JP2852812B2 - Magnetic recording media - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a magnetic recording medium such as a magnetic tape, a magnetic sheet and a magnetic disk.

BACKGROUND ART A magnetic recording medium is usually prepared by adding ferromagnetic particles, a hardener, a dispersant, an abrasive, a lubricant, a matting agent, an antistatic agent, and other various additives to a binder, and further adding as needed. A magnetic paint to which a suitable solvent has been added is coated on a support and then dried.

Conventionally, in order to improve the chemical stability of these magnetic substances, nickel, cobalt, titanium, silicon, aluminum, etc. are added to this in a small amount in the form of a metal element, salt or oxide, or surface treatment is performed. It is known.

By the way, in the multilayer magnetic recording medium using a magnetic material surface-treated with silicon, aluminum, etc., since the same magnetic material is conventionally used for both the upper and lower layers, the upper and lower magnetic materials such as silicon, aluminum, etc. Were equal.

However, such a magnetic recording medium causes stains on a roll for transporting the magnetic recording medium (hereinafter, also simply referred to as a transport roll) in a process after a drying process at the time of manufacturing, and causes the magnetic recording medium at high temperature and high humidity. However, there is a disadvantage that the running property of the magnetic recording medium is poor.

An object of the present invention is to provide a magnetic recording medium that does not cause contamination on a roll that conveys a magnetic recording medium in a process subsequent to a drying process during manufacturing, and that has good running properties of the magnetic recording medium itself even at high temperature and high humidity. It is to provide a recording medium.

Another object of the present invention is to have electromagnetic conversion characteristics,
An object of the present invention is to provide a magnetic recording medium which has both low sliding noise and low dropout (D / O), and is particularly preferably used as a video tape.

DISCLOSURE OF THE INVENTION The magnetic recording material of the present invention has a first magnetic layer (lower layer) containing a magnetic material on a nonmagnetic support and a second magnetic layer (upper layer) thereon. In this magnetic recording medium, at least one of Si and Al is contained in 100 parts by weight of the magnetic layer of the first magnetic layer.
2 parts by weight, and the second magnetic layer contains B parts by weight of Si or Al contained in 100 parts by weight of the magnetic material, and is configured so that A <B. A or B is Si in each magnetic material
And the sum of parts by weight of Al.

The magnetic material used for each layer may include only Si or only Al, or may be a mixture thereof. Also Si
And Al.

It is preferable that A and B satisfy 0 <A <0.1 and 0.05 <B <2.0. B is preferably 0.1 or more, and particularly preferably 1.0 or less.

If A is more than 0.1, the running properties of the magnetic recording medium, particularly its tape, at the time of high temperature and high humidity are slightly deteriorated, and the tape edge tends to be damaged.

In addition, when B is 0.1 or less, dirt on the transport roll during the drying step and dirt on the heat roll during the calendering step are likely to occur.

On the other hand, when B is 2 or more, the transport roll and the heat roll are easily soiled. In a magnetic recording medium in which A is smaller than B, in order to improve the dispersion state of the ferromagnetic powder in the magnetic layer and to obtain excellent electromagnetic conversion characteristics and running properties, the magnetic material of the second magnetic layer is made of Si as 0.3%. -1.0wt%, 0.05 as Al
It preferably contains -0.3 wt% Si or Al.

The magnetic material used in the magnetic recording medium of the present invention includes, for example, γ-Fe ₂ O ₃ , Co-containing γ-Fe ₂ O ₃ or Co-coated γ.
_{-Fe 2 O Co-γ-Fe} 2 O 3 , such as _3, Fe ₃ O _4, CO-containing Fe ₃ O ₄ or Co-gamma-Fe as Co-coated _{Fe 3 O 4 3 O 4;} Cro 2 Oxide magnetic materials such as Fe; Ni, Fe-Ni alloy, Fe-Co
Alloy, Fe-Ni-P alloy, Fe-Ni-Co alloy, Fe-Mn-Zn alloy, Fe-Ni-Zn alloy, Fe-Co-Ni-Cr alloy, Fe-Co-Ni
Various ferromagnetic materials such as a metal magnetic material containing Fe, Ni, and Co as main components, such as a -P alloy, a Co-P alloy, and a Co-Cr alloy.

Of the above, a Co-containing oxide magnetic material is preferably used.

These magnetic materials contain Si or Al as necessary.

The magnetic material containing Si and Al according to the present invention is, for example, using an iron oxide magnetic material as a starting material, for example, mixing and heating a needle-like magnetic iron oxide and an Si or Al salt in an alkaline aqueous solution,
This is obtained by washing and drying, and then heat-treated in a non-reducing atmosphere such as air or N ₂ gas.

It is presumed that Si or Al exists on the surface of the completed magnetic material, and thus the above-mentioned preparation method is called surface treatment of the magnetic material.

The magnetic material used for the lower layer does not necessarily need to contain Si or Al. The difference between B and A is preferably 0.05 to 2.0.

The magnetic material used in the magnetic layer is a particle having an average major axis length of 0.35 μm or less and an axial ratio of 8 to 11 in the first magnetic layer,
In the second magnetic layer, the average major axis length is 0.25 μm or less, and the axial ratio is 7 mm.
To 8 is preferable. If the average major axis length is too small, it will be easily broken as needle-like particles, so it is preferably 0.1 μm or more.

Preferable examples of the binder used in the present invention include polyurethane having wear resistance. It has strong adhesive strength to other substances, is mechanically tough against repeated stress or bending, and has good wear resistance and weather resistance.

Also, if a cellulose resin and a vinyl chloride copolymer are used in combination with the polyurethane, the dispersibility of the magnetic powder in the magnetic layer is improved, and the mechanical strength is increased. However, the layer becomes too hard with only the cellulose resin and the vinyl chloride copolymer, but this can be prevented by the presence of the above-mentioned polyurethane.

Cellulose ether,
Cellulose inorganic acid esters and cellulose organic acid esters can be used. The above-mentioned polyurethane and vinyl chloride copolymer may be partially hydrolyzed. As the vinyl chloride copolymer, preferably, a copolymer containing vinyl chloride-vinyl acetate or vinyl chloride-vinyl acetate-
Copolymers containing vinyl alcohol are exemplified.

Phenoxy resins can also be used. Phenoxy resin has high mechanical strength, excellent dimensional stability,
It has advantages such as good heat resistance, water resistance, chemical resistance and good adhesiveness. These advantages are complementary to the above-described polyurethane, and can significantly improve the stability over time of the physical properties of the magnetic recording medium.

Further, in addition to the binder described above, a mixture with a thermoplastic resin, a thermosetting resin, a reactive resin, and an electron beam irradiation-curable resin may be used.

As the binder used in the present invention, a resin modified by introducing a functional group or a functional group forming an internal salt from the viewpoint of dispersion of magnetic particles and other fillers, particularly a modified vinyl chloride resin, a modified polyurethane resin Or a modified polyester resin is preferred.

As the functional group in the resins, for example, -SO
₃ M, -OSO ₂ M, -COOM and And the like.

Wherein M is a hydrogen atom, one of lithium and sodium, M ¹ and M ² each represent a hydrogen atom, it is either lithium, potassium, sodium atom or an alkyl group. Also the M ¹ and M ^2, may be different from each other, it may be the same.

These functional groups are, for example, a resin such as a vinyl chloride resin, a polyester resin, and a polyurethane resin. M, M ¹ and M ² have the same meanings as defined above. For example, it can be obtained by condensing a compound containing a negative functional group and chlorine in the molecule by a dehydrochlorination reaction.

Among the resins thus obtained, a resin obtained by introducing a negative functional group into a vinyl chloride resin or a polyurethane resin is preferable.

In the magnetic recording medium of the present invention, it is particularly preferable that the second magnetic layer contains a resin having a functional group as a binder.

Examples of the resin containing a functional group to be used include those described above. Among them, a resin obtained by introducing a negative functional group into a vinyl chloride resin or a polyurethane resin is preferable.
Preferred examples thereof include potassium sulfone-containing vinyl chloride resins and sodium sulfonate-containing polyurethane resins.

The amount of the resin containing the functional group in the second magnetic layer is preferably 1 to 20 parts by weight based on 100 parts by weight of the magnetic substance.

As long as the effects of the present invention are not impaired, the second magnetic layer may contain an optional binder together with the resin containing the above functional group.

Note that a resin containing a functional group used for the second magnetic layer may be used for the first magnetic layer.

In this type of magnetic recording medium, carbon black is usually used as a component for a light shielding agent or an antistatic agent.

In the present invention, carbon black is used for the above-mentioned purpose. In the present invention, it is preferable that the second magnetic layer contains carbon black having an average particle diameter of 50 μm or more to improve sliding noise. It is valid.

 The upper limit of the average particle size is preferably about 100 μm.

The addition amount is preferably 0.1 to 1.5 parts by weight based on 100 parts by weight of the magnetic powder.

The average particle size of the carbon black contained in the first magnetic layer is preferably 5 to 25 mμm, and the amount added is
The amount is preferably 2 to 15 parts by weight based on 00 parts by weight.

The thickness of the second magnetic layer of the magnetic recording medium of the present invention is 1.0 μm
It is preferable to set the following. The lower limit is preferably 0.1 μm or more. The thickness of the first magnetic layer is preferably from 1.0 to 3.8 μm.

In order to improve the durability of the magnetic layer of the magnetic tape of the present invention, various hardeners, for example, isocyanate, can be contained in the magnetic paint.

Examples of the aromatic isocyanate include tolylene diisocyanate (TDI) and the adduct thereof with an isocyanate-active hydrogen compound, and those having an average molecular weight of 100 to 3,000 are preferable.

Examples of the aliphatic isocyanate include hexamethylene diisocyanate (HMDI) and the like, and adducts of these isocyanates and active hydrogen compounds. Among these, those having a molecular weight of 100 to 3,000 are preferred. Among the aliphatic isocyanates, non-alicyclic isocyanates and adducts of these compounds with active hydrogen compounds are preferred.

Generally, a dispersant is used in the magnetic paint used to form the magnetic layer, and additives such as a lubricant, an abrasive, a matting agent, and an antistatic agent may be contained as necessary.

Examples of the dispersant used in the present invention include phosphate esters,
Amine compounds, alkyl sulfates, fatty acid amides,
There are higher alcohols, polyethylene oxide, sulfosuccinic acid, sulfosuccinate, other surfactants, and the like, and salts thereof, and use a salt of a polymer dispersant having a negative organic group such as -COOH. You can also.
These dispersants may be used alone or in combination of two or more. In some cases, it is not necessary to use a dispersant as in the examples described later, and this is one of the most preferable embodiments.

Examples of the lubricant include silicone oil, graphite, carbon black graft polymer, molybdenum disulfide, tungsten disulfide, lauric acid, myristic acid, a monobasic fatty acid having 12 to 16 carbon atoms and the number of carbon atoms of the fatty acid. Fatty acid esters composed of monohydric alcohols having a total of 21 to 23 carbon atoms can be used.

These lubricants are used in an amount of 0.05 to 100 parts by weight of the binder.
To 10 parts by weight.

In the present invention, preferred as the lubricant are stearic acid, myristic acid, palmitic acid and oleic acid. The lubricant is preferably added in an amount of 0.5 to 5.0 parts by weight for the upper layer and 0.5 to 8.0 parts by weight for the lower layer based on 100 parts by weight of the magnetic powder.

Examples of the abrasive include commonly used materials such as fused alumina, various aluminas such as α-alumina, silicon carbide, chromium oxide, corundum, artificial corundum, artificial diamond, garnet, emery (main components: corundum and magnetite)
Etc. are used. These abrasives have an average particle diameter of 0.05 to 5 μm, and are particularly preferably 0.1 to 5 μm.
1 to 2 μm. These abrasives are added in an amount of 1 to 20 parts by weight based on 100 parts by weight of the binder.

As the matting agent, an organic powder or an inorganic powder is used individually or as a mixture.

Examples of preferred organic powders include acrylic styrene resin, benzoguanamine resin powder, melamine resin powder, and phthalocyanine pigment. In addition, polyolefin-based resin powder, polyester-based resin powder, polyamide-based resin powder, polyimide-based resin powder, polyfluoroethylene resin powder, and the like can also be used.

As inorganic powder, silicon oxide, titanium oxide, aluminum oxide, calcium carbonate, barium sulfate, zinc oxide,
Examples include tin oxide, aluminum oxide, chromium oxide, silicon carbide, calcium carbide, α-Fe ₂ O ₃ , talc, kaolin, calcium sulfate, boron nitride, zinc fluoride, and molybdenum dioxide.

Examples of antistatic agents include conductive powders such as carbon black, graphite, tin oxide-antimony oxide compounds, titanium oxide-tin oxide-antimony oxide compounds; natural surfactants such as saponin; alkylene oxides; glycerin And glycidol-based nonionic surfactants; higher alkylamines, quaternary ammonium salts, pyridine, other heterocycles, cationic surfactants such as phosphonium or sulfoniums; carboxylic acid, sulfonic acid, phosphoric acid, sulfuric acid Anionic surfactants containing an acid group such as an ester group or a phosphate group; and both surfactants such as amino acids, aminosulfonic acids, and sulfuric acid or phosphate esters of amino alcohol.

Examples of the solvent to be blended in the paint or a diluting solvent at the time of applying the paint include ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone;
Alcohols such as methanol, ethanol, propanol, and butanol; esters such as methyl acetate, ethyl acetate, butyl acetate, ethyl lactate, and ethylene glycol monoacetate; ethers such as glycol dimethyl ether, glycol monoethyl ether, dioxane, and tetrahydrofuran; , Toluene and xylene; and halogenated hydrocarbons such as methylene chloride, ethylene chloride, carbon tetrachloride, chloroform and dichlorobenzene.

Examples of the support include polyethylene terephthalate, polyesters such as polyethylene-2, 6-naphthalate, polyolefins such as polypropylene, cellulose triacetate, cellulose derivatives such as cellulose diacetate, polyamides, and plastics such as polycarbonate. Metals such as Cu, Al, and Zu, glass, boron nitride, and ceramics such as Si carbide can also be used.

The thickness of these supports is about 3 to 100 μm, preferably 5 to 50 μm in the case of a film or a sheet, about 30 μm to 10 mm in the case of a disc or a card, and is cylindrical in the case of a drum. The type is determined depending on the recorder used.

An intermediate layer for improving adhesion may be provided between the support and the magnetic layer.

Further, another magnetic layer or a non-magnetic layer may be provided above, below, or between the first magnetic layer and the second magnetic layer.

Coating methods for forming the magnetic layer on the support include air doctor coating, blade coating, air knife coating, squeeze coating, impregnation coating, reverse roll coating, transfer roll coating, gravure coating, kiss coating, cast coating, and spray coating. A coat, an extrusion coat and the like can be used, but not limited thereto.

When a magnetic layer is formed on a support by these coating methods, a coating and drying step is stacked one by one, that is, a so-called wet-on-dry coating method, and the next layer is simultaneously formed on a layer in a wet state that has not been dried. Alternatively, there is a so-called wet-on-wet method in which coating is performed successively, and any method can be used for manufacturing the magnetic recording medium of the present invention.

According to such a method, the magnetic layer applied on the support is subjected to a treatment for orienting the ferromagnetic metal oxide powder in the layer if necessary, and then the formed magnetic layer is dried. In this case, the orientation magnetic field is about 500-5000 gauss in AC or DC, the drying temperature is about 50-120 ° C. for baking, and the drying time is about 0.01-10 minutes for baking. Also, if necessary, subjected to a surface smoothing treatment, or cut into a desired shape,
The magnetic recording medium of the present invention is manufactured.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, examples of the best mode for carrying out the present invention will be described.

It has excellent electromagnetic conversion characteristics and low both sliding noise and dropout (D / O).
Particularly, a magnetic recording medium suitably used as a video tape is described.

This magnetic recording medium has a first magnetic layer (lower layer) and a second magnetic layer (upper layer) on a nonmagnetic support, and the magnetic material in the second magnetic layer is surface-treated with Al and / or Si. 2
The binder contained in the magnetic layer is a resin containing a functional group, the second magnetic layer contains carbon black having an average particle size of 50 μm or more, and the thickness of the second magnetic layer is 1.0 μm or more. .

 The effect is shown below.

After well kneading the magnetic paints for the first magnetic layer and the second magnetic layer having the compositions shown in Tables I and II, a polyisocyanate [Coronate L, trade name, manufactured by Nippon Polyurethane Co., Ltd.] was used as a curing agent. ] Was added by weight to prepare a magnetic coating material for application.

Subsequently, the magnetic coating material for the first magnetic layer and the magnetic coating material for the second magnetic layer are applied on the support in a multilayer manner according to a conventional method.
A magnetic recording medium was manufactured by performing a magnetic field orientation treatment and a drying treatment.

The electromagnetic conversion characteristics and other characteristics of the magnetic recording medium thus obtained were measured.

Table III shows the processing conditions of the surface treatment agent such as Si and Al of the magnetic material used for the second magnetic layer and other manufacturing conditions, and Table IV shows the measurement results of the characteristics of the obtained magnetic recording medium.

As is clear from the results in Table VI, Samples 1 to 4
Chroma output, Y-CN and other electromagnetic conversion characteristics are excellent, sliding noise and dropout are small, and each characteristic maintains a high level in a well-balanced manner. On the other hand, samples 5 to 9
For example, even if the electromagnetic conversion characteristics and the dropout are good, the sliding noise after 10 passes is large (sample 6), and the electromagnetic conversion characteristics and the sliding noise are relatively good. Many dropouts (Sample 6)
In any case, it cannot be said that each of the above characteristics maintains a high level in a well-balanced manner.

Example After kneading each of the magnetic paints having the structures shown in Tables 1 and 2 to disperse each component well, the viscosity of the paint is adjusted to 1 to 20 centipoise with a solvent (methyl ethyl ketone, toluene). . Thereafter, 5 parts by weight of polyisocyanate was added to prepare a coating magnetic coating material of Example 1.

Hereinafter, except that the magnetic powder and the lubricant used for each of the first magnetic layer and the second magnetic layer are different from each other, the composition for the upper layer and the lower layer is exactly the same as that of the magnetic paint described in Tables 1 and 2. After preparing each magnetic paint (the magnetic paint used in each example of Example 2 and Comparative Examples 1 to 5), each magnetic paint for the lower layer and the magnetic paint for the upper layer in each example were coated in a multilayer manner on a support according to a conventional method. And dried to produce a magnetic recording tape.

The thicknesses of the first magnetic layer and the second magnetic layer are 0.5 μm and 3.0 μm, respectively.
μm.

Next, the characteristics of the obtained magnetic recording tape were measured. The amounts of the surface treating agent and the lubricant used in the first and second magnetic layers in each example are shown in Tables 3-4, and the properties of the obtained tapes are shown in Tables 5 and 6.

Next, in Example 1, the polyurethane of the second magnetic layer was replaced with a sodium sulfonate-containing polyurethane resin,
Example 5 was a magnetic sample in which the average particle diameter of carbon black was changed to 65 μm, and Example 6 was a magnetic sample in which the average particle diameter of carbon black in Example 5 was changed to 65 μm and the second magnetic layer was changed to 0.6 μm. Table 7 shows the results of chroma output, Y-CN, sliding noise, and dropout.

In both Examples 5 and 6, the transport roll stain, calender roll stain, RF output reduction, tape edge damage, and high-temperature dynamic friction coefficient in the drying process were almost the same as those in Example 1.

 The measurement method in the examples and comparative examples will be described.

RF output decrease Tape is packed in a VHS cassette, and NV at 40 ° C and 80% RH
Using a -6200 (made by Matsushita Electric) deck, we ran 50 passes repeatedly. Then, the RF output fluctuation was measured.

Tape edge damage The tape after running 50 passes was visually evaluated.

Dynamic friction coefficient μk 23 ° C, 50% RH, Yokohama System Co., Ltd.
T-300-D) and set the inlet tension (T ₁ )
A sample tape was wrapped around a stainless steel pin having a diameter of 3.8 mm at 180 ° C. and allowed to run at 3.3 cm / sec. After one minute, the outlet tension (T ₀ ) was measured and determined by the following equation.

μk = ln (T ₀ / T ₁ ) / π Chroma output 500 KHz, the output during reproduction of an RF signal was determined.

Y-CN (1) Fast-forward until the winding thickness of the magnetic recording tape on the winding side becomes about 3 mm, and record 6 MHz for 10 minutes from here. This is played three times. The recording level is + 20% of the optimum recording level of the comparison tape.

(2) In the part where 6MHZ was recorded / reproduced in (1) above,
Record the 6.8 MHz signal for approximately 3 minutes each. The recording level is + 20% of the optimum recording level of each comparative tape. RF
The reproduction output (referred to as C) and C / N are compared with the value of the comparison tape.

Sliding noise (1) Perform playback without running the tape, and measure the system noise with a spectrum analyzer.

(2) Regenerate the sample tape for 1 minute and measure the sliding noise with a spectrum analyzer (1 pass).

(3) Play the sample tape 10 times for 1 minute,
Measure the sliding noise with a spectrum analyzer (10 passes).

(4) The noise level around 9 MHz is read with reference to the system noise (OdB), and the noise value of each of 1-pass and 10-pass is read.

* All measurements are performed at room temperature of 20 ± 2 ° C and relative humidity of 10 ± 2% with the top cover of the VTR removed.

Fixed section on dropout magnetic recording medium (10m-30
m), a 100% white signal is input and played back.
The dropout is measured at 10 dB at a level of 14 dB in units of 1 minute using a counter (model VHO1BZ) of Shibasoku Co., Ltd., and the average value of the entire length of the measured portion is used as the measured value.

Industrial Applicability The magnetic recording medium of the present invention, during its manufacturing process,
In particular, in the process after the drying process, the transport rolls and the like are not stained, and once the product has been commercialized, even when the tape is run under severe conditions such as high temperature and high humidity, the conventional method is used. It does not cause any serious driving failure.

It is useful as a magnetic recording material, especially as a video tape, and is easy to manufacture.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Shigehisa Kawabe 1 Konica Corporation, Sakura-cho, Hino-shi, Tokyo (72) Inventor Takeshi Nakajima 1-knee Sakuracho, Hino-shi, Tokyo In Konica Corporation (72) Inventor Takeshi Nakajima 1 Sakura-cho, Hino-shi, Tokyo Examiner Takuya Hayakawa (56) References JP-A-2-227820 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G11B 5 / 716 G11B 5/706

Claims (13)

(57) [Claims] 1. A first magnetic layer having a first magnetic layer on a nonmagnetic support and a second magnetic layer on the first magnetic layer, wherein the first magnetic layer contains at least one kind of Si or Al in 100 parts by weight of a magnetic material and A parts by weight. Containing, second
A magnetic recording medium wherein the magnetic layer contains at least one kind of Si or Al in B parts by weight in 100 parts by weight of the magnetic material, and A is smaller than B. 2. The value of A and B is 0 ≦ A <0.1, respectively.
2. The magnetic recording medium according to claim 1, wherein 0.1 <B <2.0. 3. The magnetic recording medium according to claim 2, wherein 0.1 <B ≦ 1.0. 4. The magnetic material of the second magnetic layer is 0.3 to 1.0% by weight of Si and 0.05 to 0.3% by weight of Al with respect to the magnetic material.
2. The magnetic recording medium according to claim 1, containing i or Al. 5. The magnetic recording medium according to claim 1, comprising at least one of a vinyl chloride resin containing a negative functional group and a polyurethane resin. 6. The magnetic recording medium according to claim 1, wherein the second magnetic layer contains at least one of a vinyl chloride resin containing a negative functional group and a polyurethane resin. 7. The magnetic recording medium according to claim 5, wherein the negative functional group is a sulfonate. 8. The magnetic recording medium according to claim 1, wherein the first magnetic layer and the second magnetic layer contain a lubricant. 9. The binder contained in the second magnetic layer is a resin containing a functional group, and the second magnetic layer has an average particle diameter of 40 μm.
The above-mentioned carbon black is contained, and the thickness of the second magnetic layer is
2. The magnetic recording medium according to claim 1, which has a thickness of 1.0 μm or less. 10. The magnetic recording medium according to claim 9, wherein the resin having a functional group is a vinyl chloride-based resin or a polyurethane resin. 11. The magnetic recording medium according to claim 10, wherein the functional group is a sulfonate. 12. The magnetic recording medium according to claim 9, wherein the particle size of the carbon black is 50 mμ or more. 13. The first magnetic layer has a thickness of 1.0 μm to 3.8 μm.
2. The magnetic recording medium according to claim 1, wherein the thickness of the second magnetic layer is 0.1 μm or 1.0 μm.