JP4984115B2 - Iron compound particle powder and magnetic recording medium using the same - Google Patents

Description

  The present invention relates to an iron compound particle powder suitable for a nonmagnetic particle powder constituting a nonmagnetic layer of a multilayer coating type magnetic recording medium, and a magnetic recording medium using the same.

  In recent years, in magnetic recording media, an increase in recording capacity and an improvement in reliability are further desired. Therefore, the structure of the tape is a multilayer in which a nonmagnetic layer (lower layer) is provided as an intermediate layer on the base film, and a magnetic layer (upper layer) is provided on the base film, instead of providing a magnetic layer directly on the conventional base film. Structured magnetic tapes have been developed. By adopting the multilayer structure, the electromagnetic conversion characteristics are improved, and a higher recording density is realized. Further, the durability of the tape is improved and the reliability is also improved.

  Household video tapes and the like are listed as the multilayer coating type magnetic recording media used in the immediate vicinity, and in particular, data storage applications are widely used recently. The increase in the amount of information has been remarkable recently, and attempts to record a large amount of information in a limited area are constantly being made, and have reached the present.

  In order to write as much information as possible on the medium, it is conceivable to increase the number of windings of the tape or to increase the density of the medium. To cope with an increase in the number of windings of the tape, it is necessary to make the tape thickness as thin as possible, and to cope with an increase in the density of the medium, it is necessary to make the recording area as narrow and small as possible. In order to construct a multilayer coating type magnetic recording medium corresponding to these, not only the performance of the magnetic material is improved, but also the nonmagnetic layer (lower layer) between the magnetic layer and the base film is strongly required to improve the characteristics.

  One of the important required characteristics required for the lower layer is “surface smoothness” when applied to the base film. By smoothing the surface of the lower layer, the upper magnetic layer becomes smoother, which makes it possible to realize a magnetic recording medium with excellent electromagnetic conversion characteristics, leading to higher recording density. It has been well known from conventional research that the surface smoothness of a magnetic layer depends on the nonmagnetic particles constituting the nonmagnetic layer immediately below the magnetic layer (for example, Non-Patent Document 1).

In Patent Document 1, hematite particles having a particle size distribution with a minor axis diameter and a geometric standard deviation value σ _g of a major axis diameter of 1.5 or less as a major axis and 1.3 or less as a minor axis are used as a lower layer material. It is described that when used, a medium having excellent surface smoothness can be obtained. Patent Documents 2, 3 and the like describe that a magnetic recording medium having excellent surface smoothness can be obtained by defining the geometric standard deviation value σ _g of the long axis length. Patent Document 4 is capable of forming an underlayer with excellent surface smoothness by mixing two types of powder groups having different average major axis lengths and deliberately degrading the particle size distribution and using it for the lower nonmagnetic layer. Is described. Patent Documents 5 and 6 describe that the particle size distribution is changed by adding two or more types of non-magnetic layers having a particle size.

JP 2000-143250 A JP-A-6-060362 JP-A-9-170003 JP 2001-160212 A JP-A-5-242455 JP-A-6-267059 IEICE Technical Report, Vol.95, No.383 (MR95), p.44-55

  In order to improve the surface smoothness of the magnetic recording medium, many of the conventional methods including Patent Document 1 are directed to a uniform particle, but the surface smoothness of the resulting medium is still unsatisfactory. Not enough. That is, there is a limit to improve the surface smoothness by homogenizing the particle size distribution, and drastic improvement by other methods is desired. Further, in the case of Patent Document 1, even if the geometric standard deviation value is set to a specific value or less, the actual surface area value when the shape is approximated from the TEM image is large (when the density is low) compared with the actual measurement. When the value of the specific surface area is high, the surface property of the medium, which is considered to be derived from the high viscosity, is deteriorated. Therefore, the effect of changing the surface properties of the particles due to the “densification” of the particles rather than the shape of the particles is taken into account, and a technique for improving the surface smoothness with only the shape of the particles is sufficient. It is hard to say that it is disclosed in

  According to Patent Document 4 that discloses a method of mixing two or more kinds of different particles, it is taught that two or more particles to be mixed preferably have a small geometric standard deviation value. Even when mixed, it is considered that the geometric standard deviation value is not so large. In the technique disclosed in Patent Document 4, although the “apparent” geometric standard deviation value is large, it cannot be said that packing is improved, and as a result, the surface smoothness of the medium surface is sufficiently improved. I can't say that.

  An object of the present invention is to provide a powder capable of stably and remarkably improving the surface smoothness of a magnetic recording medium when used in a lower layer.

  As a result of detailed studies, the inventors have found that the above object can be achieved by a powder having a long axis length or a short axis length variation somewhat increased in the particle size distribution obtained from a TEM (transmission electron microscope) image. It was. That is, the present invention provides an iron compound particle powder having a long axis geometric standard deviation greater than 1.5 determined from a TEM image, or an iron compound particle powder having a short axis geometric standard deviation larger than 1.35. More preferably, these are satisfied at the same time.

Here, the iron compound particle powder satisfies the following formula (1) when L is an average major axis length (nm) obtained from a TEM image and D is the same minor axis length (nm) , or The expressions (1) and (2) are satisfied .
[Cumulative number of particles whose major axis length is L / 2 or less] / [cumulative number of particles whose major axis length is 2L or less] ≧ 0.5 (1)
[Cumulative number of particles whose minor axis length is D / 2 or less] / [cumulative number of particles whose minor axis length is 2D or less] ≧ 0.5 (2)

  Of these, those having a powder pH of less than 8 as defined in JIS K5101, those containing rare earth elements (Y and Sc are also treated as rare earth elements) or P in the iron compound particles are suitable. It becomes a target. Examples of iron compound particles include hematite and iron oxyhydroxide. Such a nonmagnetic particle powder is suitable for a nonmagnetic layer of a coating type magnetic recording medium.

  According to the present invention, it is possible to stably improve the surface smoothness of the nonmagnetic layer (lower layer) in the multilayer coating type magnetic recording medium by using a powder that appropriately defines the distribution relating to the shape of the iron compound particles. . Thereby, the surface smoothness of the medium after the magnetic layer (upper layer) is applied is improved, and the recording density can be further improved and the reliability can be improved.

The iron compound particle powder of the present invention defines the geometric standard deviation of the major axis and the minor axis calculated using the values of the major axis length and minor axis length obtained by particle size measurement by a TEM image as described later. is there. That is,
i) The geometric standard deviation of the major axis is greater than 1.5,
ii) the minor axis geometric standard deviation is greater than 1.35;
An iron compound particle powder satisfying at least one of the following.

  Such a broad particle size distribution seems to be extremely effective in improving the surface smoothness when applied as a paint. At this time, it is considered that the effect that the small particles enter between the large particles to fill the gap increases, and as a result, the surface smoothness is improved. However, if the particle size distribution is too broad, the difference in particle size between small and large particles becomes too large, and the effect of filling the gap may be reduced. Therefore, the geometric standard deviation of the long axis exceeds 1.5 to 3 It is preferably in the range of 0.0, and more preferably 1.6 to 2.5. The minor axis geometric standard deviation is preferably in the range of more than 1.35 to 3.0, more preferably 1.4 to 2.5.

Also the particle size distribution, the average major axis length was determined L from TEM images (nm), the average minor axis length D when the (nm), the following equation (1) was filled, or even below (2) to satisfy the equation.
[Cumulative number of particles whose major axis length is L / 2 or less] / [cumulative number of particles whose major axis length is 2L or less] ≧ 0.5 (1)
[Cumulative number of particles whose minor axis length is D / 2 or less] / [cumulative number of particles whose minor axis length is 2D or less] ≧ 0.5 (2)

When the formula (1) is not satisfied, the ratio of the number of small particles is insufficient, and most of the small particles are present in the form of being simply buried in the gaps between the large particles, thereby filling the gaps. The effect of improving the surface smoothness due to is not sufficiently exhibited, and on the contrary, the surface tends to become rough. The same applies when the expression (2) is not satisfied. Or the present invention satisfies at least (1), and to satisfy the expression (1) and (2) below. In both formulas (1) and (2), the right side is more preferably at least 0.55, and even more preferably at least 0.6.

  The iron compound particle powder having such a specific particle size distribution can be obtained by subjecting the compound to pulverization in the step of pulverizing the compound. For example, when hematite is used as the iron compound particles, iron oxyhydroxide is fired in the air to form hematite, and then the fired hematite is mechanically pulverized. At that time, instead of pulverizing all of the total amount of particles, a certain amount can be pulverized, and the ratio can be variously changed to variously adjust the particle size distribution. A powder containing relatively coarse particles in the same lot can be produced in one step. In addition to that, the particle size distribution can be similarly adjusted to an arbitrary state by changing the particle crushing method (apparatus). By performing such an operation, a powder having the above specific particle size distribution can be obtained.

  This will be described more specifically. Although there is no particular limitation on the pulverizing device, a hammer-type fine powder pulverizer or a similar device is particularly preferable for obtaining the powder defined in the present invention. Specific examples include atomizers, pulperizers, circoplexes, particulate mills, micron particle mills, and the like. Among them, an atomizer capable of combining grinding with a rotating blade and grinding with a swing hammer and a pulverizer having only a swing hammer are convenient for adjusting the particle size distribution.

  In addition, it is also possible to apply those that compress and crush by applying pressure to the particles, and among these pulverizers having such properties, a roll pulverizer is particularly preferable. Among these, a roller mill that can obtain a shearing effect under a compressive force, a roll mill that causes compression fracture using a plurality of rollers, and the like are suitable. In these methods, the compressive force mainly acts on the sample and the characteristics of the particles may change. Therefore, it is preferable to use either the hammer pulverization method or the compression pulverization method, or both depending on the required characteristics, depending on the required characteristics.

In order to obtain the particles according to the present invention, it is desirable to repeat the pulverization process a plurality of times and appropriately change the situation. The following method can be illustrated. For example, the first powder is first pulverized first, and then the first pulverized powder is divided into amounts that do not exceed 80% by mass of the whole, and each divided powder (called divided powder) is divided. Regrind. Then, the final powder is obtained by mixing the re-ground powder. At that time, the particle size distribution of the final powder can be controlled by changing the amount of each divided powder to different amounts or changing the re-pulverization time for each divided powder.
When the coarsely pulverized particles are pulverized, the first pulverization is performed on an amount not exceeding 80% by mass of the whole, and then the particles are mixed again. The pulverization process is performed again for the amount not exceeding 80% by mass of the total mixed particles thus produced, and the particles are mixed in the same manner as in the first time. Such a pulverization operation may be repeated a plurality of times by repeating the pulverization operation in an amount not exceeding 80% by mass of the whole.

  In addition, as a method for obtaining particles having a large particle size distribution, another method of increasing the particle size distribution from the iron oxyhydroxide formation stage is conceivable. In this case, not only the particle size but also the shape of the particles is considered. May be exacerbated. In the present invention, variation in particle shape must be avoided in order to maintain surface smoothness, and thus such a method is not taken. Therefore, the present invention is also characterized in that no change from the iron oxyhydroxide formation stage is required in order to increase the particle distribution.

The particles according to the present invention are more suitable for a magnetic recording medium if they also have the following characteristics.
The major axis length is desirably 10 to 200 nm, preferably 20 to 180 nm, and more preferably 25 to 160 nm. This is the particle size as a result of considering the influence on the dispersibility of the particles. When the particle diameter is smaller than 10 nm, aggregation occurs and the surface property of the medium is lowered. On the other hand, when the particle diameter is large, the particles become three-dimensionally large, and in this case as well, the smoothness of the medium surface is lowered, which adversely affects the electromagnetic conversion characteristics.

BET specific surface area is preferably a 30~250m ² / g, preferably from 40 to 200 m ² / g, more preferably 50 to 150 m ² / g. When the BET value is small, it can be estimated that interparticle sintering occurs. In this case, it is not preferable to disperse the particles in the paint because the surface smoothness decreases. If inter-particle sintering has progressed, particles will remain in the form of so-called “dama” when converted into a medium paint as in the case of the above aggregation, which will have a significant adverse effect on the smoothness of the medium surface. It is not preferable. On the other hand, when the BET value is large, there is a possibility that the particles themselves are considerably finely divided and / or there are remarkably many pores on the particle surface. When the particles are finely divided, as described above, aggregation occurs when the paint is formed, and thus the smoothness of the surface cannot be maintained. Moreover, when there are many pores on the surface of the particle, bubbles remain in the pores when made into a paint, which is not preferable because it may adversely affect the storage stability of the medium.

When using hematite (α-iron oxide) as the powder of the present invention, the stearic acid adsorption amount (STA) representing the fatty acid adsorption amount is desirably 0.01 to 3.0 mg / m ² , more preferably. Is 0.01 to 2.0 mg / m ² , more preferably 0.01 to 1.0 mg / m ² . It is preferably a 0.01 to 5.0 mg / m ² in the case of using a powder based on iron oxyhydroxide as the powder of the present invention, more preferably 0.01~4.0mg / m ² More preferably, it is 0.01-3.0 mg / m < ² >. When the amount of fatty acid consumed is large, when the powder is dispersed in the paint, there may be a (neutralization) reaction between the surface of the powder particles and the fatty acid. This means that the added lubricant (fatty acid) is consumed by the neutralization reaction to the powder particles, and may not function as a lubricant. Therefore, a magnetic recording medium using a powder with a large amount of stearic acid adsorption deteriorates in storage stability, and is unsuitable for a medium for data storage. That is, the smaller the stearic acid adsorption amount as the physical property value of the powder, the better.

The TAP density is preferably 0.50 g / cm ³ or more, and more preferably 0.60 g / cm ³ or more. If the TAP density is too small, the surface smoothness of the medium when it is made into a paint and converted into a medium is deteriorated, and physical properties such as gloss are lowered, which is not preferable.

  The powder pH of the particles is preferably weakly basic or acidic. When examining the familiarity of the non-magnetic particles with the resin, the pH of the particles is an important factor. That is, the pH change of the lower layer powder affects the adsorption behavior of fatty acids present in the resin. More specifically, fatty acids called lubricants are usually added to a coating material containing and dispersing a lower layer powder and a magnetic powder for producing a coating type magnetic recording medium. This lubricant serves to reduce the interference between the tape surface and the head in the state of a coating film, and has the function of improving the durability of the tape. As the lubricant, fatty acids that are acidic substances are generally used. Therefore, when the powder pH of the lower layer powder is on the alkali side, a neutralization reaction with the acidic lubricant proceeds in the paint. If this reaction occurs, the original lubricating action of the lubricant cannot be achieved. For this reason, it is desirable to adjust the powder for the lower layer so that it has a surface property that does not cause adsorption with fatty acids, that is, the pH of the powder surface is acidic so that it has a surface property similar to that of fatty acid. It is desirable to adjust to.

  However, if the particles are too strongly acidic, a harmful effect occurs. For example, when a strongly acidic lower layer material is used, the metal magnetic powder in the upper layer is corroded, and there is a possibility that the storage stability as a medium may be significantly adversely affected. Considering the balance between the above characteristics and the corrosion of the upper layer and other constituent materials, the powder pH is preferably 3 or more, and more preferably 4 or more. The range of the preferable powder pH of the powder for lower layers of this invention is 3-9, More preferably, it is 3-7, More preferably, it is the range of 4-7.

  The powder pH is measured according to the boiling method described in JIS K5101, which shows methods for measuring various physical properties of pigments. By using the boiling method, carbon dioxide gas that has been exposed to the atmosphere and adsorbed on the powder surface can be volatilized and removed, so that the powder pH of the powder itself can be known, which is preferable.

  In addition, when the iron oxyhydroxide which coat | covered the phosphorus compound is baked, the powder particle which satisfy | fills said conditions is easy to be obtained. In this case, the lower layer material for a magnetic recording medium is obtained which does not deteriorate the action of the lubricant, is effective in improving the compatibility (compatibility) with the paint, and has improved surface smoothness. Moreover, it has been confirmed that the tape has a favorable effect on the durability of the tape, and it can be said that it is preferable to contain phosphorus as the particle composition.

  There are other reasons why inclusion of phosphorus improves the properties of the media. When iron oxyhydroxide is baked to form iron oxide, it is normal that sintering occurs between the particles or the shape of the particles themselves collapses. As a method for improving this point, an anti-sintering agent typified by Al or Si is originally incorporated in the iron oxyhydroxide or coated on the surface. When obtaining iron oxide from iron hydroxide, shape breakage occurs, and it has been found by conventional studies on the applicant of the present application that the original particle shape of iron oxyhydroxide cannot be maintained. The addition of P makes it easy to ensure the independence of the particles by giving the particles an anti-sintering effect, and the surface smoothness of the medium is ensured when the medium is formed. For this reason, the addition of P is extremely effective.

  Examples of phosphorus compounds that can be used include orthophosphoric acid, metaphosphoric acid, diphosphoric acid, and other phosphates such as ammonium phosphate and ammonium dihydrogen phosphate. The phosphorus coating amount is preferably in the range of 0.1 to 5.0% by mass as the content of phosphorus element in iron oxide, regardless of the type of phosphorus compound. If it is less than 0.1% by mass, the effect of preventing sintering by the phosphorus coating becomes insufficient and interparticle sintering occurs, so that a lower layer with excellent surface smoothness cannot be obtained, and sufficient coating strength can be obtained. I can't. On the other hand, if the amount of phosphorus exceeds 5.0% by mass, the effect of preventing sintering can be obtained sufficiently, but it becomes iron oxide powder with a high specific surface area. In addition to being unsuitable as a powder, free phosphorus compounds are produced when a coating is formed. These free phosphorus compounds cause chemical reactions with the components in the resin, resulting in phosphorus compounds having different compositions. Such a composition is not preferred because it may adversely affect the storage stability of the coating film.

In the present invention, as a means for adding P, a method of adhering a phosphorus compound to flat needle-like iron oxyhydroxide as a starting material can be employed. A rare earth element can also be added together with P. In that case, a method of adhering a phosphorus compound and a rare earth element compound to a flat needle-like iron oxyhydroxide as a starting material can be employed. Here, Y and Sc are also treated as rare earth elements. Coating these compounds with flat needle iron oxyhydroxide can be carried out as follows.
A suspension containing ferric hydroxide colloid is formed at 10 to 90 ° C. by adding 1.0 to 3.5 equivalents of an aqueous alkali hydroxide solution to the ferric salt aqueous solution, and then 2 to 20 hours. When using a method of producing iron oxyhydroxide powder by hydrolysis after aging, the liquid in which iron oxyhydroxide after the hydrolysis reaction is dispersed and suspended is brought into a vigorously stirred state. In addition, an aqueous alkali hydroxide solution of an equivalent amount or more is added to the ferrous salt aqueous solution to produce a suspension containing ferrous hydroxide colloid, and an oxygen-containing gas is passed through the liquid at a temperature of pH 11 to 80 ° C. Then, when using the method of generating iron oxyhydroxide by causing an oxidation reaction, the liquid in which the iron oxyhydroxide after the oxidation reaction is dispersed and suspended is vigorously stirred.
By adding a phosphorus-containing aqueous solution having a specified concentration to the vigorously stirred liquid, flat needle-like iron oxyhydroxide coated with a phosphorus compound can be obtained. Further, stirring is continued for a predetermined time, and when a specified amount of a sulfuric acid aqueous solution of rare earth elements (Y and Sc are also treated as rare earth elements) is added, flat needle iron oxyhydroxide coated with a phosphorus compound and a rare earth element compound can be obtained. it can. In this way, when adding a liquid containing phosphorus or a rare earth element, it is preferable from the standpoint that the adhesion to the particles can be made uniform by increasing the strength of stirring and keeping the stirring time after the addition long. .

  In addition to the above-described deposition treatment, a method of depositing a phosphorus compound or a rare earth element compound on iron oxyhydroxide prepared in advance may be employed. In this case, iron oxyhydroxide powder as a starting material is added to water and suspended by vigorous stirring. Phosphorus or a rare earth element is added to the vigorously stirred liquid as described above. The deposition process can be carried out by adding the contained liquid. However, in this case, since the liquid is suspended in pure water, the liquidity is near neutral. In that case, since a hydroxide is not produced | generated at the time of deposition of a rare earth element compound, it may be unable to deposit. At that time, it is necessary to add a suitable alkali (the alkali may be an alkali hydroxide or an alkali carbonate) to the liquid, and to carry out the treatment after making the suspension alkaline.

  The rare earth element that can be used is not particularly limited and the effect of the present invention can be confirmed by using any element. However, when Y, La, Sc, or Nd is used among the rare earth elements, sintering with phosphorus is prevented. This is preferable because the effect is remarkable. The content of rare earth elements (denoted as R) in iron oxide is an atomic weight percentage with respect to iron, and R / Fe is preferably 0.1 to 10 at%. If the content is less than 0.1 at%, the effect of preventing sintering by rare earths is insufficient, so that the lower layer is not excellent in surface smoothness, and sufficient coating strength cannot be obtained. On the other hand, if it exceeds 10 at%, aggregation between particles is likely to occur due to the influence of the deposited rare earth element, and inter-particle sintering occurs at the time of firing in the subsequent process, which is not preferable.

  Conventionally, there has been an attempt to improve the sintering prevention effect by solid-dissolving Al or the like in iron oxyhydroxide, which is a precursor of iron oxide, but other elements are dissolved in the interior. Then, the dehydration temperature from iron oxyhydroxide to iron oxide easily shifts to the high temperature side, and it is necessary to perform firing at a higher temperature. With iron oxyhydroxide whose surface is coated with phosphorus and further rare earth elements, the dehydration reaction proceeds from a low temperature range, so the firing temperature can be set low, and as a result, sintering between particles is suppressed, and the particle shape Can be maintained.

  If other elements (for example, Al or Si) coexist in the liquid during the production of iron oxyhydroxide, these other elements act as an iron oxyhydroxide growth inhibitor and produce a deformed iron oxyhydroxide. In some cases, the shape is not recovered even if it is baked, so that non-uniform particles are formed as the particle shape. However, in the present invention, since it is not necessary to dope such other elements to form iron oxyhydroxide, the precursor iron oxyhydroxide is also a needle-like shape and is uniform in shape and fired. It is possible to obtain iron oxide powder having a uniform needle-like shape that maintains the shape of the homogeneous needle-like iron oxyhydroxide that is a precursor.

  The iron oxide powder obtained in this way has good dispersion with respect to the resin component for forming the lower layer since sintering is prevented. In addition, since each particle is needle-shaped, it becomes easy to form a wide surface by overlapping the particles at the time of applying the tape, which makes the surface smooth and further reduces the component in the direction perpendicular to the base film surface. In addition, since it is densely oriented in the tape in-plane direction, there is an effect that the tape strength is improved in addition to the improvement of the surface smoothness. Furthermore, phosphorus present on the surface of the acicular iron oxide particles, and further rare earth elements modify the surface properties of the iron oxide particles to improve the dispersibility in the resin and the adhesion to the resin. Contributes to improving coating strength.

In a magnetic recording medium having a multilayer structure, when a lower layer is formed using a non-magnetic powder (for example, hematite) having a needle shape or a needle-like shape according to the present invention, a magnetic powder, a coating composition, which forms an upper layer, The following can be illustrated as a base film.
The magnetic powder constituting the upper layer is Co: 5 to 50 at.%, Al: 0.1 to 50 at.%, Rare earth elements (Y and Sc are also treated as rare earth elements): 0.1 to 30 at.% Periodic Table 1a group elements (Li, Na, K, etc.): 0.05% by mass or less, Periodic Table 2a Group elements (Mg, Ca, Sr, Ba, etc.): 0.1% by mass or less, Ferromagnetic powder with most of the remainder as iron, average major axis length of 10 to 200 nm, specific surface area of 30 to 150 m ² / g when measured by BET method, X-ray crystal grain size (Dx) of 5 to 20 nm Needle-shaped ferromagnetic powder satisfying the following shape condition: coercive force (Hc) of 79.6 to 238.9 kA / m (1000 to 3000 Oe), saturation magnetization (σs) of 10 to 200 Am ² / kg Examples are magnetic powders having magnetic properties and shapes of (10-200 emu / g) Kill.

  Examples of the base film include polyesters such as polyethylene terephthalate and polyethylene 2-6-naphthalate, polyolefins such as polypropylene, cellulose derivatives such as cellulose triacetate and cellulose diacetate, and plastics such as polyamide and polycarbonate.

  The magnetic coating component for forming the upper magnetic layer includes 100 parts by mass of metal magnetic powder, 0.5 parts by mass of carbon black, 5 parts by mass of alumina, 12 parts by mass of vinyl chloride resin (MR-110), and polyurethane resin (UR-8200). ) Magnetic paint having a composition comprising 8 parts by mass, 0.3 part by mass of stearic acid, 0.3 part by mass of acetylacetone, 107 parts by mass of methyl ethyl ketone, and 107 parts by mass of cyclohexanone.

As the nonmagnetic coating component forming the lower nonmagnetic layer, 80 parts by mass of acicular Fe ₂ O ₃ (hematite) powder, 0.5 part by mass of carbon black, 1 part by mass of alumina, vinyl chloride resin (MR-110) 12 Examples thereof include a non-magnetic paint comprising 8 parts by mass of polyurethane resin (UR-8200), 90 parts by mass of methyl ethyl ketone, 90 parts by mass of cyclohexane, and 100 parts by mass of toluene.

  At the time of preparing the upper layer or the lower layer, a material is blended so as to have a predetermined composition, and then kneaded and dispersed using a kneader and a sand grinder to create a magnetic or nonmagnetic coating material. The application to these non-magnetic supports is preferably performed by a so-called wet-on-wet method in which the upper magnetic layer is applied as quickly as possible while the lower layer is wet. It doesn't matter. As a wet-on-wet multilayer coating method, a known method can be used.

An evaluation method of each characteristic value employed in each example and comparative example described later will be described.
[Long axis length and short axis length of particles]
These were obtained by observation with a transmission electron microscope (TEM). Specifically, it is as follows.
The observation sample was prepared by adding about 0.005 g of a measurement sample into 10 mL of a 2% collodion solution, applying a dispersion treatment, and dropping 1 to 2 drops of the solution into water to form a collodion film. This was performed by adhering to one side of the grid and allowing it to dry naturally, followed by carbon deposition to strengthen the coating.

  About this sample, TEM (the JEOL Co., Ltd. make, JEM-2010) was used, and the observation in a bright field was performed with the acceleration voltage of 200 kV. The average major axis length and minor axis length values are obtained by printing a TEM photograph (50000 times) stretched three times in the vertical and horizontal directions, respectively, and the major axis length for each of 500 or more particles shown in this photograph. The short axis length was measured, and the average value was calculated. However, the particles present on the electron micrographs are not only monodispersed particles, but also have various modes such as particles that are bonded (sintered, continuous crystal) between particles, and overlapping particles. In performing the measurement, it is necessary to establish a reasonable and reasonable standard in advance which particle is to be measured and how. The standard was as follows.

-Long axis length and short axis length measurement standards-
The long axis length refers to a value obtained by measuring the longest portion in the longitudinal direction of the particle. The minor axis length is a value obtained by measuring the longest portion in the width direction of the particle.

Among the particles shown on the transmission electron micrograph, the selection criteria for the particles to be measured were as follows.
[1] Do not measure particles that are partially outside the field of view of the photograph.
[2] Measure particles that are well-defined and isolated.
[3] Measure particles that are not acicular, but are independent and can be measured as single particles.
[4] Particles that overlap each other but whose boundaries are clear and whose shape can be determined are measured as individual particles.
[5] Particles that overlap but do not have clear boundaries and do not know the full shape of the particle are not measured as the particle shape cannot be determined.

The presence or absence of bonding between the particles, that is, whether the particles were just overlapping or sintered was determined as follows.
(B) Prepare multiple photos with different focus, and from the photo where the fringe (note: the boundary line where the substance changes in the bright field of the electron microscope) often appears, The boundary part was judged.
(B) In the overlapping particle, observe the part where the outlines of the two intersect, and if the outlines of both of them intersect with each other in a rounded shape, it is judged that they are sintered. If they intersect at a point with an angle regardless of the other contour line, it is judged that they only overlap.
(C) When it is difficult to judge whether the boundary exists or not, it is not judged that inter-particle sintering has occurred, and it is measured as individual particles, and the particles are greatly estimated. .

[Geometric standard deviation of major and minor axes]
There are roughly two methods for calculating the geometric standard deviation indicating the particle size distribution of the major axis length and minor axis length. One is to convert the major and minor axis lengths to natural logarithm and to calculate the standard deviation mechanically. The other is to log normal probability paper, and the major axis or minor axis length is the horizontal axis. In other words, this is a method of calculating by drawing a graph with the cumulative number of particles on the vertical axis. Here, the latter method was adopted. Specifically, after magnifying the TEM photograph of the particle and measuring the particle size (major axis length and minor axis length) shown in the field of view of the photograph, comparison with the magnification at the time of photographing and magnification Based on the statistical method on the log-normal probability paper, the actual particle diameter calculated by the above is the particle diameter on the horizontal axis, and the cumulative number of particles belonging to each of the predetermined particle diameter sections on the vertical axis (under accumulated sieve) Is plotted as a percentage. And the value of the particle diameter corresponding to the number of particles of 50% and 84.13% is read from this graph,
Geometric standard deviation value = particle diameter at 84.13% under integrated sieve / particle diameter at 50% under integrated sieve (geometric mean diameter)
The value calculated according to Here, the geometric standard deviations of the long axis and the short axis obtained in this way are indicated as Lσ _g and Dσ _g , respectively.

[Particle size distribution]
The particle size distribution was prepared as follows based on the measurement data of the major axis length of 500 or more particles by the transmission electron microscope described above. The number of particles belonging to each particle size segment divided every 0.5 nm was determined, and the ratio (%) of the number of particles belonging to each particle size segment to the total number of measured particles, that is, the frequency (%) was calculated. Based on the data of the particle size distribution, the value on the left side of the equations (1) and (2) was determined. Here, these values are referred to as an X value and a Y value as follows.
X value = [cumulative number of particles whose major axis length is L / 2 or less] / [accumulated number of particles whose major axis length is 2L or less]
Y value = [cumulative number of particles whose minor axis length is D / 2 or less] / [cumulative number of particles whose minor axis length is 2D or less]

〔Specific surface area〕
It was measured by the BET method.
[TAP density]
The measurement was performed by putting the magnetic powder in a glass sample cell (5 mm diameter × 40 mm height) and performing the test 200 times with a tap height of 10 cm.
[Stearic acid adsorption amount (STA)]
After the sample powder was dispersed in a 2% stearic acid solution (solvent is MEK), the sample powder was settled by a centrifuge, and the concentration of the supernatant was determined to calculate the amount of adsorption per specific surface area.

[Viscosity of paint]
Using a viscometer (R110 type) manufactured by Toki Sangyo Co., Ltd., the viscosity of the nonmagnetic paint in which the sample powder was dispersed was measured.

The following evaluation was performed on an intermediate product stage tape (hereinafter referred to as “nonmagnetic layer tape”) in which a nonmagnetic coating material in which sample powder was dispersed was applied to a base film to form a nonmagnetic layer. [Surface smoothness]
−Surface roughness−
Evaluation was performed by measuring the arithmetic average roughness Ra of the coating film surface using a three-dimensional fine shape measuring machine (ET-30HK) manufactured by Kosaka Laboratory. Ra value was displayed in nm.
-Glossiness-
About the nonmagnetic layer tape, the glossiness (angle 60 degrees) of the nonmagnetic layer coating film surface was measured with the gloss meter.

[Steel ball sliding]
Affix the tape to the glass plate with the coating surface facing up, place the glass plate on a horizontal surface, place a 5 mm diameter stainless steel ball on the coating surface of the tape, and apply a 5 g load in the vertical direction. To. From this state, the glass plate is reciprocated horizontally at a low speed of 2320 mm / min and one way of 20 mm. In this sliding motion, the number of sliding times until the coating film peeled off was measured.

Next, the following evaluation was performed on the finished product tape in which the magnetic layer (upper layer) was formed on the nonmagnetic layer (lower layer) of the nonmagnetic layer tape.
[Electromagnetic conversion characteristics]
The electromagnetic characteristics were measured by attaching an interactive head and an MR head to a drum tester, writing with the interactive head, and reproducing with the MR head. As a recording input, a rectangular wave is amplified by a function generator, and a digital signal is recorded at a recording wavelength of 0.35 μm. Further, the output from the MR head was amplified by a preamplifier and then input to the spectrum analyzer. The carrier value of 0.35 μm is set as the output C, and the integral value of the value calculated as the integral value of the value obtained by subtracting the output and system noise from the spectral component corresponding to the recording wavelength or more when the rectangular wave is written is the particle. It is calculated as a characteristic noise Np value. Further, the carrier-to-particle noise ratio is calculated by taking the difference between the two.
[Steel ball sliding]
The measurement was performed in the same manner as for the nonmagnetic layer tape described above.
〔Young's modulus〕
It measured according to the measuring method of the elastic modulus of a magnetic tape described in JIS-X-6172-2000 and others. That is, a tape test piece of 178 mm or more is fixed at a length of 102 mm, and the fixing jig is pulled at a speed of 5 mm / min. The elastic modulus was calculated from the slope of tension when stretched 0% and 1%.
[Surface smoothness]
The surface roughness and gloss were measured by the same method as the nonmagnetic layer tape described above. Furthermore, the surface roughness by a stylus type was measured.
-Surface roughness by stylus type-
JIS-X-6172-2000 7.9. As described elsewhere, a stylus with a radius of 12.5 μm was measured with a load of 20 mg and a cutoff of 254 μm. This corresponds to the surface roughness expressed in units of μm in the table below.

[Cupping]
As described in JIS-X-6172-2000, etc., the tape is cut to 1.0 ± 0.1 mm and hung so that both sides of the tape are exposed to the atmosphere of the test environment, and left for 3 hours or more. A test piece having a length of 25 mm is cut from the central portion of the tape. The test piece is placed in a cylinder with a height of 25 mm or more and an inner diameter of 13.0 ± 0.2 mm. The test piece is placed on an optical comparator, and both edges of the test piece are aligned with the cross hair of the comparator. It was determined by measuring the distance to the center.

[Elongation load]
It was determined according to a method described in JIS-X-6172-2000 and others. That is, a tape test piece having a length of 178 mm or more measured by a tensile tester capable of displaying a load with an accuracy of 2% is attached to a fixed jig so as to have a length of 102 mm. The film is stretched at a speed of 51 mm / min until an elongation of at least 10% is reached. The tension when the elongation is 3% is defined as the elongation load.

[Example 1]
(Production of acicular iron oxyhydroxide)
2000 g of pure water was put in a 5 L reaction vessel and kept at 30 ° C. using a temperature controller, and 1000 g of a 4.50 mass% ferrous sulfate aqueous solution was added thereto. Subsequently, 1000 g of a 5% by mass aqueous ammonium sulfate solution was added. After that, sodium carbonate is added in an amount equivalent to 5.0 equivalents in terms of CO ₂ / Fe, and then the temperature is raised to 40.0 ° C., and Air is aerated at 100 mL / min for 15 minutes to form nucleus crystals. It was. Subsequently, the temperature was raised to 47.0 ° C., and Air was kept flowing at 100 mL / min. After 90 minutes, 300 g of Y solution (a solution of yttrium oxide dissolved in dilute sulfuric acid, containing 2.5% by mass as Y) was added, and the reaction was further continued to complete the oxidation. The completion of the oxidation is confirmed by confirming that the solution does not change by extracting a small amount of the filtrate of the reaction, acidifying the solution with hydrochloric acid, adding a small amount of iron potassium hexacyanoate solution, and then completing the reaction.

(Sintering prevention treatment of iron oxyhydroxide)
After the iron oxyhydroxide obtained above is separated by filtration, 1.5 L of an orthophosphoric acid aqueous solution diluted to a P concentration of 0.01 mol / L is added to the cake to deposit phosphorus on the slurry surface. It was. Then, after washing with 50 L or more of pure water, heat treatment was performed at 300 ° C. to obtain an iron compound powder composed of particles on which phosphorus and Y were adhered. Drying was performed until the water content was 0.5 mass% as measured by the Karl Fischer method.

(Α-Iron oxide treatment)
After the sintering-prevented treatment and the heat-treated powder at 300 ° C. are introduced into an air-permeable bucket, the powder is placed in a through-type firing furnace and fixed, and then the temperature is increased to 590 ° C. at 60 ° C./min in nitrogen. Warm up. Thereafter, oxygen is introduced to form a mixed gas of nitrogen and oxygen, and water vapor is adjusted so as to have a concentration of 5 vol% with respect to the total gas ratio, and then heated for 20 minutes to produce iron compound particles. Was changed to α-iron oxide (haematite, mainly α-Fe ₂ O ₃ ) to obtain α-iron oxide powder.

(Crushing process of α-iron oxide)
100 g of α-iron oxide (hematite) produced under the above conditions was pulverized using an impact mill. First, the entire amount was pulverized (initial pulverization). Next, 1/16 (6.3 g) of the whole powder was sampled from the powder after the initial pulverization, and the pulverization treatment was performed again on the collected powder (first charge for re-grinding). Next, 1/16 (6.3 g) was newly fractioned from the initial total weight (100 g) from the powder that had been initially ground, and the ground powder was again ground. (Second charge of regrind). Finally, all the remaining powder (that is, 14/16 (87.5 g) with respect to the initial total weight (100 g)) was subjected to pulverization again (third charge for re-grinding). . Then, three types of powders that were reground by each charge were mixed to obtain a sample powder having a total amount of 100 g.

(Manufacture of upper and lower layer paints)
The paint of the upper magnetic layer and the lower nonmagnetic layer having the following composition was prepared.
<Magnetic layer (upper layer) paint>
Ferromagnetic metal powder (Co / Fe = 24 (at%), coercive force Hc = 160.8 kA / m, average major axis length 35 nm, crystallite size D110 = 12 nm, σs = 102 A · m ² / kg, Δσs = 8%): 100 parts Vinyl chloride copolymer, Nippon Zeon Co., Ltd. MR-555: 12 parts Polyurethane resin, Toyobo Co., Ltd. UR-8200: 8 parts α-Al ₂ O ₃ (alumina ): 5 parts Carbon black: 0.5 parts Cyclohexanone, methyl ethyl ketone, toluene (mixing ratio = 1: 1: 1): 214 parts Acetylacetone: 0.3 parts N-butyl stearate: 0.3 parts

  A mixture comprising 194 parts of a mixed solvent obtained by mixing 12 parts of vinyl chloride resin, 8 parts of polyurethane resin, and cyclohexanone / methyl ethyl ketone / toluene was dispersed using a kneader for 5 hours. Further, 20 parts of a mixed solvent was mixed and dispersed for another 20 minutes to prepare a magnetic paint.

<Non-magnetic layer (lower layer) paint>
Sample powder (here, acicular hematite obtained above): 80 parts Vinyl chloride copolymer, MR-110 manufactured by Nippon Zeon Co., Ltd .: 12 parts Polyurethane resin, UR-8200 manufactured by Toyobo Co., Ltd. : 8 parts α-Al ₂ O ₃ (alumina): 1 part Carbon black: 0.5 part Cyclohexanone, methyl ethyl ketone, toluene (mixing ratio = 1: 1: 1): 280 parts N-butyl stearate: 0.3 parts

  A mixture composed of 12 parts of vinyl chloride resin, 8 parts of polyurethane resin, and a mixed solvent obtained by mixing cyclohexanone / methyl ethyl ketone / toluene was dispersed using a kneader for 5 hours. Further, 20 parts of a mixed solvent was mixed and dispersed for another 20 minutes to prepare a nonmagnetic paint.

(Manufacture of non-magnetic layer tape (intermediate product))
The obtained coating solution for the nonmagnetic layer was applied onto a polyethylene terephthalate support having a thickness of 7 μm using an applicator so as to have a thickness of 3 μm after drying, and calendering was performed. An underlayer medium of layers was obtained.

(Manufacture of multilayer coating type magnetic recording media (finished product))
While the coating layer for nonmagnetic layer is still wet, wet simultaneous multilayer coating is performed so that the dry thickness of the magnetic layer is 0.15 μm thereon, and both layers are still wet. Longitudinal orientation was applied through an orientation device. In this case, the orientation magnet is set to 5500 kilogauss. After that, a magnetic tape was obtained by using a calendar device composed of a metal roll.
(Characteristic evaluation)
The obtained sample powder was a nonmagnetic powder having the following characteristics.
・ Long axis length: 40nm
TAP density: 0.69 g / cm ³
-Geometric standard deviation of long axis: 1.98
・ Geometric standard deviation of short axis: 1.53
・ X value: 0.73
・ Y value: 0.64
Tables 1 and 4 show the characteristics of the sample powder other than those described above, and the characteristics of the tape manufactured using the powder. In Table 1, the columns of the first processing, the second processing, and the third processing of the pulverization processing amount mean the processing amounts in the first charge, the second charge, and the third charge of re-pulverization (Example of Table 1). 2-24, Examples 35-58 in Table 2 and Comparative Examples 4-9, 13-18, 22-27, 31-36 in Table 3).

[Example 2]
In the pulverization process, the amounts of powder used for the first charge, the second charge, and the third charge of re-grinding are respectively 1/16 (6.3 g), 1/8 (12.6 g), and the remaining amount. It was the same as Example 1 except having set it to 13/16 (81.3g).
The obtained sample powder was a nonmagnetic powder having the following characteristics.
・ Long axis length: 43nm
TAP density: 0.73 g / cm ³
-Geometric standard deviation of long axis: 1.87
・ Geometric standard deviation of short axis: 1.51
・ X value: 0.68
・ Y value: 0.62
Tables 1 and 4 show the characteristics of the sample powder other than those described above, and the characteristics of the tape manufactured using the powder.

Example 3
In the pulverization process, the amounts of powder used for the first charge, the second charge, and the third charge of re-grinding were respectively 1/16 (6.3 g), 1/4 (25 g), and the remaining 11 / 16 (68.8 g), except that the same as Example 1.
The obtained sample powder was a nonmagnetic powder having the following characteristics.
・ Long axis length: 49nm
TAP density: 0.63 g / cm ³
-Geometric standard deviation of long axis: 1.72
・ Geometric standard deviation of short axis: 1.52
・ X value: 0.63
・ Y value: 0.57
Tables 1 and 4 show the characteristics of the sample powder other than those described above, and the characteristics of the tape manufactured using the powder.

[Examples 4 to 12]
In the pulverization treatment, the same procedure as in Example 1 was conducted except that the amounts of the powders used for the first charge, the second charge, and the third charge of the pulverization were changed to the ratios shown in Table 1, respectively. Tables 1 and 4 show the characteristics of the sample powder and the tape manufactured using the sample powder.

[Examples 13 to 24]
In the pulverization process, Examples 1 to 12 were repeated except that the pulverizer was changed from an impact mill to a roll crusher. Tables 1 and 4 show the characteristics of each sample powder and the tape manufactured using the sample powder.

[Examples 25-29]
Example 1 was repeated except that the pulverization treatment was changed as follows.
The total amount of 100 g of α-iron oxide produced under the conditions of Example 1 was pulverized with a roll crusher (initial pulverization). Next, a part of the powder after the initial pulverization was separated, and the pulverized treatment was performed again with an impact mill (first charge for re-pulverization). Finally, all of the remaining powder after the initial pulverization (that is, the powder remaining after the above-described sorting) was pulverized again with an impact mill (second charge for re-pulverization). Then, the above-mentioned two kinds of powders that had been reground were mixed to obtain a sample powder having a total amount of 100 g.
Tables 1 and 4 show the characteristics of each sample powder and the tape manufactured using the sample powder. In this case, the columns of the first processing, the second processing, and the third processing of the pulverization amount in Table 1 mean the processing amounts in the first charge and the second charge of the first pulverization and the second charge, respectively (Table 2). The same in Examples 59 to 63).
[Examples 30 to 34]
Example 1 was repeated except that the pulverization treatment was changed as follows.
A part of 100 g (initial powder) of α-iron oxide produced under the conditions of Example 1 was fractionated, and this fractionated product was pulverized by an impact mill (first pulverized first). charge). Next, the remaining initial powder was pulverized by an impact mill (second charge for initial pulverization). Thereafter, the two kinds of powders after the initial pulverization were mixed, and all of the mixed powders were pulverized again with a roll crusher (re-pulverization). A total amount of 100 g of powder that had been reground was used as sample powder.
Tables 1 and 4 show the characteristics of each sample powder and the tape manufactured using the sample powder. In this case, in Table 1, the columns of the first processing, the second processing, and the third processing of the pulverization processing amount mean the processing amounts in the first charge, the second charge, and re-pulverization of the initial pulverization, respectively (Table 2). The same in Examples 64-68).

[Examples 35 to 68]
Iron oxyhydroxide powder not subjected to the heat treatment at 300 ° C. as the first pulverized product (After the sintering prevention treatment of Example 1 was performed, a drying treatment of 180 ° C. × 12 h was performed in the atmosphere. In the same manner as in Example 1 to 34, but the process of Examples 1 to 34 was repeated. Tables 2 and 5 show the characteristics of each sample powder and the tape manufactured using the sample powder.

[Comparative Example 1]
In the pulverization process, the same procedure as in Example 1 was conducted except that the sample powder was obtained by performing the operation of pulverizing the entire amount (100 g) of the object to be pulverized with an impact mill only once.
The obtained sample powder was a nonmagnetic powder having the following characteristics.
・ Long axis length: 93nm
TAP density: 0.58 g / cm ³
-Geometric standard deviation of the long axis: 1.07
・ Geometric standard deviation of short axis: 1.23
・ X value: 0.48
・ Y value: 0.49
Tables 3 and 6 show the characteristics of the sample powder other than those described above and the characteristics of the tapes manufactured using the same.

[Comparative Example 2]
In the pulverization process, the entire amount (100 g) of the object to be pulverized is first pulverized by impact mill (initial pulverization), and then all the powders after the initial pulverization are pulverized again (re-pulverization). The procedure was the same as Example 1 except that sample powder was obtained.
The obtained sample powder was a nonmagnetic powder having the following characteristics.
・ Long axis length: 75nm
TAP density: 0.55 g / cm ³
-Geometric standard deviation of long axis: 1.05
・ Geometric standard deviation of short axis: 1.23
・ X value: 0.36
・ Y value: 0.48
Tables 3 and 6 show the characteristics of the sample powder other than those described above and the characteristics of the tapes manufactured using the same. In this case, in Table 3, the columns of the first processing and the second processing of the pulverization processing amount mean the processing amounts in the initial pulverization and re-pulverization, respectively.
[Comparative Example 3]
In the pulverization process, the entire amount (100 g) of the material to be pulverized is first pulverized with an impact mill (first pulverization), and then all the powder after the initial pulverization is pulverized again (second pulverization), Finally, the same procedure as in Example 1 was performed, except that the sample powder was obtained by subjecting all of the powders that had undergone the second pulverization to pulverization again (third pulverization). Tables 3 and 6 show the characteristics of each sample powder and the tape produced using the sample powder. In this case, in Table 3, the columns of the first processing, the second processing, and the second processing of the pulverization processing amount mean the processing amounts in the first pulverization, the second pulverization, and the third pulverization, respectively.

[Comparative Examples 4 to 9]
The processing amount of the powder to be crushed was the same as in Example 1 except that the conditions shown in Table 3 were used. Tables 3 and 6 show the characteristics of each sample powder and the tape produced using the sample powder.
[Comparative Examples 10-18]
The processes of Comparative Examples 1 to 9 were repeated in the same manner except that the apparatus for performing the pulverization process was changed to a roll crusher. Tables 3 and 6 show the characteristics of each sample powder and the tape produced using the sample powder.

[Comparative Examples 19 to 36]
Iron oxyhydroxide powder not subjected to the heat treatment at 300 ° C. as the first pulverized product (After the sintering prevention treatment of Example 1 was performed, a drying treatment of 180 ° C. × 12 h was performed in the atmosphere. The process of Comparative Examples 1-18 was repeated similarly except having employ | adopted. Tables 3 and 6 show the characteristics of each sample powder and the tape produced using the sample powder.

As can be seen from the above results, the surface roughness of the example having a large major axis geometric standard deviation Lσ _g and a minor axis geometric standard deviation Dσ _g is lower than that of the comparative example having a small value. Thus, a medium with improved surface smoothness could be obtained.
Further, by performing re-grinding on the whole preparative material of 80% by mass or less, a particle size distribution having the X value and Y value larger than 0.5 can be obtained. In addition, surface smoothness and accompanying property improvements are achieved.

More specifically, the change when the atomizer crushing strength is changed can be confirmed by comparing Examples 1 and 3 and Comparative Examples 1 and 2. Moreover, it turns out that the particle | grains which satisfy | fill this range can form the lower layer single layer excellent in surface smoothness.
In Comparative Example 1, pulverization was always performed in a strong state. In such a case, as a result, the geometric standard deviation value as shown in the publicly known document is shown, but the medium is slightly inferior in smoothness.

Claims (10)

An iron compound particle powder having a geometric standard deviation of a major axis obtained from a TEM image larger than 1.5 , and when L is an average major axis length (nm) obtained from a TEM image, the following equation (1) is satisfied. Iron compound particle powder.
[Cumulative number of particles whose major axis length is L / 2 or less] / [cumulative number of particles whose major axis length is 2L or less] ≧ 0.5 (1) An iron compound particle powder having a minor axis geometric standard deviation obtained from a TEM image larger than 1.35 , and when L is an average major axis length (nm) obtained from the TEM image, the following equation (1) is satisfied. Iron compound particle powder.
[Cumulative number of particles whose major axis length is L / 2 or less] / [cumulative number of particles whose major axis length is 2L or less] ≧ 0.5 (1) An iron compound particle powder having a major axis geometric standard deviation obtained from a TEM image larger than 1.5 and a minor axis geometric standard deviation larger than 1.35 , wherein L is an average major axis length obtained from the TEM image. An iron compound particle powder satisfying the following formula (1) when (nm).
[Cumulative number of particles whose major axis length is L / 2 or less] / [cumulative number of particles whose major axis length is 2L or less] ≧ 0.5 (1) The iron compound particle powder according to any one of claims 1 to 3, which satisfies the following formula (2) when D is an average minor axis length (nm) determined from a TEM image.
[Cumulative number of particles whose minor axis length is D / 2 or less] / [cumulative number of particles whose minor axis length is 2D or less] ≧ 0.5 (2) The iron compound particle powder according to any one of claims 1 to 4, wherein the iron compound particle contains a rare earth element (Y is also treated as a rare earth element). The iron compound particles according to any one of claims 1 to 5, wherein the iron compound particles contain P. The iron compound particle powder according to any one of claims 1 to 6, wherein the iron compound particle is hematite. The iron compound particle powder according to any one of claims 1 to 7, wherein the powder is a non-magnetic particle powder for a magnetic recording medium. The iron compound particle powder according to any one of claims 1 to 7, wherein the powder is a nonmagnetic particle powder used for a nonmagnetic layer of a coating type magnetic recording medium. A magnetic recording medium using the iron compound particle powder according to claim 8 or 9 .