JP2008181916A - Magnetic powder and manufacturing method thereof, and magnetic recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a manufacturing method of magnetic powder containing nitrogen having high coercive force and a narrow coercive force distribution regardless of an extremely small particle size and a spherical or elliptical shape, and to provide a magnetic recording medium having a narrow coercive force distribution using the magnetic powder. <P>SOLUTION: When manufacturing spherical or elliptical magnetic powder that has at least iron and nitrogen as components where an average particle size containing at least an Fe<SB>16</SB>N<SB>2</SB>phase is 10-20 nm and is nitriding iron magnetic powder containing rare earth elements, aluminum, and/or silicon, a goethite particle that becomes a raw material is manufactured and is subjected to hydrothermal treatment in advance, thus obtaining a goethite particle having a narrow particle size distribution and a smooth surface. The nitriding iron powder using the goethite particle as a raw material has a narrow coercive force distribution and SFD as a magnetic paint film becomes appropriate. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a spherical or elliptical magnetic powder containing at least iron and nitrogen as constituent elements, a method for producing the same, and a magnetic recording medium using the magnetic powder, and more specifically, a digital video tape, a backup tape for a computer, and the like. The present invention relates to a magnetic powder particularly suitable for ultra-high density recording, a manufacturing method thereof, and a magnetic recording medium using the magnetic powder.

  A coating type magnetic recording medium having a magnetic layer formed by applying a magnetic coating containing a magnetic powder and a binder on a non-magnetic support, as the recording / reproducing system shifts from an analog system to a digital system, A higher recording density is required. This demand has been increasing year by year, particularly in computer backup tapes for high recording density.

  In order to cope with the short wavelength recording which is indispensable for improving the recording density, it is necessary to reduce the thickness loss at the time of recording. One effective method for this purpose is to reduce the thickness of the magnetic layer to 300 nm or less. It is to make it 100nm or less. Further, as a reproducing magnetic head corresponding to such a high recording density medium, an MR head capable of obtaining a high output is generally used, and a GMR head having higher sensitivity will be used in the future.

  Also, in order to reduce noise, the particle size of magnetic powder has been made finer year by year, and at present, acicular metal magnetic powder having a particle diameter of about 100 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 is increased year by year, and a coercive force of about 238.9 kA / m (3000 Oe) is realized by iron-cobalt alloying (Patent Document 1). (Especially page 4), patent document 2 (especially page 3) and patent document 3 (especially page 2)).

  However, in the acicular magnetic particles, since the coercive force is based on the shape magnetic anisotropy based on the acicular shape of the particles, it has become difficult to further reduce the particle size from about 45 nm.

In metal magnetic particles, it is necessary to form an oxide protective layer with a certain thickness or more on the particle surface in order to give oxidation resistance, but the particle size of the acicular metal particles with the oxide protective layer formed As a result, the saturation magnetization is significantly lowered. In order to suppress this significant increase in specific surface area, it is effective to make the needle ratio as small as possible. However, when the acicular ratio is reduced, the particle shape approaches a sphere, so that the coercive force is remarkably reduced due to the reduction in shape magnetic anisotropy.
As described above, the acicular metal magnetic particles have an essential problem that the coercive force is reduced as the particle size is reduced.

Therefore, an iron nitride magnetic powder having a BET specific surface area of 10 m ² / g or more with a Fe ₁₆ N ₂ phase as a main phase and a medium using the same are proposed as magnetic powders completely different from the needle-shaped magnetic powder. (See Patent Document 4 (particularly, page 3, FIG. 4)). In Patent Document 4, iron nitride-based magnetic powder is produced by reducing iron oxide powder to obtain metallic iron powder, and nitriding the obtained metallic iron powder (Patent Document 4, paragraph [0010]). ). The iron nitride magnetic powder having a BET specific surface area of 10 m ² / g or more and having the Fe ₁₆ N ₂ phase as the main phase has a high coercive force, but has a large particle size and is not suitable for a high-density recording medium.

Also proposed is a magnetic recording medium using spherical or elliptical magnetic powder with a particle size of 5 to 50 nm, which contains at least one element of rare earth elements, aluminum and silicon and Fe ₁₆ N ₂ phase containing iron and nitrogen as constituent elements (Patent Document 5). This magnetic powder is manufactured by reducing iron-based oxide or hydroxide and then nitriding (Patent Document 5, paragraphs [0028] to [0033]). Although this magnetic recording medium has a high coercive force even though it is a fine particle, it has been shown to exhibit excellent high-density recording characteristics.
Japanese Patent Laid-Open No. 3-49026 JP-A-10-83906 JP-A-10-340805 JP 2000-277311 A JP 2004-273094 A

Patent Document 4 manufactures and uses an iron nitride-based magnetic powder having a BET specific surface area of 10 to 22 m ² / g in Examples, but the particle size is too large and high noise reduction is aimed at reducing noise. Not suitable for magnetic powder for high density magnetic recording media.

  The magnetic powder of Patent Document 5 is essentially a magnetic powder suitable for high-density recording. However, in order to maximize the inherent characteristics, it is necessary to further narrow the particle size distribution.

  Accordingly, an object of the present invention is to provide a method for producing a nitrogen-containing magnetic powder having a high coercive force and a narrow coercive force distribution, although the particle size is very small and the shape is spherical or elliptical. Furthermore, the present invention provides a magnetic powder produced by such a production method, and a magnetic recording medium having an excellent SFD (anisotropic magnetic field distribution) using the magnetic powder.

As a result of intensive studies to achieve the above object, the present inventor has a spherical or elliptical shape having an average particle size of 10 to 20 nm containing at least iron and nitrogen as constituent elements and containing at least an Fe ₁₆ N ₂ phase, and further a rare earth In a method for producing a magnetic powder containing at least one element selected from the group consisting of elements, aluminum and silicon, goethite particles as raw materials are produced, and this goethite is subjected to a heat reduction treatment, followed by nitriding treatment In the method for producing iron nitride magnetic powder, the goethite particles used as a raw material are subjected to hydrothermal treatment before the heat reduction treatment, so that the fine goethite particles are dissolved and removed, and the surface roughness of the goethite particles is partially It was found that goethite particles having a smooth surface were obtained by dissolution and re-precipitation. As a result, it was found that by using these hydrothermally treated goethite particles as raw materials, a magnetic powder having a narrow particle size distribution and a good coercive force distribution can be obtained, and the present invention has been completed.

That is, the present invention includes a step of subjecting the goethite particles to heat reduction treatment and nitriding treatment, comprising at least iron and nitrogen as constituent elements, and having an average particle size of at least 10 to 20 nm including an Fe ₁₆ N ₂ phase. And a method for producing an iron nitride magnetic powder containing at least one element selected from the group consisting of rare earth elements, aluminum and silicon, wherein the goethite particles are subjected to hydrothermal treatment before the heat reduction treatment A method for producing an iron nitride magnetic powder is provided.
The present invention also provides an iron nitride magnetic powder obtained by the method for producing an iron nitride magnetic powder and a magnetic recording medium using the iron nitride magnetic powder.

  The magnetic powder production method of the present invention is characterized in that the particle size distribution of the goethite particles themselves that are the raw material of the iron nitride magnetic powder can be narrowed. That is, for example, goethite particles prepared by an aqueous solution reaction are first subjected to hydrothermal treatment to dissolve fine particles and narrow the particle size distribution, while at the same time partially dissolving and reprecipitating the surface of goethite particles to smooth the surface. It is characterized by obtaining goethite particles. The iron nitride magnetic powder obtained by subjecting such goethite particles to reduction treatment and nitriding treatment has a good particle size distribution. Further, since the obtained iron nitride magnetic powder has a narrow coercive force distribution, a magnetic recording medium using this magnetic powder can realize a high SNR (signal to noise ratio).

Basically, iron nitride magnetic powders containing at least iron and nitrogen as constituent elements and containing at least an Fe ₁₆ N ₂ phase are known (Patent Documents 4 and 5). However, the inventors aimed to further improve the magnetic properties of the iron nitride magnetic powder, improved the manufacturing method, synthesized various magnetic powders, and investigated their shapes and magnetic anisotropy. As a result, the magnetic powder having a spherical or elliptical particle shape with an average particle size of 10 to 20 nm containing at least iron and nitrogen as constituent elements and containing at least an Fe ₁₆ N ₂ phase is highly magnetically anisotropic. It was found to show sex.
The average particle size referred to in the present invention is obtained by measuring the particle size of a photograph taken with a transmission electron microscope (TEM) and calculating an average value of 300 particles.

  Furthermore, by containing at least one element selected from the group consisting of rare earth elements, aluminum and silicon in the magnetic powder, a magnetic powder with less sintering that retains the shape of the raw material goethite particles can be obtained.

In the magnetic powder of the present invention, the ratio of nitrogen to iron is preferably 1.0 to 20.0 atomic%, more preferably 2.0 to 18.0 atomic%. If the ratio of nitrogen to iron is too small, the amount of Fe ₁₆ N ₂ phase formed will be small, and the effect of increasing the coercive force will be small. On the other hand, if the ratio of nitrogen to iron is too large, nonmagnetic nitrides are easily formed, so the effect of increasing the coercive force is reduced and the saturation magnetization is excessively lowered.

The ratio of at least one element selected from the group consisting of rare earth elements, aluminum and silicon in the magnetic powder is preferably 0.05 to 20.0 atomic% with respect to iron. If the ratio of these elements is smaller than this range, the shape retention effect is small, while if it is too large, the amount of nonmagnetic components increases and the saturation magnetization decreases too much.
As the rare earth element, yttrium, samarium and / or neodymium is preferable.

In the magnetic powder of the present invention, the portion where the Fe ₁₆ N ₂ phase exists is not particularly limited, but it is preferable that the magnetic powder is present inside the magnetic powder and the rare earth element, aluminum and / or silicon are mainly present in the outer layer portion. By having such a core-shell structure, the magnetic powder becomes more chemically stable when used as a magnetic powder for a magnetic recording medium.

The magnetic powder of the present invention does not need to have only the Fe ₁₆ N ₂ phase as the iron nitride phase, and may contain, for example, an Fe ₁₆ N ₂ phase and another phase, for example, an α-Fe phase. When the Fe ₁₆ N ₂ phase and the α-Fe phase are present, there is an advantage that the desired coercive force can be easily adjusted by adjusting the ratio between the two, and the design of the magnetic recording medium can be widened. In addition, other phases such as α-Fe phase may be present inside the magnetic powder, in the outer layer portion containing rare earth element, aluminum and / or silicon, or Fe ₁₆ N ₂ phase and outer layer. It may be present at the interface. In particular, when the α-Fe phase is formed at the interface between the inner Fe ₁₆ N ₂ phase and the outer layer containing rare earth elements, aluminum and / or silicon, the chemical stability becomes better.

Unlike the conventional acicular particles, the magnetic powder of the present invention has a spherical or elliptical shape with an average particle size of 10 to 20 nm. For this reason, each individual particle volume is smaller than that of a conventional acicular magnetic powder. Therefore, when used in a magnetic recording medium, the particle noise is reduced.
The “spherical or elliptical shape” referred to in the present invention refers to those having an average value of the axial ratio [ratio of major axis length (major axis) to minor axis length (minor axis)] of 1 or more and 2 or less. .

  When recording a signal on a magnetic recording medium, it is desirable that the recording bit boundary is as sharp as possible. When the acicular magnetic powder is used, the acicular magnetic powder straddles the boundary between the recording bits, so that the boundary line becomes zigzag and causes an increase in noise. This phenomenon becomes more prominent as the recording density increases. Since the magnetic powder powder of the present invention has a spherical or elliptical shape, the boundary between recording bits becomes sharp in a magnetic recording medium using the same, and even if the particle volume is the same as the acicular magnetic powder, Noise level is lower. This is also one of the effects of the present invention.

The magnetic powder of the present invention has a very small particle size of 10 to 20 nm. For this reason, if fine particles are present, the fine particles have a large surface area, so that they are easily oxidized excessively during the oxidation stabilization treatment. When the fine particles are excessively oxidized in this manner, the particle size of iron nitride having a composition of Fe ₁₆ N ₂ is substantially reduced, and the coercive force is reduced by superparamagnetism. As a result, the coercive force distribution is widened and the SNR is lowered. Further, the saturation magnetization is excessively lowered.

Therefore, narrowing the particle size distribution is indispensable for obtaining a high SNR when a magnetic recording medium is used. Usually, in a thermal process such as reduction, the particle size distribution deteriorates due to coarsening due to sintering of particles. Therefore, sintering is prevented by optimizing the kind of sintering treatment agent, its addition amount, addition method, reduction temperature and time.
However, these methods aim to maintain the shape of the raw material particles as much as possible. In other words, it is essentially impossible to obtain a material having a size distribution larger than that of the raw material particles. One of the most important things in obtaining a magnetic powder having a narrow particle size distribution is to use raw material particles having a narrow particle size distribution.

  In the present invention, first, goethite particles as raw materials are prepared, and then the goethite particles are subjected to hydrothermal treatment to dissolve and remove fine goethite particles contained in the raw material goethite particles, and the surface of the goethite particles. By partially dissolving and reprecipitating, goethite particles having a smooth surface are obtained. That is, in the method for producing goethite by a normal aqueous solution reaction, there are more or less fine particles. Therefore, the goethite particles are further subjected to hydrothermal treatment, and such fine particles are dissolved and completely disappeared. Furthermore, the surface of the goethite particles has irregularities in a state produced by an aqueous solution reaction, and the irregularities cause sintering in a thermal process such as a reduction process. Therefore, the surface of the remaining goethite particles can be partially dissolved and re-precipitated by a hydrothermal treatment so that the surface can be made smooth and difficult to sinter.

Next, the manufacturing method of the iron nitride magnetic powder of the present invention will be described.
First, goethite particles as a starting material are prepared.
Goethite particles generally tend to grow into needle-like shapes, but when prepared by a coprecipitation method, they are prevented from growing into needle-like shapes by adjusting the alkali concentration and the dropping method of the iron salt solution and the alkali solution. To do. In this case, the shape of goethite tends to be indefinite and has a wide particle size distribution in which fine particles are present. Although it is possible to use goethite particles in this state as raw materials to form iron nitride particles, in the present invention, as described above, the goethite particles are subjected to hydrothermal treatment in advance, and the fine particles are dissolved and disappeared. In addition, the surface of the goethite particles is partially dissolved and re-precipitated to make the surface smooth, so that sintering is unlikely to occur.

  The raw material goethite particles are preferably prepared in the presence of at least one element selected from the group consisting of rare earth elements, aluminum and silicon. These rare earth elements, elements such as aluminum and silicon are present in the goethite particles even after hydrothermal treatment. In addition, it is possible to subject the goethite subjected to hydrothermal treatment to at least one element selected from the group consisting of rare earth elements, aluminum and silicon. In any case, in order to prevent sintering between particles in the subsequent heating step, at least one element selected from the group consisting of rare earth elements, aluminum, and silicon is used as iron nitride particles. It is preferable to make it exist in the ratio of 0.1-20 atomic% with respect to.

  The shape of the goethite particles is not particularly limited because the particle surface is partially dissolved and reprecipitated by hydrothermal treatment, and the shape is shaped. A non-acicular shape such as a shape is preferred. The average particle size is not particularly limited, but is preferably about 5 to 30 nm. If the particle size is too small, inter-particle sintering is likely to occur during the reduction treatment, and if it is too large, the size of the magnetic powder itself obtained after reduction and nitridation increases and the noise level increases.

  The hydrothermal treatment is preferably performed at 100 to 200 ° C. When the temperature is lower than 100 ° C., the effect of hydrothermal treatment is small, and the shape of the particle shape is insufficient. On the other hand, even if the temperature is higher than 200 ° C., there is no problem, but the particle shaping effect by hydrothermal treatment is saturated. The pH during hydrothermal treatment is preferably neutral to alkaline. By carrying out on the alkali side, the surface of the goethite particles is likely to be partially dissolved, and the goethite particles having a smooth surface can be easily obtained.

  By this hydrothermal treatment, the fine goethite particles present in the raw material goethite particles are dissolved and become goethite particles having a narrow particle size distribution. As a result, a magnetic powder having a narrow coercive force distribution is finally obtained. Further, the surface of the goethite particles is partially dissolved by the hydrothermal treatment, and the surface irregularities are removed to make the surface smooth. As a result, it becomes difficult to sinter in a subsequent heating step such as a reduction step, the particle size distribution becomes narrow, and a magnetic powder with a narrow coercive force distribution can be obtained.

  The time for the hydrothermal treatment is not particularly limited, but is preferably 1 to 20 hours. If it is longer than this, the remaining goethite particles may be excessively dissolved. On the other hand, if the hydrothermal treatment time is too short, fine goethite particles remain without being sufficiently dissolved. An appropriate hydrothermal treatment time may be determined by performing a simple preliminary experiment.

  Next, the hydrothermally treated goethite particles are heated and reduced in a reducing gas stream. The type of reducing gas is not particularly limited, but hydrogen gas is preferably used. The reduction temperature is preferably 300 to 550 ° C, more preferably 350 to 500 ° C. When the reduction temperature is lower than 300 ° C., the reduction reaction does not proceed sufficiently, and when it is higher than 550 ° C., the powder particles are easily sintered.

  By subjecting the reduced particles to nitriding treatment, the magnetic powder containing iron nitride of the present invention can be obtained. The nitriding treatment is desirably performed using a gas containing ammonia. In addition to ammonia gas alone, a mixed gas using hydrogen gas, helium gas, nitrogen gas, argon gas or the like as a carrier gas may be used. Nitrogen gas is particularly preferable as a carrier gas because it is inexpensive.

The nitriding temperature is preferably 100 to 300 ° C, more preferably 110 to 250 ° C. If the nitriding temperature is too low, nitriding does not proceed sufficiently and the effect of increasing the coercive force is small. If it is too high, nitriding will be promoted excessively, and the proportion of Fe ₄ N or Fe ₃ N phase will increase, so the coercive force will decrease and the saturation magnetization will tend to decrease excessively.

In such nitriding treatment, it is desirable to select the nitriding treatment conditions so that the ratio of nitrogen to iron in the obtained magnetic powder is 1.0 to 20.0 atomic%. When the proportion of nitrogen is too small, the effect of improving the coercive force is reduced because the amount of Fe ₁₆ N ₂ phase produced is small. On the other hand, when the proportion of nitrogen is too large, Fe ₄ N or Fe ₃ N phases are easily formed, the coercive force is lowered, and the saturation magnetization is further lowered excessively.

  Next, the magnetic powder after the nitriding treatment is subjected to stabilization treatment as desired. The stabilization treatment can be performed by heating and oxidizing in nitrogen gas containing a small amount of oxygen (for example, 10 to 10,000 ppm), and this treatment forms an oxide film on the surface of the magnetic powder. Although the oxidation conditions are not particularly limited, the oxidation is usually performed at 30 to 100 ° C., usually for about 2 to 50 hours.

By the above treatment, at least the Fe ₁₆ N ₂ phase is contained, the average particle size is 10 to 20 nm, the saturation magnetization is 40 to 120 Am ² / kg (40 to 120 emu / g), and the coercive force is 215.2 to 318.4 kA / m (2000 Magnetic powder (˜4000 oersted) is obtained.

  The magnetic recording medium of the present invention can be produced using the above magnetic powder according to a conventional method for producing a coating type magnetic recording medium. For example, by mixing magnetic powder with a binder and an organic solvent so as to have an appropriate content, and further applying a magnetic coating material prepared by a dispersion method optimum for this magnetic powder on a nonmagnetic support and drying. A magnetic recording medium can be manufactured.

  Further, if the above-mentioned magnetic coating material is formed on an undercoat layer containing non-magnetic particles such as α-hematite and titanium dioxide and a binder to form a multilayer coating film, the characteristics of the magnetic powder of the present invention can be further exhibited. High SNR.

The magnetic recording medium of the present invention will be described in more detail.
As the nonmagnetic support used in the present invention, any conventionally used nonmagnetic support for magnetic recording media can be used. For example, films made of resins such as polyesters such as polyethylene terephthalate and polyethylene naphthalate, polyolefins, cellulose triacetate, polycarbonate, polyamide, polyimide, polyamideimide, polysulfone, aramid, and aromatic polyamide are preferably used. The thickness of the nonmagnetic support is usually 2 to 15 μm, particularly 2 to 7 μm.

  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. The optimum thickness of the magnetic layer is determined from the relationship with the recording wavelength used. For example, when the magnetic recording medium is used in a system having a shortest recording wavelength of 0.3 m or less, the thickness of the magnetic layer is particularly important and is preferably 10 to 200 nm. When the magnetic layer thickness is 300 nm or more, the reproduction output becomes small due to the thickness loss, or the product of the residual magnetic flux density and the thickness becomes too large, and a high sensitivity head such as an MR head or GMR head is used for the reproduction head. If this occurs, distortion of the reproduction output due to saturation is likely to occur. If it is less than 10 nm, it is difficult to obtain a uniform magnetic layer.

  The coercive force of the magnetic layer is preferably 159.2 to 398.0 kA / m (2,000 to 5,000 Oe). If the coercivity of the magnetic layer is less than 159.2 kA / m, if the recording wavelength is shortened, the output is likely to decrease due to demagnetization due to the demagnetizing field, and if it exceeds 398.0 kA / m, recording by the magnetic head becomes difficult.

  In the case of a magnetic tape, the product of the saturation magnetic flux density in the longitudinal direction and the thickness is preferably in the range of 0.0015 to 0.05 μTm. If this product is less than 0.0015 μTm, the playback output is small even when a high sensitivity head such as an MR head or GMR head is used as the playback head, while if it exceeds 0.05 μTm, the playback head tends to be saturated, and It tends to be difficult to obtain a high output due to reproduction demagnetization in the target short wavelength region.

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

  When the magnetic layer has a square shape (Br / Bm), when oriented in the longitudinal direction, Br / Bm in the longitudinal direction is 0.65 to 0.95. It is preferable that Bm is 0.60 to 0.90, and that when used as a non-oriented medium, Br / Bm in the longitudinal direction is 0.55 to 0.85.

  The magnetic field orientation of the magnetic layer is preferably properly used according to the application. For example, longitudinal orientation is preferable when particularly high output is required in the low wavelength region to medium wavelength region, non-orientation is preferable when output is required over the entire region, and particularly high output is required in the short wavelength region. In such a case, vertical alignment is preferable. Since the magnetic powder of the present invention is spherical or elliptical in shape and has little shape anisotropy, the magnetic powder can be oriented in any direction without deteriorating the surface property (smoothness) of the tape. This is one of the major features of the present invention that is different from recording media using needle-like magnetic powder or plate-like magnetic powder.

  The magnetic layer preferably contains a conductive material such as carbon black for improving conductivity and surface lubricity, and an abrasive such as alumina for improving polishability. Conventionally known materials can be used as the conductive material and the abrasive.

  In order to improve the durability of the magnetic recording medium, an undercoat layer is preferably provided between the nonmagnetic support and the magnetic layer. The thickness of the undercoat layer is preferably 0.1 to 3.0 μm. When the thickness of the undercoat layer is 0.1 μm or less, the durability of the magnetic tape is not effectively improved, and when it is 3.0 μm or more, not only the durability improvement effect of the magnetic tape is saturated, but also the recording medium The total thickness is increased, the tape length per roll is shortened, and the storage capacity per magnetic tape roll is reduced.

  The binder used for the undercoat layer and the magnetic layer is not particularly limited, and binders usually used for magnetic recording media can also be used in the present invention. 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.

  The backcoat layer is not always necessary, 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, 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 the running property is insufficient. On the other hand, if the thickness exceeds 0.8 μm, the total thickness of the tape is increased and the storage capacity per roll is reduced. The back coat layer preferably contains carbon black having an average particle size of 5 to 400 nm, such as acetylene black and furnace black. The amount added is preferably 60 to 98% by weight, more preferably 70 to 95% by weight. For the purpose of improving the strength, an additive such as iron oxide or aluminum oxide having an average particle size of 0.05 to 0.6 μm may be included. In this case, the addition amount is preferably 2 to 40% by weight, and more preferably 2 to 30% by weight. Moreover, as a binder, the same thing as what is used for a magnetic layer and undercoat can be used.

  For the preparation of the magnetic paint, the undercoat paint and the backcoat paint, any organic solvent conventionally used 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 magnetic coating materials, base coating materials, and back coating materials, conventionally known coating manufacturing processes can be used, and it is particularly preferable to employ a kneading process or a primary dispersion process using a kneader. 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.

  In addition, conventionally known coating methods such as gravure coating, roll coating, blade coating, and extrusion coating may be used to apply a magnetic coating material, a base coating material, and a desired back coating material on a nonmagnetic support. Used. In particular, as a method of applying the undercoat paint and the magnetic paint, the undercoat paint is applied on a non-magnetic support and dried, and then the magnetic paint is applied. The sequential multilayer coating method, or the undercoat paint and the magnetic paint are applied simultaneously. Any of the simultaneous multilayer coating methods (wet on wet) can be adopted. Considering the leveling of the thin magnetic layer at the time of application, it is particularly preferable to adopt a simultaneous multilayer coating method in which the magnetic paint is applied while the undercoat paint is wet.

Hereinafter, examples of the present invention will be described in more detail.
The effect that appears most prominently when the iron nitride magnetic powder is obtained by subjecting the goethite particles to hydrothermal treatment is that the particle size distribution becomes narrow. As a result, the coercive force distribution becomes narrow when a magnetic tape is used. Therefore, in the present invention, a magnetic coating film was prepared using the obtained iron nitride magnetic powder, and the coercive force distribution, orientation and coercive force were evaluated. SFD (anisotropic magnetic field distribution) was measured as the coercive force distribution, and square (Br / Bm) was measured as the orientation. The SFD is obtained by measuring a demagnetization curve using a vibrating sample magnetometer (VSM) and dividing the half-value width of the differential curve of this magnetization curve by the coercive force value. The smaller the SFD, the narrower the coercive force distribution, indicating that the magnetic powder is preferable as the magnetic powder for magnetic tape. Moreover, it shows that it is a magnetic powder with favorable orientation, so that Br / Bm is large.

<Example 1>
(A) Production of goethite particles
To 4 liters of 0.2 mol / L FeSO4 aqueous solution, 0.5 liters of 12 mol / L NaOH aqueous solution and an amount of sodium aluminate such that Al / Fe = 19 atomic% were added. While maintaining this suspension at 30 ° C., air was blown at a flow rate of 300 mL / min for 3 hours to precipitate goethite particles in which Al was dissolved. The obtained goethite particles had fine irregularities on the surface, but were almost spherical with an average particle size of 18 nm. Next, the goethite particles were washed with water and filtered, and then 10 g of the goethite particles were redispersed in 80 g of water without drying, and 0.8 g of NaOH was added to the dispersion to dissolve it. This dispersion was put into an autoclave and heat-treated at 180 ° C. for 5 hours. After the hydrothermal treatment, the obtained goethite particles were washed with water, filtered, and dried in air at 110 ° C. The BET specific surface area of the goethite particles subjected to the hydrothermal treatment was 75 m ² / g.

(B) Production of iron nitride-based magnetic powder The goethite particles produced in the above step (A) were crushed in a mortar, then heated and reduced at 450 ° C. for 5 hours in a hydrogen stream, and then in a state of flowing hydrogen gas at 140 ° C. When the temperature reached 140 ° C., the gas was switched from hydrogen gas to ammonia gas, and nitriding was performed for 20 hours while maintaining the temperature at 140 ° C. Thereafter, the temperature was lowered from 140 ° C. to 40 ° C. with ammonia gas flowing, and at 40 ° C., the ammonia gas was switched to an oxygen / nitrogen mixed gas (oxygen concentration: 1000 ppm), and a stabilization treatment was performed for 4 hours. Further, with the mixed gas flowing, the temperature is lowered to 30 ° C., held at 30 ° C. for another day, then cooled to room temperature, switched from mixed gas to air at room temperature, taken out into the air, and iron nitride magnetic powder Got.

With respect to the iron nitride magnetic powder thus obtained, the proportions of aluminum and nitrogen were measured by fluorescent X-ray and found to be 16.2 atomic% and 9.3 atomic%, respectively. Further, the X-ray diffraction pattern of the magnetic powder showed a profile corresponding to the Fe ₁₆ N ₂ phase.

Furthermore, when the particle shape was observed with a high-resolution analytical transmission electron microscope, it was an almost spherical particle with an average particle size of 17 nm, and the inside was an oxide layer composed mainly of aluminum with iron nitride of Fe ₁₆ N ₂ structure. It was found to be a core-shell structure composed of The BET specific surface area was 72.1 m ² / g.

The magnetic powder had a saturation magnetization measured by applying a magnetic field of 1,270 kA / m (16 kOe) of 94.2 Am ² / kg (94.2 emu / g) and a coercive force of 226.1 kA / m (2,840 Oersted). It was.

(C) Preparation of magnetic coating film Using the iron nitride magnetic powder obtained in the above step (B), the following composition is placed in a 100 cc pot for a planetary ball mill and dispersed for 12 hours using a planetary ball mill. A paint was made. This magnetic paint was applied onto a PET base film using an applicator and dried in a magnetic field of 398.0 kA / m (5 kOe).
4.0 g of magnetic powder (iron nitride magnetic powder obtained in step (B))
Binder (Nippon Zeon MR-110) 1.0g
Cyclohexanone 7.0g
7.0g of toluene
(D) Evaluation of magnetic coating film About a magnetic coating film, SFD, square shape (Br / Bm), and coercive force were measured using VSM. The results are shown in Table 1.

<Example 2>
In the production process of goethite particles in Example 1 (A), the goethite particles after hydrothermal treatment were washed with water, sodium aluminate was further added so that Al / Fe = 4 atomic%, and the pH was adjusted to 7-8 with an aqueous NaOH solution. The mixture was stirred at room temperature for about 2 hours, and Al was further applied to the goethite particles. The goethite particles had a BET specific surface area of 78 m ² / g. The goethite particles were treated by the same method as the production of the iron nitride magnetic powder in Example 1 (B) to produce an iron nitride magnetic powder.

The contents of aluminum and nitrogen in the iron nitride magnetic powder were 19.4 atomic% and 9.2 atomic%, respectively. Further, the X-ray diffraction pattern of the magnetic powder showed a profile corresponding to the Fe ₁₆ N ₂ phase. Furthermore, when the particle shape was observed with a high-resolution analytical transmission electron microscope, it was an almost spherical particle with an average particle size of 17 nm, and the inside was an oxide layer composed mainly of aluminum with iron nitride of Fe ₁₆ N ₂ structure. It was found to be a core-shell structure composed of The BET specific surface area was 74.3 m ² / g.

In addition, this magnetic powder had a saturation magnetization measured by applying a magnetic field of 1,270 kA / m (16 kOe) of 92.5 Am ² / kg (92.5 emu / g) and a coercive force of 229.2 kA / m (2,880 Oersted). It was. Using this iron nitride magnetic powder, a magnetic coating film was produced in the same manner as in the magnetic coating film production process of Example 1 (C), and SFD, square (Br / Bm) and coercive force were measured. The results are shown in Table 1.

<Example 3>
Goethite particles were produced in the same manner as in the production of goethite particles in Example 1 (A), except that the hydrothermal treatment conditions were changed from 180 ° C. for 5 hours to 130 ° C. for 5 hours. The BET specific surface area of the goethite particles subjected to the hydrothermal treatment was 81 m ² / g. Further, the goethite particles were treated by the same method as the production of the iron nitride magnetic powder in Example 1 (B) to produce an iron nitride magnetic powder.

The contents of aluminum and nitrogen in the iron nitride magnetic powder were 16.5 atomic% and 9.5 atomic%, respectively. Further, the X-ray diffraction pattern of the magnetic powder showed a profile corresponding to the Fe ₁₆ N ₂ phase. Furthermore, when the particle shape was observed with a high-resolution analytical transmission electron microscope, it was an almost spherical particle with an average particle size of 17 nm, and the inside was an oxide layer composed mainly of aluminum with iron nitride of Fe ₁₆ N ₂ structure. It was found to be a core-shell structure composed of The BET specific surface area was 75.6 m ² / g.

The magnetic powder measured by applying a magnetic field of 1,270 kA / m (16 kOe) had a saturation magnetization of 90.5 Am ² / kg (90.5 emu / g) and a coercive force of 214.1 kA / m (2,690 oersted). It was. Using this iron nitride magnetic powder, a magnetic coating film was produced in the same manner as in the magnetic coating film production process of Example 1 (C), and SFD, square (Br / Bm) and coercive force were measured. The results are shown in Table 1.

<Comparative Example 1>
In the preparation of goethite particles in Example 1 (A), hydrothermal treatment was omitted. The BET specific surface area of the goethite particles not subjected to the hydrothermal treatment was 140 m ² / g. Using the goethite particles that were not subjected to the hydrothermal treatment, the iron nitride magnetic powder was produced by treating in the same manner as the production of the iron nitride magnetic powder in Example 1 (B).

The content ratios of aluminum and nitrogen in the iron nitride magnetic powder were 16.9 atomic% and 9.0 atomic%, respectively. Further, the X-ray diffraction pattern of the magnetic powder showed a profile corresponding to the Fe ₁₆ N ₂ phase. Furthermore, when the particle shape was observed with a high-resolution analytical transmission electron microscope, it was an almost spherical particle with an average particle size of 18 nm, an iron layer with an Fe ₁₆ N ₂ structure inside, and an oxide layer mainly composed of aluminum on the surface It was found to be a core-shell structure composed of The BET specific surface area was 83.2 m ² / g.

In addition, this magnetic powder had a saturation magnetization measured by applying a magnetic field of 1,270 kA / m (16 kOe) of 82.0 Am ² / kg (82.0 emu / g) and a coercive force of 210.9 kA / m (2,650 oersted). It was. Using this iron nitride magnetic powder, a magnetic coating film was produced in the same manner as in the magnetic coating film production process of Example 1 (C), and SFD, square (Br / Bm) and coercive force were measured. The results are shown in Table 1.

<Comparative example 2>
In the preparation of goethite particles in Example 2, hydrothermal treatment was omitted. That is, when goethite particles were prepared, sodium aluminate in an amount of Al / Fe = 19 atomic% was added to precipitate goethite particles in which Al was dissolved, and the goethite particles were further subjected to Al / Fe without performing hydrothermal treatment. Al was added by adding sodium aluminate so that Fe = 4 atomic%. The BET specific surface area of the goethite particles was 180 m ² / g. The goethite particles were treated by the same method as the production of the iron nitride magnetic powder in Example 1 (B) to produce an iron nitride magnetic powder.

The content ratios of aluminum and nitrogen in this iron nitride magnetic powder were 20.8 atomic% and 8.8 atomic%, respectively. Further, the X-ray diffraction pattern of the magnetic powder showed a profile corresponding to the Fe ₁₆ N ₂ phase. Furthermore, when the particle shape was observed with a high-resolution analytical transmission electron microscope, it was an almost spherical particle with an average particle size of 18 nm, an iron layer with an Fe ₁₆ N ₂ structure inside, and an oxide layer mainly composed of aluminum on the surface It was found to be a core-shell structure composed of The BET specific surface area was 86.1 m ² / g.

This magnetic powder has a saturation magnetization of 80.6 Am ² / kg (76,4 emu / g) and a coercive force of 216.5 kA / m (2,720 Oersted) measured by applying a magnetic field of 1,270 kA / m (16 kOe). there were. Using this iron nitride magnetic powder, a magnetic coating film was produced in the same manner as in the magnetic coating film production process of Example 1 (C), and SFD, square (Br / Bm) and coercive force were measured. The results are shown in Table 1.

  As is clear from the results of Table 1 above, the iron nitride magnetic powders of Examples 1 to 3 using the goethite particles hydrothermally treated before reduction and nitriding as raw materials, The BET specific surface area is small compared to the iron nitride magnetic powders of Comparative Examples 1 and 2 using goethite particles not subjected to heat treatment. This is because fine particles disappeared by the hydrothermal treatment, and at the same time, the surface of the goethite particles was partially dissolved and smoothed. In addition, as a result of the smooth surface of the goethite particles as described above, iron nitride magnetic powder having a narrow particle size distribution without sintering was obtained, and although the BET specific surface area was small compared to the magnetic powder of the comparative example. This is because the particle size is reduced.

  Further, the SFDs of the magnetic coating films of Examples 1 to 3 are clearly smaller than the SFDs of the magnetic coating films of Comparative Examples 1 and 2. This is because the particle size distribution of the magnetic powder is narrow and the coercive force distribution of the particles is narrow. Moreover, also in Br / Bm, the magnetic coating film of Examples 1-3 showed a high value compared with the magnetic coating film of Comparative Examples 1 and 2, and has shown that orientation is favorable. This is because the dispersibility of the magnetic powder is improved as a result of the smooth particle surface.

Claims (5)

Including a step of subjecting the goethite particles to a heat reduction treatment and a nitriding treatment, comprising at least iron and nitrogen as constituent elements, and having an average particle size of at least 10 to 20 nm including an Fe ₁₆ N ₂ phase, a spherical or elliptical shape, and a rare earth element, In the method for producing an iron nitride magnetic powder containing at least one element selected from the group consisting of aluminum and silicon, the goethite particles are subjected to a hydrothermal treatment before the heat reduction treatment, wherein the iron nitride magnetic powder is characterized in that Manufacturing method.   The method for producing an iron nitride magnetic powder according to claim 1, wherein the hydrothermal treatment is performed at a temperature of 100 to 200 ° C.   By containing in advance at least one element selected from the group consisting of rare earth elements, aluminum and silicon in the goethite particles before hydrothermal treatment, or in the goethite particles after hydrothermal treatment from rare earth elements, aluminum and silicon The iron nitride magnetic powder contains at least one element selected from the group consisting of rare earth elements, aluminum and silicon by depositing at least one element selected from the group consisting of: The manufacturing method of the iron nitride magnetic powder of magnetic powder of description. The average particle size produced by subjecting the goethite particles to hydrothermal treatment, and then subjecting the hydrothermally treated goethite particles to heat reduction treatment and nitriding treatment, comprising at least iron and nitrogen as constituent elements and at least Fe ₁₆ N ₂ phase An iron nitride magnetic powder containing at least one element selected from the group consisting of rare earth elements, aluminum and silicon, having a spherical or elliptical shape of 10 to 20 nm.   5. A magnetic recording medium comprising a nonmagnetic support and a magnetic layer containing the magnetic powder and a binder on the nonmagnetic support, wherein the magnetic powder is the iron nitride magnetic powder according to claim 4.