JP2011181116A - Magnetic recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a magnetic recording medium suitable for high density recording and achieving a high SNR by materializing low noise intrinsic in pulverized hexagonal ferrite magnetic powder. <P>SOLUTION: Plate-like hexagonal ferrite magnetic powder of which the plate ratio is in the range of 1-2, the average particle size is 10-20 nm, the coercivity is in the range of 79.6-318.4 kA/m (1,000 to 4,000 oersted), and the amount of saturation magnetization is in the range of 20-60 Am<SP>2</SP>/kg (20-60 emu/g) is made to be contained in the magnetic recording medium as magnetic powder. In particular, the hexagonal ferrite magnetic powder with a small plate ratio of 1-2 is used, contrary to plate-like hexagonal ferrite magnetic powder with a high plate ratio, which is conventionally used. The hexagonal ferrite magnetic powder is at least one selected from barium ferrite or strontium ferrite. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

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

  Coating type magnetic recording media, that is, magnetic recording media having a magnetic layer containing a magnetic powder and a binder on a non-magnetic support, have a higher recording density as the recording / reproducing system shifts from an analog system to a digital system. Improvement is required. 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.

  Further, in order to reduce noise, the magnetic powder is becoming finer every year, and at present, acicular metal magnetic powder having a particle diameter of about 45 nm is put into practical use. Furthermore, in order to prevent a decrease in output due to demagnetization during short wavelength recording, a high coercive force has been achieved year by year, and a coercive force of about 238.9 kA / m (3,000 oersted) has been realized by alloying with iron-cobalt. (See Patent Documents 1-3). However, in magnetic recording media using acicular magnetic particles, the coercive force depends on the shape magnetic anisotropy based on the acicular shape of the particles, so that it is difficult to further reduce the particle size from the above particle diameter. Yes.

  In other words, 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 particles become finer. , The coercive force decreases. This reduction in coercive force becomes a fatal problem in increasing 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.

  Accordingly, hexagonal ferrite magnetic powders that are plate-like and have an easy axis of magnetization in a direction perpendicular to the plate surface have been proposed as magnetic powders that are completely different from the needle-like magnetic powders (Patent Documents 4 to 6). ).

  Since this plate-shaped hexagonal ferrite magnetic powder is based on the magnetocrystalline anisotropy of the coercive force, it can maintain a high coercive force even when it becomes a fine particle. It has been shown that the magnetic powder is low and, as a result, has a high noise-to-power ratio (SNR) and is suitable for high-density recording media.

Japanese Patent Laid-Open No. 3-49026 JP-A-10-83906 Japanese Patent Laid-Open No. 10-34085 JP-A-6-290924 Japanese Patent Laying-Open No. 2005-340690 JP 2002-298331 A

  Since the hexagonal ferrite magnetic powders of Patent Documents 4 to 6 have fine coercive force despite being fine particles, they have excellent characteristics as magnetic powders for high-density recording. On the other hand, recently, the sensitivity of the reproducing head has been remarkably increased, and a high reproduction output value can be obtained relatively easily. However, since the noise output also increases at the same time, there arises a problem that a high SNR cannot be obtained as a result. That is, 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 Currently, low noise commensurate with the above has not been 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.

As a result of intensive studies on the above object, the present inventors have found that the magnetic powder has a plate-like ratio in the range of 1 to 2, an average particle size of 10 to 20 nm, and a coercive force of 79.6 to 318. A plate-shaped hexagonal ferrite magnetic powder having a saturation magnetization of 20 to 60 Am ² / kg (20 to 60 emu / g) in a range of 0.4 kA / m (1,000 to 4,000 Oersted), It has been found that it is possible to achieve the original low noise and achieve a high SNR in conformity with the above-mentioned purpose. In particular, a conventional plate-shaped hexagonal ferrite magnetic powder having a high plate ratio was used, whereas in the present invention, a hexagonal ferrite magnetic powder having a small plate ratio of 1-2 is used. There is a special feature.

  As described above, according to the present invention, the magnetic powder that is a defect of the plate-shaped hexagonal ferrite magnetic powder by using the hexagonal ferrite magnetic powder having a plate-like ratio of 1 to 2 as the magnetic powder. It is possible to prevent lamination and aggregation between each other, achieve the inherent low noise of hexagonal ferrite magnetic powder, and when this magnetic recording medium is applied to a system using a high-sensitivity head such as a GMR head, a high SNR can be obtained. .

The magnetic layer in the present embodiment is a 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, and a coercive force of 79.6 to 318.4 kA / m (1, 000 to 4,000 Oersted), by adding a plate-shaped hexagonal ferrite magnetic powder having a saturation magnetization of 20 to 60 Am ² / kg (20 to 60 emu / g). When this magnetic recording medium is applied to a system that realizes low noise inherent in powder and uses a high-sensitivity head such as a GMR head, a high SNR can be obtained.

  In addition, the plate-like ratio mentioned here shows a value (length / thickness) obtained by dividing the maximum length in the planar direction of the plate-like particles by the thickness. The average particle size is obtained by measuring the particle size of a photograph taken with a transmission electron microscope and calculating the average value of 300 particles.

  When the plate ratio of the hexagonal ferrite magnetic powder is larger than 2, the magnetic powders are easily laminated with each other, and as a result, the laminated magnetic powder behaves as one magnetic powder, resulting in increased noise. The magnetic powder having a plate ratio of 1 includes spherical and legitimate ones.

  On the other hand, when the average particle size of the hexagonal ferrite magnetic powder is smaller than 10 nm, it becomes difficult to uniformly disperse, and the effect of noise reduction becomes small. On the other hand, if it is larger than 20 nm, even if it can be uniformly dispersed, the particle size of one magnetic powder itself is too large, so that noise increases.

As the magnetic characteristics of the magnetic recording medium of the present invention, for example, when a longitudinally oriented medium is used, the coercive force (Hc) in the longitudinal direction is 119.4 to 258.2 kA / m (1,500 to 4,500). Oersted), the squareness ratio (Br / Bm) is 0.65 to 0.92, and the product of residual magnetization (Mr) and magnetic layer thickness (t) is Mr · t of 0.1 to 2.0 memu / It is preferable to set the magnetic layer thickness and the filling degree so as to be in the range of cm ² . When a vertical alignment medium is used, the coercive force (Hc) in the vertical direction is 79.6 to 318.4 kA / m (1,000 to 4,000 Oersted), and the squareness ratio (Br / Bm) is 0.8. It is preferable to set the magnetic layer thickness and the degree of filling so that Mr · t is in the range of 0.05 to 1.5 memu / cm 2 at 60 to 0.85. Further, when the non-oriented medium is used, the coercive force (Hc) in the longitudinal direction is 79.6 to 318.4 kA / m (1,000 to 4,000 Oersted), and the squareness ratio (Br / Bm) is 0.8. It is preferable to set the magnetic layer thickness and the degree of filling so that Mr · t is in the range of 0.08 to 1.8 memu / cm ² at 40 to 0.65.

In any magnetic recording medium in any orientation state, a magnetic recording medium having low noise and high SNR can be obtained by using the hexagonal ferrite magnetic powder of the present invention.

  The hexagonal ferrite magnetic powder is preferably at least one selected from the group consisting of barium ferrite magnetic powder and strontium ferrite magnetic powder, and barium ferrite magnetic powder is particularly preferable. Since these hexagonal ferrite magnetic powders have a large magnetic anisotropy, there is a feature that a decrease in output can be suppressed particularly in a short wavelength region. The above hexagonal ferrite magnetic powder includes Al, Si, S, Sc, Ti, V, Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ta, W in addition to the predetermined elements. , 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, although the manufacturing method of the hexagonal ferrite magnetic powder of the present invention will be described, it is not particularly limited, and a conventionally known method can be used. As such a production method, for example, an aqueous solution of a metal salt containing at least one metal salt of barium (Ba) salt or strontium (Sr) salt, which is a constituent element of hexagonal ferrite magnetic powder, and an iron salt A coprecipitate is formed by adding an alkaline aqueous solution. Next, a precursor of hexagonal ferrite magnetic powder is synthesized by hydrothermally treating the coprecipitate. As barium salts, strontium salts, and iron salts, chlorides, sulfates, nitrates, and carbonates of these metals are preferably used.

  At this time, by adding an appropriate amount of metal ions such as cobalt (Co), titanium (Ti), nickel (Ni), zinc (Zn), manganese (Mn) together with these metal salts, particles can be formed simultaneously with the coercive force. The size can be controlled arbitrarily. In particular, in order to obtain a hexagonal ferrite magnetic powder having a small particle size, the addition of these metal ions is indispensable.

  As the alkali, sodium hydroxide is usually used, and it is preferable that the excess alkali concentration is 0.01 mol / L or more with a molar equivalent or more of the metal salt to be added. 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 equal to or less than the molar amount with respect to the metal salt, it becomes easy to produce other than hexagonal ferrite magnetic powder.

  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, and according to the subsequent heat treatment conditions, While maintaining the particle shape, it is effective to increase the crystallinity and finish the magnetic powder having the desired magnetic properties.

  The hydrothermal treatment is usually performed using an autoclave, and 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 the desired shape cannot be obtained, and if it is too high, there is no particular problem, but it does not make much sense because the energy efficiency is poor. There is no. Also, the processing time has the same tendency as the processing temperature. If the processing time is too short, a precursor of the hexagonal ferrite magnetic powder having the desired shape cannot be obtained, and if it is too long, it is particularly problematic. No, but it doesn't make much sense just because it is less energy efficient.

  The thus-prepared hexagonal ferrite magnetic powder precursor is then heat treated at a temperature equal to or higher than the melting point of the flux by using a flux, thereby producing a hexagonal ferrite magnetic powder in the melted flux. The powder is produced by crystal growth. 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 intended particle shape and magnetic properties and a sharp particle size distribution can be obtained.

  The flux used here is preferably one that melts at 500 to 1,000 ° C. and does not form a solid solution with the hexagonal ferrite particles. The heat treatment becomes insufficient, the crystallinity of the hexagonal ferrite particles is sufficiently improved, and the magnetic properties cannot be improved. On the other hand, when the melting temperature is high, the crystal growth of the hexagonal ferrite particles in the flux becomes excessive, and the particles tend to become coarse. In addition, a solid solution with hexagonal crystal ferrite particles is not preferable because the amount of saturation magnetization tends to decrease. As such a flux, for example, sulfites of sodium chloride (Na), potassium (K), lithium (Li), chloride, bromide, iodide, boric acid and the like are preferably used. Since KCl and KBr dissolve well in water, washing with water after heat treatment facilitates removal of these fluxes and is preferable because they do not remain as impurities in the magnetic powder.

  The heat treatment with the 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 becomes insufficient, and hexagonal crystals The crystallinity of the system 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 the saturation magnetization will tend to be lowered among the magnetic properties.

  As another manufacturing method, the above-described hexagonal ferrite constituent element and a flux mixed with a flux are rapidly cooled to suppress the growth of hexagonal ferrite particles, and the rapidly cooled material is heated at 400 to 700 ° C. By treatment, hexagonal ferrite particles can be obtained by growing crystals in hexagonal ferrite particles to an appropriate size in the flux and then dissolving and removing the flux. 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 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 problem of output reduction 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 the GMR head is used, the reproduction output is easily distorted due to the saturation of the reproduction output. On the other hand, if the magnetic layer thickness is less than 10 nm, it is difficult to obtain a uniform magnetic layer.
As the magnetic characteristics, for example, when a longitudinally oriented medium is used, the longitudinal coercive force (Hc) is 119.4 to 258.2 kA / m (1,500 to 4,500 oersted), and the squareness ratio ( Br / Bm) is 0.65 to 0.92, and 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. It is preferable to set the thickness of the magnetic layer and the filling degree. In the case of a vertical alignment medium, the coercive force (Hc) in the vertical direction is 79.6 to 318.4 kA / m (1,000 to 4,000 Oersted), and the squareness ratio (Br / Bm) is 0. It is preferable to set the magnetic layer thickness and the filling degree so that Mr · t is in the range of 0.05 to 1.5 memu / cm ² at .60 to 0.85. Further, when the non-oriented medium is used, the coercive force (Hc) in the longitudinal direction is 79.6 to 318.4 kA / m (1,000 to 4,000 Oersted), and the squareness ratio (Br / Bm) is 0.8. It is preferable to set the magnetic layer thickness and the degree of filling so that Mr · t is in the range of 0.08 to 1.8 memu / cm ² at 40 to 0.65.
These coercive force ranges are preferable because if the coercive force is too low, output is likely to decrease due to demagnetization in the short wavelength region, and if the coercive force is too high, recording by the magnetic head becomes difficult. Because. 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 this case, 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 1-2 according to the present invention, the inherent low noise of the magnetic powder can be obtained, As a result, a high SNR can be obtained.

In addition, the average surface roughness of the magnetic layer is preferably in the range of Ra of 1.0 to 3.2 nm. In this range, contact with the head is improved and high SNR is obtained. On the other hand, when Ra is below this range, the slidability tends to decrease due to sticking of the head or the like, and above this range, the contact of the head becomes poor and the output tends to decrease.
The magnetic layer preferably contains carbon black for the purpose of improving electrical conductivity and surface lubricity, and alumina or the like for the purpose of improving polishing. Conventionally known carbon black and alumina can be used.

  The undercoat layer is not an essential component, 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 is 0.1 μm or less, the durability of the magnetic tape may be deteriorated. If the thickness is 3.0 μm or more, the durability of the magnetic tape is improved. Not only is the effect saturated, but the total tape thickness is increased, the tape length per roll is shortened, and the storage capacity is reduced.

The inorganic particles to be included in the undercoat layer are not particularly limited. For example, when non-magnetic iron oxide is used, the needle-like one preferably has an average length of 50 to 200 nm. Amorphous particles having an average particle diameter of 5 to 200 nm are preferably used. When the magnetic layer is vertically oriented and used as a perpendicular recording medium, it is preferable to use magnetic particles for the undercoat layer. At this time, the type of magnetic particles is not particularly limited, and iron oxide, metal, or alloy can be used. However, the purpose is to generate a strong magnetic flux only from the surface by closing the magnetic flux from the magnetic layer with an undercoat layer. The magnetic particles used in the undercoat layer are preferably those having as small a coercive force as possible and a large saturation magnetization.
Further, when used as a perpendicular recording medium, an intermediate layer can be further formed between the magnetic layer of the undercoat layer and the upper magnetic layer for signal recording. 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 for the undercoat layer and the magnetic layer is not particularly limited, and those usually used for magnetic recording media can be used. For example, 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-maleic anhydride copolymer resin, chloride There is a combination of a polyurethane resin and at least one selected from vinyl chloride-based resins such as vinyl-hydroxyl group-containing alkyl acrylate copolymer resins, nitrocellulose, and epoxy resins. In particular, it is preferable to use a vinyl chloride resin and a polyurethane resin in combination.
In addition to these binders, it is desirable to use in combination with a thermosetting crosslinking agent that crosslinks by bonding with a functional group contained in the binder. Examples of the crosslinking agent include tolylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, reaction products of these isocyanates with a plurality of hydroxyl groups such as trimethylolpropane, and condensation products of the above isocyanates. Various polyisocyanates are preferred.

  Conventionally known fatty acids, fatty acid esters, fatty acid amides and the like are used for the lubricant contained in the magnetic layer and the undercoat layer. Among them, 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 not an essential component, but in the case of a magnetic tape, it is desirable to form the 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, and more preferably 0.3 to 0.8 μm. This range is preferable because if the thickness 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 solvents can be used as the solvent. For example, 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, carbonate esters such as dimethyl carbonate and diethyl carbonate Solvents and alcohol solvents such as ethanol and isopropanol can be used, and various organic solvents such as hexane, tetrahydrofuran and dimethylformamide are also used.

  In preparing the magnetic coating material, the undercoat coating material, and the back coating material, a conventionally known coating manufacturing process can be used, and it is particularly preferable to use a kneading process using a kneader or the like and a primary dispersion process. In the primary dispersion step, it is desirable to use a sand mill because the surface properties can be controlled as well as the dispersibility of the magnetic powder and the like.

  Further, when applying a magnetic paint, undercoat paint, or backcoat paint on the nonmagnetic support, conventionally known coating methods such as gravure coating, roll coating, blade coating, and extrusion coating are used. In particular, the coating method of the primer coating and the magnetic coating is such that the primer coating is applied to a non-magnetic support and dried, and then the magnetic coating is applied, or the successive coating method or the primer coating and the magnetic coating are applied simultaneously. 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 application, it is particularly preferable to adopt the simultaneous multilayer coating method in which the magnetic coating is applied while the undercoat is wet.

Hereinafter, examples of the present invention will be described in more detail. In the following description, “parts” means parts by weight. In addition, an example in which a barium ferrite magnetic powder represented by the general formula BaO · 6Fe ₂ O ₃ is used as the hexagonal ferrite magnetic powder used as the magnetic powder will be described, but it is not limited to the barium ferrite magnetic powder. Needless to say.

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

  This barium ferrite precursor is sufficiently washed with water until the pH is 8 or less, then settled so that the total volume including the barium ferrite precursor becomes 1 L, a suspension is formed, and the supernatant is removed. 500 g NaCl as a flux was added to the suspension and stirred to dissolve. Next, the barium ferrite precursor suspension in which NaCl was dissolved was put in a hat having a large area and heated to 100 ° C. with a dryer to evaporate water.

  The mixture of barium ferrite precursor and NaCl obtained in this way was crushed and mixed into a crucible, first heated at 830 ° C. for 20 minutes to dissolve NaCl, which is a flux, and then heated to 800 ° C. The mixture was then heated to 800 ° C. for about 10 hours, and then cooled to room temperature. Next, NaCl was dissolved and removed by washing with water, and barium ferrite magnetic powder was taken out. The obtained barium ferrite magnetic powder had a plate ratio of about 1.5 and an average particle size of 16 nm.

Further, this barium ferrite magnetic powder, 1,270 kA / m (16,000 Oe) saturation magnetization 37.4Am ² /kg(37.4emu/g measured by applying a magnetic field), the coercive force is 137. 7 kA / m (1,730 oersted).
<Preparation of magnetic paint>
Using the above barium ferrite magnetic powder as the magnetic powder, kneading a magnetic coating component having the following composition with a kneader, and then performing a dispersion treatment with a sand mill for a residence time of 60 minutes. 5 parts by weight of “Coronate L”) was added and stirred and filtered to prepare a magnetic paint.

74 parts by weight of barium ferrite magnetic powder 13 parts by weight of vinyl chloride-hydroxypropyl acrylate copolymer resin (containing-SO ₃ Na group: 0.7 × 10 ⁻⁴ equivalent / g)
Polyester polyurethane resin 8 parts by weight
(Contained -SO ₃ Na group: 1.0 × 10 ^-4 equivalent / g)
α-alumina (average particle size: 80 nm) 4 parts by weight Cyclohexanone 156 parts by weight Toluene 156 parts by weight <Preparation of paint for lower layer>
The following lower layer paint components were kneaded with a kneader, and then dispersed in 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 by weight Alumina powder (average particle size: 80 nm) 10 parts by weight Carbon black (average particle size: 25 nm) 20 parts by weight Vinyl chloride-hydroxypropyl methacrylate copolymer resin 10 parts by weight ( -SO ₃ Na group: 0.7 × 10 ⁻⁴ equivalent / g)
Polyester polyurethane resin 5 parts by weight (containing-SO ₃ Na group: 1.0 × 10 ⁻⁴ equivalent / g)
Methyl ethyl ketone 130 parts by weight Toluene 80 parts by weight Myristic acid 1 part by weight Butyl stearate 1.5 parts by weight Cyclohexanone 65 parts by weight <Preparation of magnetic tape>
The lower layer paint 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 paint is further subjected to magnetic field orientation treatment, The coating was performed while adjusting the coating thickness so that the magnetic layer thickness after drying and calendering was 120 nm.

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

Carbon black (average particle size: 25 nm) 40 parts by weight Carbon black (average particle size: 370 nm) 1 part by weight Barium sulfate 4 parts by weight Nitrocellulose 28 parts by weight Polyurethane resin (containing -SO ₃ Na group) 20 parts by weight Cyclohexanone 100 parts by weight Part 100 parts by weight Toluene 100 parts by weight Methyl ethyl ketone 100 parts by weight The magnetic sheet thus obtained was mirror-finished with a five-stage calendar (temperature 70 ° C., linear pressure 150 kg / cm), and this was wound around a sheet core. Aging was performed at 40 ° C. and 40% RH for 48 hours. 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 hydrothermal treatment temperature was changed from 250 ° C. for 4 hours to 300 ° C. for 4 hours to prepare a precursor of barium ferrite magnetic powder. The processing conditions of the precursor in the flux are first heated at 830 ° C. for 20 minutes to dissolve the NaCl, which is the flux, then the temperature is lowered to 820 ° C., and the heat treatment is changed to 820 ° C. for about 10 hours. A barium ferrite magnetic powder was produced under the same conditions as in Example 1 except that.

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 the hydrothermal treatment temperature was changed from 250 ° C. for 4 hours to 230 ° C. for 4 hours. Thus, a precursor of barium ferrite magnetic powder was produced. The processing conditions of the precursor in the flux are first heated at 830 ° C. for 20 minutes to dissolve the NaCl, which is the flux, then the temperature is lowered to 780 ° C., and the heat treatment is changed to 780 ° C. for about 10 hours. A barium ferrite magnetic powder was produced under the same conditions as in Example 1 except that.

This 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 the hydrothermal treatment temperature was changed from 250 ° C. for 4 hours to 280 ° C. for 4 hours. Thus, a precursor of barium ferrite magnetic powder was produced. The processing conditions of the precursor in the flux are first heated at 830 ° C. for 20 minutes to dissolve the NaCl, which is the flux, then the temperature is lowered to 780 ° C., and the heat treatment is changed to 780 ° C. for about 10 hours. A barium ferrite magnetic powder was produced under the same conditions as in Example 1 except that.

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.

About each magnetic tape of said Examples 1-3 and the comparative example 1, the coercive force (Hc) of the longitudinal direction, the squareness ratio (Br / Bm), and the electromagnetic conversion characteristic were measured as a magnetic characteristic in the following way. 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 are as follows:
A spin valve type GMR head using a MIG head having a track width of 12 μm, a gap length of 0.15 μm, and Bs: 1.2 T, and having a track width of 2.5 μm and an SH-SH width of 0.15 μm. It was used. 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 uses a barium ferrite magnetic powder having a plate ratio of 1 to 2 and a barium ferrite magnetic powder having a large plate ratio of 5 as the barium ferrite magnetic powder. Since the orientation is inferior to the magnetic tape of Comparative Example 1, the output is low. 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 inherent 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 aggregation is increased, resulting in a high SNR. I can't.

  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 the lamination aggregation which is a problem of the plate-like magnetic powder, noise can be reduced, and as a result, a large SNR can be obtained.

Claims (3)

In a magnetic recording medium having a magnetic layer containing a magnetic powder and a binder on a nonmagnetic support, the magnetic powder is maintained in a plate ratio of 1 to 2 and an average particle size of 10 to 20 nm. Plate-shaped hexagon with magnetic force in the range of 79.6 to 318.4 kA / m (1,000 to 4,000 oersted) and saturation magnetization in the range of 20 to 60 Am ² / kg (20 to 60 emu / g) A magnetic recording medium comprising a crystalline ferrite magnetic powder.   2. The magnetic recording medium according to claim 1, wherein the hexagonal ferrite magnetic powder is at least one selected from barium ferrite and strontium ferrite.   3. The magnetic recording medium according to claim 1, wherein 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 A magnetic recording medium having a thickness of 0.3 μm or less.