WO2009142263A1

WO2009142263A1 - Magnetic recording medium

Info

Publication number: WO2009142263A1
Application number: PCT/JP2009/059351
Authority: WO
Inventors: 幹雄岸本; 俊明泰井; 健一郎吉田
Original assignee: 日立マクセル株式会社
Priority date: 2008-05-23
Filing date: 2009-05-21
Publication date: 2009-11-26
Also published as: JP2011181116A

Abstract

Disclosed is a magnetic recording medium comprising a non-magnetic support and a magnetic layer which is provided on the non-magnetic support and contains a magnetic powder and a binder. In the recording medium, the magnetic powder is a plate-like hexagonal ferrite magnetic powder having a plate ratio in the range of 1 to 2, an average particle size in the range of 10 to 20 nm, a coercivity in the range of 79.6 to 318.4 kA/m (1000 to 4000 oersteds), and a saturation magnetization in the range of 20 to 60 Am²/kg (20 to 60 emu/g). The magnetic powder can prevent stacking aggregation of a plate-like magnetic powder, whereby noise can be reduced and a large SNR can be obtained.

Description

Magnetic recording medium

The present invention relates to a magnetic recording medium, and more particularly, to a magnetic recording medium suitable for ultrahigh density recording such as a digital video tape and a backup tape for a computer using hexagonal ferrite powder as a magnetic powder.

A coating type magnetic recording medium having a magnetic layer containing a magnetic powder and a binder on a non-magnetic support requires a further improvement in recording density as the recording / reproducing method shifts from an analog method to a digital method. Yes. In particular, the demand for video tapes for high recording density and backup tapes for computers is increasing year by year.

In order to cope with the short wavelength recording indispensable for improving the recording density, it is effective to reduce the thickness of the magnetic layer to 200 nm or less, particularly 100 nm or less in order to reduce the thickness loss during recording. As a reproducing magnetic head used in such a high recording density medium, an MR head capable of obtaining a high output is generally used, but it is considered that a GMR head with higher sensitivity will be used in the future.

In addition, to reduce noise, magnetic powder particles have been made fine so far, and at present, acicular metal magnetic powder having a particle diameter of about 45 nm has been put into practical use. Furthermore, in order to prevent a decrease in output due to demagnetization during short wavelength recording, the coercive force has been increased conventionally, and the coercive force of about 238.9 kA / m (3,000 Oersted) has been improved by improving the iron-cobalt alloy. Magnetic force is realized (Japanese Patent Laid-Open Nos. 3-49026, 10-83906, and 10-34085).

However, in a magnetic recording medium using acicular magnetic particles, since the coercive force depends on the shape magnetic anisotropy based on the acicular shape of the particles, it is difficult to further reduce the particle size from the above particle diameter. ing.
That is, the acicular metal magnetic powder exhibits coercive force due to the shape magnetic anisotropy due to the acicular shape, but the acicular ratio (particle length / width) inevitably decreases as the particle size decreases. Thus, the coercive force is reduced. This reduction in coercive force becomes a fatal problem in improving the recording density. As described above, the acicular metal magnetic powder has an essential problem that the coercive force decreases as the particle size is reduced, and there is a limit to the particle size reduction.

Therefore, a hexagonal ferrite magnetic powder having a plate shape and having an easy axis of magnetization in a direction perpendicular to the plate surface has been proposed as a magnetic powder that is completely different from the needle-shaped magnetic powder (Japanese Patent Laid-Open No. 6-290924). JP, JP-A-2005-340690 and JP-A-2002-298331).
This plate-shaped hexagonal ferrite magnetic powder has a coercive force based on crystal magnetic anisotropy, so that it can maintain a high coercive force even when it becomes a fine particle. As a result, a high noise-to-power ratio (SNR) is obtained, indicating that the magnetic powder is suitable for high-density recording media.

The hexagonal ferrite magnetic powder disclosed in JP-A-6-290924, JP-A-2005-340690, and JP-A-2002-298331 has a high coercive force even though it is a fine particle. It has excellent characteristics as a magnetic powder. On the other hand, recently, the sensitivity of the reproducing head has been remarkably increased, and a high reproduction output can be obtained relatively easily, but at the same time, the noise output also increases, resulting in a problem that a high SNR cannot be obtained. In order to prevent this increase in noise, it is essential to make the magnetic powder fine particles. The hexagonal ferrite magnetic powder has a small calculation volume due to the fine particles, but the particles have a plate-like shape, so that the particles are easily laminated and aggregated. As a result, the volume of the particles Low noise commensurate with is not realized.

The magnetic cohesive force due to the lamination of these plate-like particles is extremely strong, and it is difficult to disintegrate in the dispersion process. Therefore, even though the particles themselves are small, a sufficiently low noise has not been realized. is there.

An object of the present invention is to obtain a magnetic recording medium suitable for high-density recording that achieves a high SNR by realizing the inherent low noise of fine-grained hexagonal ferrite magnetic powder.

In order to achieve the above object, the present invention provides a magnetic recording medium comprising a nonmagnetic support, and a magnetic layer containing a magnetic powder and a binder formed on the nonmagnetic support. Plate ratio in the range of 1 to 2, preferably 1.1 to 1.9, average particle size in the range of 10 to 20 nm, preferably in the range of 12 to 18 nm, 79.6 to 318.4 kA / m ( 1,000 to 4,000 oersted), preferably in the range of 95.5 to 302.4 kA / m (1,200 to 3,800 oersted), 20 to 60 Am ² / kg (20 to 60 emu / kg). A magnetic recording medium is provided which is a plate-shaped hexagonal ferrite magnetic powder having a saturation magnetization in the range of g), preferably 25 to 58 Am ² / kg (25 to 58 emu / g).
The remarkable feature of the present invention is that the conventional plate-shaped hexagonal ferrite magnetic powder has a high plate-like ratio, whereas the plate-like ratio of the hexagonal ferrite magnetic powder used is 1-2. It is small.

According to the present invention, by using a hexagonal ferrite magnetic powder having a plate ratio of 1 to 2 as the magnetic powder, the laminated aggregation of magnetic powders, which has been a drawback of the plate hexagonal ferrite magnetic powder, can be achieved. When the magnetic recording medium is applied to a system using a high-sensitivity head such as a GMR head, the inherent low noise of the hexagonal ferrite magnetic powder can be realized.

The magnetic layer of the magnetic recording medium according to the present invention comprises, as a magnetic powder, a plate ratio in the range of 1 to 2, an average particle size in the range of 10 to 20 nm, 79.6 to 318.4 kA / m (1,000 to By using a plate-shaped hexagonal ferrite magnetic powder having a coercive force in the range of 4,000 Oersted) and a saturation magnetization in the range of 20 to 60 Am ² / kg (20 to 60 emu / g), the magnetic powder When this magnetic recording medium is applied to a system that realizes inherently low noise and uses a high-sensitivity head such as a GMR head, a high SNR can be obtained.

Here, the plate-like ratio means a value (length / thickness) obtained by dividing the maximum length of the plate-like particles in the plane direction by the thickness. The average particle size is a value obtained by actually measuring the particle size of a photograph (magnification of 100,000 times) taken with a transmission electron microscope for 300 particles and averaging the measured values.

When the plate ratio of the hexagonal ferrite magnetic powder is larger than 2, the magnetic powders can be easily laminated with each other, and as a result, the laminated and agglomerated magnetic powder behaves as one magnetic powder, resulting in high noise. . Further, the magnetic powder having a plate ratio of 1 includes not only a hexagonal shape but also a spherical or cubic shape.

If the average particle size of the hexagonal ferrite magnetic powder is smaller than 10 nm, it becomes difficult to uniformly disperse the magnetic powder in the magnetic layer, and the noise reduction effect becomes small. If the average particle size of the hexagonal ferrite magnetic powder exceeds 20 nm, the noise becomes high because the particle size of one magnetic powder itself is too large even if it can be uniformly dispersed.

As the hexagonal ferrite magnetic powder used in the present invention, at least one hexagonal ferrite magnetic powder selected from the group consisting of barium ferrite magnetic powder and strontium ferrite magnetic powder is preferable, and barium ferrite magnetic powder is particularly preferable. Since these hexagonal ferrite magnetic powders have large magnetic anisotropy, they have a feature that can suppress a decrease in output particularly in a short wavelength region. The hexagonal ferrite-based magnetic powder includes Al, Si, S, Sc, Ti, V, Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ta, in addition to the predetermined elements. Elements such as W, Re, Au, Bi, La, Ce, Pr, Nd, P, Co, Mn, Zn, Ni, B, Ge, and Nb may be included. The addition of these elements is necessary to control the particle size and magnetic properties of the hexagonal ferrite magnetic powder.

Next, a method for producing the hexagonal ferrite magnetic powder of the present invention will be described. The production method is not particularly limited, and a conventionally known method for producing a hexagonal ferrite magnetic powder can be used. An example of such a manufacturing method is implemented as follows. For example, an alkaline aqueous solution is added to an aqueous solution of a metal salt containing one or both of barium salt and strontium salt, which are constituent elements of hexagonal ferrite magnetic powder, and an iron salt to form a coprecipitate. Next, the coprecipitate is hydrothermally treated to synthesize a precursor of hexagonal ferrite magnetic powder. As barium salts, strontium salts, and iron salts, chlorides, sulfates, nitrates, and carbonates of these metals are preferably used.

In the above production method, the particle size can be arbitrarily controlled simultaneously with the coercive force by adding an appropriate amount of metal ions such as cobalt, titanium, nickel, zinc and manganese together with the above metal salt. In particular, in order to obtain a hexagonal ferrite magnetic powder having a small particle size, it is preferable to add these metal ions.

As the alkali, any alkaline compound can be used, but sodium hydroxide is usually used. The amount of the alkaline compound is preferably an equimolar amount or more with respect to the metal salt to be added, and the excess alkali concentration is preferably 0.01 mol / L or more. This alkali concentration is particularly important for obtaining a hexagonal ferrite magnetic powder having a small plate ratio. If the alkali concentration is too high, the plate ratio tends to increase. On the other hand, when the alkali concentration is low, those having a small plate ratio are likely to be produced, but the crystallinity is low and the magnetic properties tend to be lowered. In particular, when the amount is less than an equimolar amount with respect to the metal salt, other than hexagonal ferrite magnetic powder is likely to be generated.

Therefore, in order to obtain a target hexagonal ferrite magnetic powder having a plate ratio of 1 to 2, a precursor of a hexagonal ferrite magnetic powder is first synthesized at a relatively low alkali concentration, followed by hydrothermal treatment. It is effective to produce a magnetic powder having desired magnetic properties by increasing the crystallinity while maintaining the particle shape by adjusting the conditions.

Hydrothermal treatment is usually performed using an autoclave. The heat treatment in the autoclave is preferably performed at 200 to 350 ° C. for 1 to 6 hours. If the hydrothermal treatment temperature is too low, a precursor of a hexagonal ferrite magnetic powder having a desired shape cannot be obtained. If the hydrothermal treatment temperature is too high, no particular problem occurs, but the energy efficiency is deteriorated. If the hydrothermal treatment time is too short, a precursor of hexagonal ferrite magnetic powder having a desired shape cannot be obtained. If the hydrothermal treatment time is too long, no particular problem occurs, but the energy efficiency deteriorates.

The thus-prepared hexagonal ferrite magnetic powder precursor is then heat-treated with a flux at a temperature equal to or higher than the melting point of the flux. Thereby, crystals of hexagonal ferrite magnetic powder grow in the melted flux. The flux is a base material for crystal growth of the hexagonal ferrite magnetic powder, and at the same time prevents sintering of the hexagonal ferrite magnetic powder. As a result, a hexagonal ferrite magnetic powder having the desired particle shape and magnetic properties and a narrow particle size distribution can be obtained.

As the flux used, those which melt at 500 to 1,000 ° C. and do not dissolve in hexagonal ferrite particles are preferably used. When the melting temperature is lower than this, the heat treatment of the hexagonal ferrite particles becomes insufficient, so that the crystallinity of the hexagonal ferrite particles can be sufficiently improved and the magnetic properties cannot be improved. On the other hand, when a flux having a high melting temperature is used, crystal growth of hexagonal ferrite particles in the flux becomes excessive, and the particles tend to be coarse. Further, when a flux that is solid-solved with the six-strength crystal ferrite particles is used, the saturation magnetization amount of the obtained magnetic powder tends to be lowered. Preferred examples of such fluxing agents include sulfates of sodium, potassium and lithium, chlorides, bromides and iodides, boric acid and the like, and especially NaCl, KCl and KBr are well soluble in water. After the heat treatment, washing with water is preferable because these fluxes can be easily removed and do not remain as impurities in the magnetic powder.

The heat treatment using a flux is preferably performed at a temperature in the range of 750 to 900 ° C. for 1 to 4 hours. If the treatment temperature is too low or the treatment time is too short, the heat treatment is insufficient, the crystallinity of the hexagonal ferrite magnetic powder is not sufficiently improved, and the magnetic properties are not sufficiently improved. On the other hand, if the treatment temperature is too high or the treatment time is too long, the flux will adhere to the particle surface and tend to lower the magnetic properties, particularly the saturation magnetization.

In another method for producing a hexagonal ferrite magnetic powder, the above-described hexagonal ferrite constituent element metal salt, a flux, and a mixture of a mixture are rapidly cooled to suppress the growth of hexagonal ferrite particles. The rapidly cooled product is heat-treated at 400 to 700 ° C. to grow crystals into hexagonal ferrite particles in an appropriate size in the flux, and then the hexagonal ferrite particles are obtained by dissolving and removing the flux. You can also. By such a method, a tabular hexagonal ferrite magnetic powder having an average particle size of 10 to 20 nm can be obtained.

The magnetic recording medium of the present invention will be described below.

As the nonmagnetic support used in the magnetic recording medium of the present invention, any conventionally used nonmagnetic support for magnetic recording media can be used. For example, the thickness composed of polyesters such as polyethylene terephthalate and polyethylene naphthalate, polyolefins, cellulose triacetate, polycarbonate, polyamide, polyimide, polyamideimide, polysulfone, aramid, aromatic polyamide, etc. is usually 2 to 15 μm, especially 2 to 7 μm. The plastic film is preferably used.

The thickness of the magnetic layer is preferably 300 nm or less in order to solve the decrease in output due to demagnetization, which is an essential problem in longitudinal recording. When the magnetic layer thickness is 300 nm or more, the reproduction output decreases due to the thickness loss, or Mr · t, which is the product of the residual magnetization (Mr) and the magnetic layer thickness (t), becomes too large. When a GMR head is used as the head, reproduction output distortion is likely to occur due to reproduction output saturation. If the magnetic layer thickness is less than 10 nm, it is difficult to obtain a uniform magnetic layer.

Regarding the magnetic properties, for example, when a longitudinally oriented medium is used, the longitudinal coercive force (Hc) is in the range of 119.4 to 358.2 kA / m (1,500 to 4,500 oersted), preferably 95. In the range of 5 to 318.4 kA / m (1,200 to 4,000 oersted), the squareness ratio (Br / Bm) in the range of 0.65 to 0.92, preferably in the range of 0.62 to 0.90, Mr · t, which is the product of remanent magnetization (Mr) and magnetic layer thickness (t), is in the range of 0.1 to 2.0 memu / cm ² , preferably in the range of 0.12 to 1.98 memu / cm ^2. Thus, it is preferable to adjust the magnetic layer thickness and the filling degree. When the vertical alignment medium is used, the coercive force (Hc) in the vertical direction is in the range of 79.6 to 318.4 kA / m (1,000 to 4,000 Oersted), preferably 95.5 to 302.5 kA / m (1,200 to 3,800 oersted), squareness ratio (Br / Bm) in the range of 0.60 to 0.85, preferably in the range of 0.62 to 0.83, and Mr · t of 0.1. It is preferable to adjust the thickness of the magnetic layer and the degree of filling so that it is in the range of 05 to 1.5 memu / cm ² , preferably in the range of 0.06 to 1.4 memu / cm ² . Further, when the non-oriented medium is used, the longitudinal coercive force (Hc) is in the range of 79.6 to 318.4 kA / m (1,000 to 4,000 Oersted), preferably 95.5 to 302.5 kA. / M (1,200 to 3,800 oersted), squareness ratio (Br / Bm) in the range of 0.40 to 0.65, preferably in the range of 0.42 to 0.63, and Mr · t is 0. .08 ~ 1.8memu / cm ^2, preferably in the range to be in the range of 0.09 ~ 1.7memu / cm ^2, it is preferable to adjust the magnetic layer thickness and degree of filling.

When the coercive force is too low, output is likely to decrease due to demagnetization in the short wavelength region, and when the coercive force is too high, recording with a magnetic head becomes difficult, so the above coercive force range is preferable. If Mr · t is smaller than the above range, the reproduction output is small even when a high-sensitivity head such as a GMR head is used. If Mr · t is larger than the above range, a high-sensitivity head such as a GMR head is used. In some cases, the output is saturated and distorted easily.

In any orientation of the magnetic recording medium, by using the hexagonal ferrite magnetic powder having a low plate ratio of 1 to 2 according to the present invention, the inherent low noise of the magnetic powder can be realized, As a result, a high SNR can be obtained.

The average surface roughness Ra of the magnetic layer is preferably in the range of 1.0 to 3.2 nm. When Ra is within this range, contact with the head is improved, and high SNR is obtained. If Ra is smaller than this range, the slidability tends to decrease due to sticking of the head or the like, and if it exceeds this range, the contact of the head becomes poor and the output tends to decrease.

The magnetic layer preferably contains an additive such as carbon black for improving conductivity and surface lubricity and alumina for improving polishing. Conventionally known carbon black and alumina can be used.

In the magnetic recording medium of the present invention, the undercoat layer is not essential, but is preferably provided between the nonmagnetic support and the magnetic layer for the purpose of improving durability. The thickness of the undercoat layer is preferably 0.1 to 3.0 μm. If the thickness of the undercoat layer is less than 0.1 μm, the durability of the magnetic tape may deteriorate, and if it exceeds 3.0 μm, not only the effect of improving the durability of the magnetic tape is saturated but also the entire tape As the thickness increases, the tape length per roll becomes shorter and the storage capacity per roll becomes smaller.

The inorganic particles included in the undercoat layer are not particularly limited. For example, when nonmagnetic iron oxide is used, acicular iron oxide preferably has an average length of 50 to 200 nm, and granular or amorphous iron oxide preferably has an average particle size of 5 to 200 nm. Have.

When the magnetic layer is vertically oriented and used as a perpendicular recording medium, it is preferable that the undercoat layer contains magnetic particles. In this case, the type of magnetic particles is not particularly limited, and magnetic iron oxide, magnetic metal, or magnetic alloy can be used, but the magnetic flux from the magnetic layer can be closed by the undercoat layer to generate a strong magnetic flux only from the surface. For this purpose, it is preferable that the magnetic particles used in the undercoat layer have as small a coercive force as possible and a large saturation magnetization.

When used as a perpendicular recording medium, an intermediate layer may be further formed between the magnetic layer of the undercoat layer and the upper magnetic layer for recording signals. This intermediate layer is effective for controlling the magnetic interaction between the upper layer and the undercoat layer and more effectively utilizing the perpendicular magnetization component.

The binder used in the undercoat layer and the magnetic layer is not particularly limited, and those used in conventional magnetic recording media can be used. Examples of binders are polyurethane resin, vinyl chloride resin, vinyl chloride-vinyl acetate copolymer resin, vinyl chloride-vinyl alcohol copolymer resin, vinyl chloride-vinyl acetate-vinyl alcohol copolymer resin, vinyl chloride-vinyl acetate- It is a combination with at least one selected from vinyl chloride resins such as maleic anhydride copolymer resin, vinyl chloride-hydroxyl group-containing alkyl acrylate copolymer resin, nitrocellulose, epoxy resin and the like. In particular, it is preferable to use a vinyl chloride resin and a polyurethane resin in combination.

It is desirable to use in combination with a binder a thermosetting crosslinking agent that is bonded to a functional group contained in the binder to crosslink. Preferred examples of the crosslinking agent include isocyanates such as tolylene diisocyanate, hexamethylene diisocyanate and isophorone diisocyanate, reaction products of these isocyanates with polyols such as trimethylolpropane, and polyisocyanates such as condensation products of the above isocyanates. is there.

As the lubricant to be included in the magnetic layer and the undercoat layer, any known fatty acid, fatty acid ester, fatty acid amide and the like are used. Among these, it is particularly preferable to use a fatty acid having 10 or more carbon atoms, preferably 12 to 30 carbon atoms, and a fatty acid ester having a melting point of 35 ° C. or lower, preferably 10 ° C. or lower.

The backcoat layer is also not essential in the magnetic recording medium of the present invention, but in the case of a magnetic tape, it is desirable to form a backcoat layer on the surface opposite to the magnetic layer forming surface of the nonmagnetic support. The thickness of the back coat layer is preferably 0.2 to 0.8 μm, more preferably 0.3 to 0.8 μm. If the thickness of the backcoat layer is less than 0.2 μm, the effect of improving running performance is insufficient, and if it exceeds 0.8 μm, the total thickness of the tape is increased and the storage capacity per roll is reduced.

In preparing the magnetic paint, the undercoat paint and the back coat paint, any conventionally used organic solvent can be used as the solvent. Examples of such organic solvents include aromatic solvents such as benzene, toluene and xylene, ketone solvents such as acetone, cyclohexanone, methyl ethyl ketone and methyl isobutyl ketone, ester solvents such as ethyl acetate and butyl acetate, dimethyl carbonate, Carbonate ester solvents such as diethyl carbonate and alcohol solvents such as ethanol and isopropanol. In addition to these, organic solvents such as hexane, tetrahydrofuran, and dimethylformamide can also be used.

The magnetic paint, undercoat paint and backcoat paint can be prepared by a conventional paint production method, and it is particularly preferable to use a kneading process or a primary dispersion process using a kneader. In the primary dispersion step, the use of a sand mill is desirable because it can improve the dispersibility of magnetic powder and the like and can control the surface properties.

Magnetic coating, undercoating and / or back coating can be applied by conventional coating methods such as gravure coating, roll coating, blade coating, and extrusion coating.
It is coated on a nonmagnetic support. In particular, as a method for applying the undercoat paint and the magnetic paint, the undercoat paint is applied on a non-magnetic support, and after drying, the magnetic paint is applied. Any of the simultaneous multilayer coating method (wet-on-wet) may be employed. Considering the leveling of the thin magnetic layer at the time of coating, it is particularly preferable to adopt a simultaneous multilayer coating method in which the magnetic coating is applied while the undercoat coating is wet.

Hereinafter, the present invention will be described more specifically with reference to examples. In the examples, “parts” means “parts by weight”. In the following example, a barium ferrite magnetic powder represented by the general formula: BaO · 6Fe ₂ O ₃ is used as the hexagonal ferrite magnetic powder, but it goes without saying that the hexagonal ferrite magnetic powder is not limited to the barium ferrite magnetic powder. .

<Preparation of barium ferrite magnetic powder>
1 mol of ferric chloride, 1/8 mol of barium chloride, 1/20 mol of cobalt chloride and 1/20 mol of titanium chloride were dissolved in 1 L of water, and the resulting solution was dissolved in 2.8 mol of The mixture was added to 1 L aqueous sodium hydroxide solution in which sodium hydroxide was dissolved and stirred. The suspension was then aged for 1 day, and the precipitate was placed in an autoclave and heated at 250 ° C. for 4 hours to obtain a barium ferrite precursor.

The barium ferrite precursor is thoroughly washed with water until the pH (of the washing liquid) is 8 or less, and then a barium ferrite precursor suspension is prepared and the supernatant liquid is removed. As a flux, 500 g of NaCl was added and stirred to dissolve NaCl. Next, the barium ferrite precursor suspension containing dissolved NaCl was placed in a vat with a wide area and heated to 100 ° C. with a dryer to evaporate water.

The mixture of barium ferrite precursor and NaCl thus obtained is crushed, put in a crucible, first heated at 830 ° C. for 20 minutes to melt NaCl as a flux, and then the temperature is lowered to 800 ° C. , And heated at 800 ° C. for about 10 hours, and then cooled to room temperature. Next, NaCl was removed by washing with water, and barium ferrite magnetic powder was recovered. The barium ferrite magnetic powder obtained had a plate ratio of about 1.5 and an average particle size of 16 nm.

Further, this barium ferrite magnetic powder, the measured saturation magnetization and coercive force by applying a magnetic field of 1,270 kA / m (16,000 Oe), respectively, 37.4Am ² /kg(37.4emu/g ) And 137.7 kA / m (1,730 oersted).

<Preparation of magnetic paint>
Using the barium ferrite magnetic powder prepared above as the magnetic powder, the magnetic coating component having the following composition was kneaded with a kneader, and then dispersed with a sand mill for a residence time of 60 minutes. Polyisocyanate (Nippon Polyurethane Industry Co., Ltd.) was dispersed in the dispersion. 5 parts of “Coronate L”) was added and stirred and filtered to prepare a magnetic paint.

Barium ferrite magnetic powder 74 parts Vinyl chloride-hydroxypropyl acrylate copolymer resin 13 parts (contained -SO ₃ Na group: 0.7 × 10 ^-4 equivalent / g)
Polyester polyurethane resin 8 parts (containing -SO ₃ Na group: 1.0 × 10 ^-4 equivalent / g)
α-alumina (average particle size: 80 nm) 4 parts cyclohexanone 156 parts toluene 156 parts

<Preparation of paint for lower layer>
The following lower layer paint components were kneaded with a kneader and then dispersed with a sand mill with a residence time of 45 minutes to prepare a lower layer paint.

Iron oxide powder (average particle size: 55 nm) 70 parts Alumina powder (average particle size: 80 nm) 10 parts Carbon black (average particle size: 25 nm) 20 parts Vinyl chloride-hydroxypropyl methacrylate copolymer resin 10 parts (containing -SO ₃ Na group: 0.7 × 10 ⁻⁴ equivalent / g)
Polyester polyurethane resin 5 parts (containing -SO ₃ Na group: 1.0 × 10 ^-4 equivalent / g)
Methyl ethyl ketone 130 parts Toluene 80 parts Myristic acid 1 part Butyl stearate 1.5 parts Cyclohexanone 65 parts

<Preparation of magnetic tape>
The lower layer coating material is applied to a polyethylene terephthalate film, which is a nonmagnetic support, so that the lower layer thickness after drying and calendering is 2 μm, and the magnetic coating material is further added to 238.8 kA / After magnetic field orientation treatment at m, the coating was dried at 90 ° C. while adjusting the coating thickness so that the magnetic layer thickness after calendering was 120 nm.

Next, a back coat layer coating is applied on the side opposite to the surface on which the non-magnetic support and the undercoat layer of the non-magnetic support are formed, and the thickness of the back coat layer after drying and calendering is 700 nm. Applied and dried. The coating material for the backcoat layer is prepared by dispersing the following backcoat coating material components with a sand mill with a residence time of 45 minutes, adding 8.5 parts of polyisocyanate, and stirring and filtering.

Carbon black (average particle size: 25 nm) 40 parts Carbon black (average particle size: 370 nm) 1 part Barium sulfate 4 parts Nitrocellulose 28 parts Polyurethane resin (containing -SO ₃ Na group) 20 parts Cyclohexanone 100 parts Toluene 100 parts Methyl ethyl ketone 100 Part

The magnetic sheet thus obtained was mirror-finished with a 5-stage calendar (temperature: 70 ° C., linear pressure: 150 kg / cm), and this was wound on a sheet core at 60 ° C. and 40% RH for 48 hours. Aged. Then, it cut | judged to the 1/2 inch width.

In the production of the barium ferrite magnetic powder in Example 1, the addition amounts of cobalt chloride and titanium chloride were both changed from 1/20 mol to 1/15 mol, and the addition amount of sodium hydroxide was changed from 2.8 mol to 2.5 mol. The precursor of barium ferrite magnetic powder was prepared in the same manner as in Example 1 except that hydrothermal treatment was performed at 300 ° C. for 4 hours. Next, the processing conditions of this precursor in the flux were first heated at 830 ° C. for 20 minutes to melt the NaCl, which is the flux, and then the temperature was lowered to 820 ° C. and heated at 820 ° C. for about 10 hours. A barium ferrite magnetic powder was produced under the same conditions as in Example 1 except that the treatment was changed.

This barium ferrite magnetic powder had a substantially cubic shape with a plate ratio of about 1.1 and an average particle size of 14 nm. The barium ferrite magnetic powder had a saturation magnetization of 35.1 Am ² / kg (35.1 emu / g) and a coercive force of 125.8 kA / m (1,580 oersted). Using this barium ferrite magnetic powder, a magnetic tape was produced in the same manner as in Example 1.

In the production of barium ferrite magnetic powder in Example 1, the amount of sodium hydroxide added was changed from 2.8 mol to 3.5 mol, and hydrothermal treatment was performed at 230 ° C. for 4 hours. A precursor of barium ferrite magnetic powder was prepared in the same manner. Next, the processing conditions of the precursor in the flux were first heated at 830 ° C. for 20 minutes to dissolve NaCl as the flux, then the temperature was lowered to 780 ° C. and heated at 780 ° C. for about 10 hours. A barium ferrite magnetic powder was produced under the same conditions as in Example 1 except that the treatment was changed.

The barium ferrite magnetic powder had a plate ratio of about 1.8 and an average particle size of 17 nm. The barium ferrite magnetic powder had a saturation magnetization of 39.1 Am ² / kg (39.1 emu / g) and a coercive force of 149.6 kA / m (1,880 Oersted). Using this barium ferrite magnetic powder, a magnetic tape was produced in the same manner as in Example 1.

[Comparative Example 1]
In the production of barium ferrite magnetic powder in Example 1, the amount of sodium hydroxide added was changed from 2.8 mol to 5.0 mol, and hydrothermal treatment was performed at 280 ° C. for 4 hours. A precursor of barium ferrite magnetic powder was prepared in the same manner. Next, the processing conditions of the precursor in the flux were first heated at 830 ° C. for 20 minutes to dissolve NaCl as the flux, then the temperature was lowered to 780 ° C. and heated at 780 ° C. for about 10 hours. A barium ferrite magnetic powder was produced under the same conditions as in Example 1 except that the treatment was changed.

This barium ferrite magnetic powder was a flat plate having a plate ratio of about 5 and an average particle size of 23 nm. The barium ferrite magnetic powder had a saturation magnetization of 42.3 Am ² / kg (42.3 emu / g) and a coercive force of 157.6 kA / m (1,980 oersted). Using this barium ferrite magnetic powder, a magnetic tape was produced in the same manner as in Example 1.

For the magnetic tapes of Examples 1 to 3 and Comparative Example 1 described above, the longitudinal coercive force (Hc), squareness ratio (Br / Bm), and electromagnetic conversion characteristics were measured as magnetic characteristics by the following methods. These results are summarized in Table 1.

<Measurement of electromagnetic conversion characteristics>
The electromagnetic conversion characteristics were measured using a rotating drum device. The measurement conditions were that a MIG head (track width: 12 μm, gap length: 0.15 μm, Bs: 1.2 T) was used as the recording head, and a spin valve type GMR head (track width: 2.5 μm) as the reproducing head. And SH-SH width: 0.15 μm). The relative speed of the tape and the head was 3.4 m / sec. Using a spectrum analyzer, the reproduction output (S) and broadband noise (N) at a recording density of 169 kfci were measured, and the SNR was obtained. The reproduction output, noise level, and SNR are shown as relative values with the value of the tape of Comparative Example 1 being 0 dB.

Each of the magnetic tapes of Examples 1 to 3 according to the present invention uses a barium ferrite magnetic powder having a low plate ratio of 1 to 2 as a barium ferrite magnetic powder, and a barium ferrite magnetic powder having a large plate ratio of 5 Compared with the magnetic tape of Comparative Example 1 using No. 1, the output is low because of poor orientation. However, the effect of noise reduction is greater than the reduction in output, and as a result, a higher SNR is obtained compared to the magnetic tape of Comparative Example 1. This is because the bar ratio of the barium ferrite magnetic powder of the present invention is small, so that the particles are not easily laminated and aggregated, and as a result, the inherently low noise of individual particles is realized.

On the other hand, the magnetic tape of Comparative Example 1 uses a conventional barium ferrite magnetic powder having a large plate ratio, and a relatively high output can be obtained. On the other hand, noise due to particle agglomeration increases, resulting in a high SNR. I can't get it.

As described above, the present invention uses, as the magnetic powder, a hexagonal ferrite magnetic powder having a low plate ratio in which the plate ratio is in the range of 1 to 2 and the average particle size is in the range of 10 to 20 nm. By preventing lamination and aggregation, which is a drawback of the plate-like magnetic powder, noise can be reduced, and as a result, a large SNR can be obtained.

Claims

In a magnetic recording medium comprising a nonmagnetic support and a magnetic layer containing a magnetic powder and a binder formed on the nonmagnetic support, the magnetic powder has a plate-like ratio in the range of 1 to 2, 10 to Average particle size in the range of 20 nm, coercivity in the range of 79.6 to 318.4 kA / m (1,000 to 4,000 Oersted), saturation in the range of 20 to 60 Am 2 / kg (20 to 60 emu / g) A magnetic recording medium, which is a plate-shaped hexagonal ferrite magnetic powder having a magnetization amount.
The magnetic recording medium according to claim 1, wherein the hexagonal ferrite magnetic powder is at least one magnetic powder selected from the group consisting of barium ferrite and strontium ferrite.
3. An undercoat layer containing at least one inorganic powder and a binder is provided between the nonmagnetic support and the magnetic layer, and the thickness of the magnetic layer is 0.3 μm or less. 2. A magnetic recording medium according to 1.
The magnetic recording medium according to claim 3, wherein the undercoat layer contains a nonmagnetic powder as an inorganic powder.
The magnetic recording medium according to claim 3, wherein the magnetic recording medium is a perpendicular recording medium, and the undercoat layer contains magnetic particles as an inorganic powder.
6. The magnetic recording medium according to claim 1, wherein the thickness of the magnetic layer does not exceed 300 nm.