance and light resistance. The dielectric material of the first shell 210 is preferably at least one selected from the group consisting of titanium dioxide, silicon dioxide, alumina, calcium carbonate, zirconium oxide, magnesium fluoride, zinc oxide, and zinc sulfide. 210) The thickness of the spheres is preferably 10 to 500 nm.

In this case, as shown in FIG. 6, the structure of the first shell 210 includes a laminated film in which two or more inorganic films having different refractive indices are stacked. 6 illustrates three dielectric films (first dielectric film 211, second dielectric film 212, and third dielectric film) in which the structure of the first shell 210 has three different refractive indices. (213) is an example showing the case of lamination, but the present invention is not limited to the number of the laminated dielectric films.

Similar to the protective film 300 of the magnetic particles I, the protective film 300 protects the shell 220 made of a metal material to prevent a decrease in reflectance. In particular, by forming the protective film 300, to prevent damage to the shell 220 due to friction with the filler during the inkling process and pressure between the rolls, the wear resistance, chemical resistance, light resistance of the magnetic particles (II) To improve. Protective As the material, one or more selected from the group consisting of titanium dioxide, silicon dioxide, alumina, calcium carbonate, zirconium oxide, magnesium fluoride, zinc oxide and zinc sulfide may be used. When silicon dioxide is used as a material of a protective film, it is preferable that the thickness is 5-160 nm, and it is more preferable that thickness is 10-50 nm. In addition, when titanium dioxide is used as a material of a protective film, it is preferable that the thickness is 5-80 nm, and it is more preferable that thickness is 10-30 nm. desirable. And the reflectance to fall is thicker than the thickness range of the protective film, the protective film thinner than a thickness in the range of the scratch resistance, chemical resistance and light resistance away, can drop the reduction preventing effect of reflectance by the protective film.

The manufacturing method of the magnetic particles (II) will be described in detail with reference to FIG. 7.

As shown in FIG. 7, the magnetic particles (II) may include forming a first shell layer including a dielectric material on an outer surface of the magnetic core (S21); Forming a second shell layer including a metal at an outer surface of the first shell layer (S22); And forming a protective film on the outer shell of the second shell layer (S23).

The forming of the first shell layer including the dielectric material and the second shell layer including the metal in the magnetic core may include impregnating the magnetic core in the metal precursor solution, and then introducing a reducing agent to form a metal film in the magnetic core, or the magnetic body. After mixing and stirring the core and the metal particles to physically form a metal film on the magnetic core, it is preferable to prepare an inorganic-magnetic core composite by oxidizing the formed metal film.

In a specific embodiment, the thickness of the shell layer may be controlled according to the reaction temperature in forming the shell of the dielectric material. When the dielectric material is titanium dioxide, the reaction temperature may be about 20 to 85 ° C.

8 is a photograph showing a protective layer formed on the outer surface of the silver film to prevent damage to the silver film. FIG. 8 (a) shows that a silicon dioxide protective layer is included, and FIG. 8 (b) is a photograph showing that a titanium dioxide protective layer is included. When such a protective layer is formed, it is possible to prevent damage to the silver film due to friction with the filler in the grinding process during the inkification process, and to prevent the silver film damage due to the pressure between the rolls.

The following is described by way of example for the detailed description of the invention, the invention is not limited to the following examples.

<On the outer shell of the magnetic core Titanium dioxide layer  Formation>

120 g of magnetic particles and 24 ml of distilled water were added to 3.6 L of ethanol, and then ultrasonic waves were dispersed. Mix 38 ml of tetrabuthoxy titanium (TBOT) and 300 ml of ethanol and add slowly for 30 minutes. Stir at a rotational speed of 300 rpm for 3 hours at room temperature. The core particle powder was separated by a magnet, washed twice with ethanol and dried.

<This Titanium oxide layer  Forming silver film on the formed magnetic core>

A reducing solution was prepared by dissolving 85 g of glucose and 5 g of potassium tartrate in 800 ml of distilled water. 12 g of sodium hydroxide (NaOH), 100 ml of ammonium hydroxide (NH ₄ OH), and 55 g of silver nitrate (AgNO ₃ ) were dissolved in 800 ml of distilled water to prepare a colorless transparent silver rock complex solution.

80 g of magnetic body particles having a titanium dioxide layer prepared in the above process was added to 2.4 L of distilled water, and the colorless transparent silver rock adsorbent solution was mixed and irradiated with ultrasonic waves. After stirring at a rotational speed of 300 rpm for 30 minutes at room temperature, the reducing solution was mixed, and ultrasonic coating and stirring were continued for 20 minutes to coat the silver film. Separated by magnet, washed twice with distilled water, and dried.

< At the outermost level  Silicon dioxide protective layer formed on magnetic particles with silver film>

120 g of magnetic particles having an outermost silver film, 30 ml of distilled water, and 150 ml of NH ₄ OH were added to 3.6 L of ethanol, and ultrasonic waves were dispersed for 3 minutes. Mix 40 ml of Tetraethly orthosilicate (TEOS) and 300 ml of ethanol and slowly add for 30 minutes. Stir at a rotational speed of 300 rpm for 3 hours at room temperature. Magnetic particle powder was separated by a magnet, washed twice with ethanol and dried.

< At the outermost level  Titanium dioxide protective layer formed on magnetic particles with silver film>

120 g of magnetic particles having 24 mm of distilled water and a distilled water were added to 3.6 L of ethanol at the outermost side, and ultrasonic waves were dispersed for 3 minutes. 38 ml of TBOT (Tetrabuthoxy titanium) and 300 ml of ethanol are mixed slowly for 30 minutes. Stir at a rotational speed of 300 rpm for 3 hours at room temperature. Magnetic particle powder was separated by a magnet, washed twice with ethanol and dried.

Experimental Example

[Abrasion Resistance Improvement Measurement Experiment]

The improvement in the wear resistance of the magnetic particles obtained according to the present invention was found by measuring the reflectance before and after in the ink grinding process. Ink prepared using magnetic particles was applied to the paper using an applicator and dried for at least 48 hours, and then the reflectance was measured by using a reflectometer (Varian, Cary 5000). FIG. 9 is a graph showing the reflectance at 555 nm before and after ink softening. Referring to FIG. 9, it can be seen that the reflectance of the magnetic particles prepared according to the present invention is reduced in a small width as compared with the case without the protective layer. have. In particular, the change in the reflectance before and after the ink softening shows that the case where the protective layer is titanium dioxide reduces the reflectance to a lesser extent than the case where the protective layer is silicon dioxide. That is, the magnetic particles formed on the outermost surface of the magnetic particles are damaged during the ink softening process, and the reflectance is rapidly decreased. However, the magnetic particles having the outermost protective layer formed on the outermost surface exhibit a gentle decrease in reflectance.

[Chemical Resistance Improvement Measurement Experiment]

It was found out by measuring the reflectance that the chemical resistance of the magnetic particles obtained according to the present invention is improved. Ink prepared using magnetic particles was applied to the paper using an applicator and dried for at least 48 hours. After measuring the reflectance using a reflectometer (Varian, Cary 5000), the chemical resistance test was performed, and the reflectance was measured again. . Chemical resistance experiments were conducted at 23 ° C. for 30 minutes using a 5% sodium hypochlorite solution. The magnetic particles having the outermost silver film had a reduced reflectivity due to damage of the silver film in sodium hypochlorite solution, whereas the magnetic particles according to the present invention had a protective layer formed at the outermost part, so that the reflectance was less reduced. FIG. 10 is a graph showing reflectance at 555 nm before and after chemical test. Referring to FIG. 10, the magnetic particles having the protective layer according to the present invention have a reflectance higher than that without the protective layer before chemical test. It is lower, but after the chemical test, the reflectance is superior to that without the protective layer, and as a result, the brightness is improved.

[Light resistance improvement measurement experiment]

The improvement in the light resistance of the magnetic particles obtained according to the present invention was examined by observing the occurrence and extent of yellowing. Ink prepared using magnetic particles was applied to the paper using an applicator and dried for at least 48 hours. After measuring the reflectance using a reflectometer (Varian, Cary 5000), the light resistance test was performed, and the reflectance was measured again. The light resistance test was conducted for 100 hours using a light resistance tester (Atlas, Ci4000 Xenon Weather-Ometer). Inks made using magnetic particles with a silver film without a protective layer at the outermost part showed yellowing as a result of light resistance test, while inks made with magnetic particles with outermost protective layers were alleviated yellowing phenomenon as a result of light resistance test. It became.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, Those skilled in the art will recognize that many modifications and variations are possible in light of the above teachings.

Accordingly, the spirit of the present invention should not be construed as being limited to the embodiments described, and all of the equivalents or equivalents of the claims, as well as the following claims, belong to the scope of the present invention .

Description of the Related Art [0002]
100: magnetic core 200: shell
300: shield
210: shell (first shell) 211 to 213 made of a dielectric material: first to third dielectric films
220: shell made of a metal material (second shell) 221 to 223: first to third metal films

Claims (15)

Magnetic core;
A shell formed on an outer shell of the magnetic core; And
Magnetic particles, characterized in that it comprises a protective film formed on the shell shell. The method of claim 1,
The shell comprises a metal or dielectric material, characterized in that the magnetic particle is a single layer or a multilayer. 3. The method of claim 2,
Wherein said shell comprises a first shell of dielectric material and a second shell of metal. 3. The method of claim 2,
The dielectric material is magnetic particles, characterized in that at least one selected from the group consisting of titanium dioxide, silicon dioxide, alumina, calcium carbonate, zirconium oxide, magnesium fluoride, zinc oxide and zinc sulfide. 3. The method according to claim 1 or 2,
The metal is magnetic particles, characterized in that at least one selected from copper, nickel, gold, platinum, silver, aluminum and chromium. 3. The method according to claim 1 or 2,
The protective film is magnetic particles, characterized in that at least one selected from the group consisting of titanium dioxide, silicon dioxide, alumina, calcium carbonate, zirconium oxide, magnesium fluoride, zinc oxide and zinc sulfide. The method according to claim 6,
Magnetic particles, characterized in that the thickness of the silicon dioxide is 5 ~ 160 nm. The method according to claim 6,
Magnetic particles, characterized in that the thickness of the titanium dioxide is 5 ~ 80 nm. Forming a first shell layer comprising a dielectric material on an outer surface of the magnetic core;
Forming a second shell layer including a metal on the outer shell of the first shell layer; And
Forming a protective film on the outer shell of the second shell layer characterized in that it comprises a magnetic particle. The method of claim 9,
The dielectric material in forming the first shell layer is at least one selected from the group consisting of titanium dioxide, silicon dioxide, alumina, calcium carbonate, zirconium oxide, magnesium fluoride, zinc oxide and zinc sulfide. . The method of claim 9,
In forming the second shell layer, the metal is a method of producing magnetic particles, characterized in that at least one selected from copper, nickel, gold, platinum, silver, aluminum and chromium. The method of claim 9,
The protective film is at least one selected from the group consisting of titanium dioxide, silicon dioxide, alumina, calcium carbonate, zirconium oxide, magnesium fluoride, zinc oxide and zinc sulfide. 13. The method of claim 12,
Magnetic particles, characterized in that the thickness of the silicon dioxide is 5 ~ 160 nm. 13. The method of claim 12,
Magnetic particles, characterized in that the thickness of the titanium dioxide is 5 ~ 80 nm. The security ink of the oil price certificate containing the magnetic particle of any one of Claims 1-4.

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