JP2005340672A - Magnetic powder for magnetic recording medium, and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic powder suitable for an application type super-high density magnetic recording medium and to provide a manufacturing method thereof. <P>SOLUTION: The magnetic powder having a specific surface area of 40-100 m<SP>2</SP>/g, the percentage of water of 0.9-4 mass%, and a variation coefficient for the percentage of water of the magnetic powder ≤ 5 % is used for the magnetic recording medium. Such a magnetic powder for the magnetic recording medium can be obtained by receiving a magnetic powder for the magnetic recording medium in a humidity controller, vaporizing a predetermined amount of water in this humidity controller at 30-80 °C, and then holding it in this humidity controller for 2-24 hours so that the powder absorbs the water and this magnetic powder for the magnetic recording medium contains the water. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a magnetic powder for a magnetic recording medium suitable for a coating-type magnetic recording medium and a method for producing the same.

The coating-type magnetic recording medium has been widely used in magnetic recording systems so far because it has excellent mass productivity and high reliability. In order to cope with the dramatic increase in information volume in the recent broadband era, it is necessary to increase the capacity and transfer rate of magnetic recording systems and to increase the density of magnetic recording media. It has been. This has been coated magnetic recording medium is already developing with areal density of 0.1 GB / in ² levels by, the development of one to several Gb / in ² or more ultra-high density magnetic recording medium capable The next goal.

In order to increase the recording density of the magnetic recording medium, the transition region of the magnetization reversal is sharp, the half width (PW ₅₀ ) of the isolated reversal waveform is narrow, and the output change from low to high is small and flat. Has been demanded. For this reason, a magnetic recording medium capable of high-density recording requires a thin magnetic layer, and recently, a magnetic recording medium having a magnetic layer thickness reduced from several hundreds of nanometers to several tens of nanometers has already been developed. Yes.

  In order to read signals recorded at high density with high sensitivity, a high-sensitivity reproducing head using MR (magnetoresistance effect) has come to be used, and the recording density in magnetic recording has been remarkably improved. For high-sensitivity MR heads, the magnetic layer of the magnetic recording medium is a thin layer with good uniformity in order to prevent head saturation and to prevent waveform distortion and pulse waveform asymmetry. Therefore, it is required that the output is small and flat from the low range to the high range.

  In a magnetic recording device using a high-sensitivity MR head, the signal-to-noise ratio of the device can be increased by reducing the medium noise. In order to reduce the medium noise, it is required to make the magnetic particles of the magnetic recording medium finer.

  Hexagonal ferrite magnetic powder is suitable as a magnetic powder suitable for finer particle size and high density recording. Magnetic recording media using hexagonal ferrite magnetic powder have been pointed out that the recorded magnetization is relatively small because the magnetization is relatively small, but a highly sensitive MR head should be used for signal reproduction. As a result, a sufficient signal output can be obtained, which is expected as an ultra-high density magnetic recording medium.

  Since the ferrite particles of the hexagonal ferrite magnetic powder have a hexagonal plate shape, the particle size and shape are determined by the average plate diameter and average plate thickness, or the average plate diameter and average plate ratio (average plate diameter (Divided by the average thickness). In order to obtain an ultra-high-density magnetic recording medium that exceeds the recording density of the above-described existing magnetic recording medium, and has a thin magnetic layer of, for example, 100 nm or less, a low medium noise, and a high signal-to-noise ratio. The hexagonal ferrite magnetic powder used in the invention has an average plate diameter of 30 nm or less, and the average plate thickness is required to be refined to about 1/3 or less.

  The coating type magnetic recording medium is manufactured by dispersing magnetic powder particles in a binder and coating it on a nonmagnetic material. When the particle size of the magnetic powder dispersed in the binder is reduced, it becomes difficult to uniformly disperse in the binder, the orientation of the magnetic powder particles decreases, and the magnetic powder formed by applying this magnetic powder together with the binder. The smoothness of the layer is lowered, and when a signal recorded in the magnetic layer is reproduced, a sufficient signal-to-noise ratio cannot be obtained.

On the other hand, it is known that the dispersibility of the magnetic powder in the binder is improved by adjusting the water content of the magnetic powder particles to a predetermined amount. In Patent Document 1 (Japanese Patent Laid-Open No. 60-1187931), pre-dispersion and mixing dispersion are performed with the water content in the magnetic powder having a specific surface area of 35 m ² / g or more being 0.8 to 1.6% by weight. It is described. However, the present invention is not an invention about a method for specifically adjusting the water content of the magnetic powder, and there is no description of how to adjust the water content of the magnetic powder.

  Further, in Patent Document 2 (Japanese Patent Laid-Open No. 4-362018), the slurry containing hexagonal ferrite particles obtained by the glass crystallization method is dried, and the amount of water contained in the dried hexagonal ferrite powder is set to 0. The manufacturing method of the magnetic powder made into 4 to 5.0 weight% is disclosed, and the method of accommodating a magnetic powder in a warm air dryer is shown as a method of moisture adjustment. In Patent Document 3 (Japanese Patent Laid-Open No. 2003-203809), although not magnetic powder for magnetic recording media, the water content of the mixture of magnetic powder and binder in the production process of the permanent magnet is 0.20 to 0.80. As methods for adjusting the weight% and moisture adjustment, a method of leaving at room temperature, a method of taking in a constant temperature and humidity chamber, and a method of adding moisture with fine water vapor such as a nebulizer are disclosed.

  However, since the magnetic powder used in the above-described ultrahigh-density magnetic recording medium is required to thoroughly suppress the aggregation of particles, the methods described in Patent Documents 2 and 3 require this requirement. I couldn't respond to it enough. In the above-described ultrahigh-density magnetic recording medium, the thickness of the magnetic layer to be applied is as thin as several tens of nm, and the magnetic powder used for this is required to have an average particle diameter of 30 nm or less. ing. In addition, there is a demand for a coercive force of the magnetic powder used in this magnetic recording medium, which makes it easy to form an aggregate.

  If aggregates of magnetic particles remain in the binder and are present in the magnetic layer of the coated magnetic recording medium, the aggregates cause disorder in the orientation of magnetic particles and a decrease in packing density, as well as surface properties of the magnetic layer. It becomes a factor disturbing. This disturbance in surface property causes the thermal asperity of the MR head that reproduces the recording signal with high sensitivity. Further, in the magnetic layer of the thinned magnetic recording medium, the presence of aggregates causes a variation in the thickness of the magnetic layer, which causes medium noise. As described above, in an ultra-high density magnetic recording medium in which the magnetic layer is a thin layer and a high surface property is required, it is necessary to avoid generation of aggregates that have not been necessary to be a problem in the past. Don't be. For this reason, there has been a strong demand for a magnetic powder that does not remain agglomerated when dispersed in a binder.

Adjustment of the moisture content of the magnetic powder plays an important role in meeting such new requirements. However, the magnetic powder whose moisture has been adjusted by the conventional methods described in Patent Documents 1 to 3 above cannot obtain a magnetic powder that does not remain agglomerated in an ultra-high density magnetic recording medium.
JP 60-1187931 A JP-A-4-362018 JP 2003-203809 A

  As described above, as magnetic powder for magnetic recording media used for ultra-high density magnetic recording media, conventional magnetic powders are not suitable for this, so that it is possible to develop magnetic powders suitable for ultra-high density magnetic recording media. This was a major issue for realizing an ultra-high density magnetic recording medium. The present invention solves this problem and provides a magnetic powder for a magnetic recording medium suitable for an ultra-high density magnetic recording medium and a method for producing the same.

  In order to solve the above-mentioned problems, the present inventors have studied in detail the moisture and the behavior in the magnetic powder for magnetic recording media, and as a result, obtained the following knowledge.

  In order to form a magnetic recording medium in which magnetic particles are uniformly dispersed and filled in a thin magnetic layer, a technique of enclosing individual magnetic particles with a polymer binder to reduce the attractive force acting between the particles is used. This polymer binder has the property of spreading the molecular chain in the coating solution to maintain the distance between the magnetic particles and preventing the aggregation of the magnetic particles, and shrinks after the coating solution is dried to fill the magnetic particles with high density. doing. In this case, if the moisture content in the magnetic powder varies, strong agglomeration occurs in the portion with a small amount of water, and on the other hand, the portion of the water with a large amount of moisture leads to the surface of the magnetic particle in the polymer binder for enclosing the magnetic particles. It was found that the adsorption of water was hindered by this moisture, and both caused a decrease in dispersibility. For this reason, in order to obtain uniform dispersion of magnetic particles, it was found that in addition to adjusting the total amount of water contained in the magnetic powder, it was necessary to highly uniform the water in the magnetic powder. .

  Based on this finding, as a result of research on a method for highly uniformly containing a predetermined amount of water in a magnetic powder, a method for realizing this was found. Also, by using this method, a magnetic powder containing a predetermined amount of moisture in the magnetic powder highly uniform is prepared, and it is found that a coated magnetic layer without aggregation can be formed by using this powder, and the present invention is summarized. I was able to.

The magnetic powder for magnetic recording media of the present invention is a magnetic powder for magnetic recording media having a specific surface area of 40 to 100 m ² / g, having a moisture content of 0.9 to 4% by mass and a coefficient of variation of the moisture content. It is characterized by being 5% or less.

  The coefficient of variation in the moisture content of the magnetic powder in the above is an amount representing the variation of the moisture content in the same lot, and is a standard for the measured moisture content of the magnetic powder sampled at 10 locations in the same lot. The value obtained by dividing the deviation by the average value of the measured water content is shown as a percentage. Any method can be used to measure the moisture content of the magnetic powder as long as it can be measured correctly. For example, the Karl Fischer method, which is a useful method for measuring a minute amount of moisture, can be used. it can.

The smaller the coefficient of variation, the higher the moisture content in the magnetic powder. In the magnetic powder for a magnetic recording medium having a specific surface area of 40 to 100 m ² / g, when this coefficient of variation exceeds 5%, an aggregate of 0.1 μm or more is confirmed, which is the cause of the above-mentioned medium defect. Thus, it was found that it could not be used for the ultra-high density magnetic recording medium.

In the present invention, the moisture content of the magnetic powder for a magnetic recording medium having a specific surface area of 40 to 100 m ² / g is preferably in the range of 0.9 to 4% by mass. When the moisture content of the magnetic powder is less than 0.9% by mass, the cohesive force between the magnetic powders appears strongly, and it becomes easy to form a firm aggregate. On the other hand, when the moisture content of the magnetic powder exceeds 4.0% by mass, the adsorption of the polymer binder to the magnetic particles is hindered by the presence of excess moisture, so that the magnetic powder in the binder Dispersibility is lowered, and agglomerates of undispersed magnetic powder are generated in the medium. For this reason, the moisture content in the magnetic powder is more preferably 1.0% by mass or more and 2% by mass or less. When the magnetic powder is a hexagonal ferrite magnetic powder, the cohesive force between the particles when the plate surfaces are in contact with each other is strong, so that the moisture content of the magnetic powder is 1.1% by mass or more. Preferably, it is more preferably less than 1.6% by mass.

The specific surface area of the magnetic powder is a value measured by the BET method and has a value in the range of 40 to 100 m ² / g. If the specific surface area of the magnetic powder is less than 40 m ² / g, the resistance received from the binder is small and the orientation of the magnetic particles is improved, but it becomes difficult to ensure the dispersion stability of the magnetic particles in the magnetic coating, and the moisture content of the magnetic powder Even if the rate is uniform, a sufficient dispersion stability effect cannot be obtained. On the other hand, when the specific surface area of the magnetic powder exceeds 100 m ² / g, the orientation of the magnetic powder particles is lowered and the filling property is lowered, which is not suitable for high density recording.

  The magnetic powder for magnetic recording medium of the present invention is preferably a hexagonal ferrite magnetic powder having an average particle diameter of 15 to 30 nm and a coercive force Hc of 2,000 to 5,000 Oe.

  The hexagonal ferrite magnetic powder in the present invention has a plate-like particle shape, and the average particle diameter of the particle is expressed by the plate diameter of the magnetic particle. The average particle size of these particles is obtained by randomly selecting the plate diameter of 500 powder particles from the transmission electron microscope image of the magnetic powder, and then arithmetically averaging these measured values. is there.

  If the average particle size of the hexagonal ferrite magnetic powder is less than 15 nm, the magnetization value of the particles becomes small and dispersion becomes difficult, and the signal-to-noise ratio when used in an ultra-high density magnetic recording medium is significantly reduced. On the other hand, when the average particle size of the hexagonal ferrite magnetic powder exceeds 30 nm, the medium noise is remarkably increased when used in an ultrahigh density magnetic recording medium. The average particle diameter is more preferably 28 nm or less, and further preferably 26 nm or less.

  In the present invention, the coercive force of the hexagonal ferrite magnetic powder having a particle size reduced to 30 nm or less for ultra-high density recording media can prevent recording demagnetization and enable high density recording, and can stabilize recording magnetization. In order to ensure the properties, it is desirable to have a larger value than before. When the coercive force is less than 2,000 Oe, the recording demagnetization is increased and the stability against fluctuation of the recording magnetization is also lowered. On the other hand, when the coercive force of the magnetic powder exceeds 5,000 Oe, it becomes difficult to magnetize and record the magnetic particles with the existing recording head.

  Magnetic recording media using hexagonal ferrite magnetic powder have been pointed out that their recorded magnetization is relatively small because of their relatively small magnetization, but use a highly sensitive MR head for signal reproduction. Thus, a sufficient signal output can be obtained. As a result, the relatively small recording magnetization is not disadvantageous as a magnetic recording medium. Therefore, taking advantage of the hexagonal ferrite magnetic powder, such as being suitable for producing fine particles and obtaining a high coercive force, application to an ultrahigh density magnetic recording medium can be expected. The present invention is useful in that it can provide a solution to the problem of strong aggregation exhibited by hexagonal ferrite magnetic powder particles when the hexagonal ferrite magnetic powder is applied to an ultra-high density magnetic recording medium.

  The hexagonal ferrite magnetic particles are treated with an aqueous solution of a strong acid, for example, an aqueous solution of strong acid such as nitric acid having a concentration of 0.2 to 2 mol / L, thereby fine particles in the magnetic powder, for example, particles having a plate diameter of 10 nm or less. It was found that it can be dissolved and removed, and its abundance can be, for example, 3% or less. By doing so, fine particles representing superparamagnetic particles in the hexagonal ferrite magnetic powder are removed, the particle size distribution is improved, the SFD of the magnetic powder is improved, and it is suitable for a coating type ultra-high density magnetic recording medium. It was found that hexagonal ferrite particles can be obtained. For this reason, it is preferable to use the hexagonal ferrite magnetic particles that have been subjected to the strong acid treatment.

The magnetic powder package for a magnetic recording medium according to the present invention is characterized in that the magnetic powder for a magnetic recording medium is filled and sealed in a moisture-proof container having a water vapor transmission rate of 0.2 g / m ² / day or less. .

  The magnetic powder for magnetic recording media of the present invention is not always used for the production of magnetic recording media immediately after the step of highly uniformly containing moisture in the magnetic powder. Rather, after this step, the magnetic powder is often transported or stored and then used in the manufacture of magnetic recording media. In the meantime, it is necessary to take an appropriate measure in order to keep the moisture of the magnetic powder uniformly absorbed.

In order to find such an appropriate means, as a result of accumulating detailed data on changes in moisture content due to storage of magnetic powder, the magnetic powder for magnetic recording media has a water vapor transmission rate of 0.2 g / m ² / day. It has been found that a highly uniform content state of moisture content can be maintained by storing it in the following moisture-proof package and hermetically storing it. It has been found that it is not preferable to fill a magnetic powder for magnetic recording media in a container having a water vapor transmission rate exceeding 0.2 g / m ² / day, because the moisture content of the powder changes with time.

Examples of the moisture-proof packaging material that enables a water vapor transmission rate of 0.2 g / m ² / day or less include a moisture-proof bag made of aluminum, a thick nylon bag, an acrylic bag, a metal can having a sealed structure, and a sealed glass. Containers can be used.

  In the method for producing a magnetic powder for a magnetic recording medium of the present invention, the magnetic powder for a magnetic recording medium is accommodated in a humidity control device, and after a predetermined amount of water is vaporized at 30 to 80 ° C. in the humidity control device, The magnetic powder for magnetic recording medium is made to contain moisture by holding in the humidity control apparatus for 2 to 24 hours and absorbing the moisture in the powder.

  By using such a method, for example, water supply using a nebulizer, there is no longer a problem such as excessive water on the surface of the powder as seen in the prior art. It was confirmed that the magnetic powder for a magnetic recording medium prepared by this method has a state in which a predetermined amount of moisture is highly uniformly contained in the magnetic powder.

  In the present invention, a predetermined amount of water is vaporized at 30 to 80 ° C. in the humidity control apparatus. When heating at less than 30 ° C., it takes a long time to evaporate water, resulting in low productivity. Moreover, when it exceeded 80 degreeC, it turned out that water evaporates quickly, a water | moisture content condenses on a magnetic powder, and this causes a magnetic powder to aggregate. And if water was vaporized in the temperature range of 30-80 degreeC, it turned out that water is absorbed with a magnetic powder with sufficient uniformity, and the productivity is also favorable. For this reason, the vaporization temperature of water is greater than atmospheric temperature + 5 ° C., specifically 35 to 70 ° C., more preferably 40 to 60 ° C.

  In the present invention, after the water to be added is vaporized in the humidity control apparatus, the magnetic powder for the magnetic recording medium is kept in the humidity control apparatus as it is for 2 hours or more to diffuse the moisture contained in the magnetic powder. Make uniform. It has been found that when the holding time is less than 2 hours, the moisture is not sufficiently diffused and homogenized, and even if the holding time exceeds 24 hours, no substantial change is observed. From the viewpoint of productivity, the holding time is preferably within 24 hours.

  Note that the lower the vaporization temperature, the longer it takes to vaporize the water, and during that time the diffusion into the magnetic powder proceeds in parallel, so the placement time after completion of vaporization may be relatively short, conversely When the temperature is high, the vaporization of water is completed in a short time, but it is necessary to take a longer time for diffusion homogenization. Therefore, at a processing amount of the same level as in the examples described later, the sum of the water vaporization time and the device holding time can be completed within approximately 24 hours. In order to ensure uniform diffusion of moisture and stably obtain the coefficient of variation of 5% or less, the holding time is preferably 10 hours or more, and more preferably 12 hours or more. On the other hand, from the viewpoint of productivity, it is preferable that the total of the water vaporization time and the above-mentioned subsequent holding time is within 24 hours.

  It is desirable that the process conditions for uniformly containing water in the magnetic powder take into account the amount of the magnetic powder. Therefore, when the total time of the moisture vaporization time and the retention time is named moisture content time and the value obtained by dividing the added moisture content per kg of magnetic powder by the moisture content time is defined as the moisture content rate, the moisture content rate is 0. It is preferably 2 to 0.6 g / hour · kg. If the moisture content rate is less than 0.2 g / hour · kg, the process takes a long time. If the moisture content rate exceeds 0.6 g / hour · kg, the variation coefficient of variation in moisture content exceeds 5%. It is not preferable. For this reason, it is more preferable that the moisture content rate is 0.3 to 0.5 g / hour · kg.

  According to the method for producing a magnetic powder for a magnetic recording medium of the present invention, it is a hexagonal ferrite magnetic powder having an average particle diameter of 15 to 30 nm, and the moisture content of the magnetic powder is set to 0.9 to 4% by mass. A magnetic powder for magnetic recording media can be obtained in which the coefficient of variation of the moisture content of the magnetic powder is 5% or less.

  By using the production method of the present invention, it has become possible to produce a magnetic powder for ultra high density magnetic recording in which aggregation of magnetic particles is sufficiently suppressed. Further, according to the present invention, it is possible to provide a coating type magnetic powder for an ultra-high density recording medium.

  Next, based on the Example implemented with respect to the M type hexagonal ferrite magnetic powder, this invention is demonstrated further in detail and concretely.

A glass matrix component composed of BaO.B ₂ O ₃ and an M-type hexagonal ferrite component represented by the composition formula BaO.Fe _{12-3 (x + y) / 2} Co _x Zn _y Nb _{(x + y) / 2} O ₁₈ Weighed and mixed well. This mixed raw material was put into a platinum crucible equipped with a nozzle and heated and melted at 1350 ° C. using a high-frequency heating measure. After melting all the raw materials, in order to homogenize the glass, the mixture was stirred for 1 hour, and the homogenized molten glass was poured onto a water-cooled twin roller rotated at a high speed, and rapidly cooled by rolling to produce an amorphous body. . This amorphous body was heat-treated in a heat-treating furnace at 700 ° C. for 5 hours for crystallization by crystallization. Subsequently, the crystallized product was pulverized, and acid treatment was performed by stirring for 4 hours in a 10% acetic acid solution while controlling the solution temperature at 80 ° C or higher. The glass matrix component dissolved by acetic acid was sufficiently washed and removed, and the remaining magnetic powder slurry was dried to obtain a hexagonal ferrite magnetic powder.

  In this cleaning step, when the electrical conductivity of the separated cleaning liquid is expressed in units of mS / m, the numerical value becomes a value equal to or less than ¼ of the mass% of the sedimented slurry concentration (magnetic powder amount). It has been found that it is preferable to perform water washing. By doing so, it was found that precipitation of impurity salts from the magnetic powder after washing can be suppressed, which is preferable as a magnetic powder for ultra-high density recording media. Further, at this time, the electric conductivity of the liquid in which the magnetic powder hexagonal ferrite magnetic powder subjected to the cleaning treatment is immersed is adjusted to 6.0 mS / m or less and 0.02 mS / m or more. From this, no salt precipitation of impurities was observed, and the dispersibility was good, and thus it was found that the powder was preferable as a magnetic powder for ultra-high density recording media.

  Further, as the hexagonal ferrite magnetic powder obtained in this way, the hexagonal ferrite magnetic powder is adjusted so that the electric conductivity of the immersion liquid in which the magnetic powder is immersed and boiled is 0.02 to 6.0 mS / m. It was found that it is preferable to obtain good dispersibility. The electrical conductivity of the immersion liquid boiled and boiled here with magnetic powder The electrical conductivity of the immersion boiled liquid is 5 g of magnetic powder placed in a 200 mL beaker, immersed in 100 mL of pure water and heated to 210 ° C. It is the electrical conductivity of this liquid measured after boiling for 40 minutes on a hot plate, sealed in a PET container and cooled with water for 15 minutes in a state where contact with air was cut off.

  The moisture content of the hexagonal ferrite magnetic powder was adjusted using the humidity controller shown in FIG. In FIG. 1, the humidity control apparatus 1 includes a humidity control container (large-scale desiccator) 2, a moisture vaporizer (hot plate) 3, and a stirring mechanism (electric fan) 4 that stirs the air in the humidity control container 2. . The magnetic powder 8 is spread on a flat powder container 5 such as a stainless steel tray to a thickness of about 2 to 20 mm, and is placed and stored on the shelf of the humidity control container 5. In addition, when the thickness of the magnetic powder spread on the flat powder container 5 exceeds 20 mm, much time exceeding 24 hours is required to homogenize the moisture contained in the magnetic powder.

  The container 6 of the moisture vaporizer 3 is filled with a required amount of water 7 so as to obtain a target moisture content according to the amount of the magnetic powder 8 contained in the humidity control container 2 and the moisture content. After the humidity control container 2 is sealed, it is heated to the above temperature and held, and the water 7 is absorbed by the magnetic powder 8. At this time, by slowly circulating the air in the humidity control container 2 by the stirring mechanism 4, it is possible to suppress variation in moisture content due to the position of the flat powder container 5 in the humidity control container 2. Here, the Karl Fischer method is used to measure the moisture content of the magnetic powder 8 before humidity control. The required amount of water 7 absorbed by the magnetic powder 8 is (required amount of water) = (required amount of magnetic powder 8). Amount) × {(target water content) − (water content held by the magnetic powder before conditioning)}.

  In the humidity control apparatus 1 shown in FIG. 1, the above-described hexagonal ferrite magnetic powder is accommodated as a magnetic powder 8 in a treatment amount of 54 kg, and water 7 is vaporized at a water vaporization temperature of 40 ° C. and absorbed by the magnetic powder 8. I let you. After the moisture vaporization was completed, the magnetic powder 8 was held as it was for 12 hours in a humidity control device, and the moisture content was adjusted.

  In this example, 1.4% by mass was set as the target value as the moisture content (% by mass) of the magnetic powder after humidity control. Therefore, by the above calculation method, the moisture content already contained in the magnetic powder is measured to obtain 0.64% by mass, and the difference between this value and the target of 1.4% by mass is 0.76% by mass. As the amount of water corresponding to%, 54 kg × 0.76 / 100 = 0.41 kg was calculated, and this was used as the amount of water 7 to be absorbed by the magnetic powder.

Part of the magnetic powder thus conditioned is used for measurement evaluation, while the remainder is filled in an aluminum moisture-proof bag with a water vapor transmission rate of 0.1 g / m ² / day, and the temperature is 50 ° C.-100% RH. A storage test was conducted for one week in the environment, and the magnetic powder after storage was measured and evaluated.

  The measurement evaluation of the magnetic powder was performed according to the conditions already described. The average particle diameter of the magnetic powder was obtained by averaging those measured using the respective transmission electron microscope images. Further, the specific surface area of the magnetic powder was measured using a measuring apparatus by the BET method. The coercive force was determined by magnetic measurement of the magnetic powder using VSM.

  The Karl Fischer method was used to measure the amount of water contained in the magnetic powder. After measuring the moisture content before conditioning, perform humidity conditioning so that the target value after conditioning is 1.4% by mass, and measure the moisture content after conditioning at 10 locations using the same method. The average moisture content and the standard deviation of the moisture content distribution were calculated, and the moisture content variation coefficient of variation was calculated from this. Furthermore, the moisture content after 1 week storage of the magnetic powder stored in the moisture-proof package was measured.

For these magnetic particles, the number of undispersed aggregates was measured. The number of undispersed agglomerates was measured by sampling from a slurry obtained by dissolving 2 g of magnetic powder in 100 mL of cyclohexanone and dispersing for 24 hours at 150 rpm with a bead mill filled with zirconia beads having a diameter of 1 mm. The number of agglomerates of 0.1 μm or more was counted for 100 fields of 40,000 times that of a transmission electron microscope.
These measurement results are shown in Table 1 together with the humidity control treatment conditions.

  A Ba ferrite magnetic powder conditioned under the same conditions as in Example 1 except that the water vaporization temperature in the humidity conditioning process was set to 70 ° C. was prepared, and a storage experiment for one week was conducted to measure and evaluate the magnetic powder. Was done. The results are shown in Table 1 together with the humidity control treatment conditions.

  A Ba ferrite magnetic powder was prepared that was conditioned under the same conditions as in Example 1 except that the stationary holding time after the completion of moisture vaporization in the process was changed to 3 hours. The powder was evaluated for measurement. The results are shown in Table 1 together with the humidity control treatment conditions.

  A Ba ferrite magnetic powder which was conditioned under the same conditions as in Example 1 was prepared except that the stationary holding time after the moisture vaporization in the humidity conditioning step was 20 hours, and a storage experiment was conducted for one week. The magnetic powder was measured and evaluated. The results are shown in Table 1 together with the humidity control treatment conditions.

  The heat treatment in the process of precipitating Ba ferrite crystals by heating and holding the amorphous body in a heat treatment furnace is held at 620 ° C. for 5 hours, and the target value after humidity control of the moisture content is 2.3 mass%. In addition to the above, Ba ferrite magnetic powder which was conditioned under the same conditions as in Example 1 was prepared, and a storage experiment for one week was conducted to measure and evaluate the magnetic powder. The results are shown in Table 1 together with the humidity control treatment conditions.

  The heat treatment in the process of precipitating Ba ferrite crystals by heating and holding the amorphous body in a heat treatment furnace is held at 740 ° C. for 5 hours, and the target value after moisture conditioning of the moisture content is 1.1 mass%. In addition to the above, Ba ferrite magnetic powder that had been conditioned under the same conditions as in Example 1 was prepared, and a storage experiment for one week was conducted to evaluate and measure the magnetic powder. The results are shown in Table 1 together with the humidity control treatment conditions.

  A Ba ferrite magnetic powder which was conditioned under the same conditions as in Example 1 was prepared except that the stationary holding time after the moisture vaporization in the humidity conditioning process was 28 hours, and a one-week storage experiment was conducted. The magnetic powder was measured and evaluated. The results are shown in Table 1 together with the humidity control treatment conditions.

(Comparative Example 1)
A Ba ferrite magnetic powder that was conditioned under the same conditions as in Example 1 except that the water vaporization temperature in the humidity conditioning step was set to 90 ° C. was prepared, and a storage experiment for one week was conducted to measure and evaluate the magnetic powder. Was done. The results are shown in Table 1 together with the humidity control treatment conditions.

(Comparative Example 2)
A Ba ferrite magnetic powder that was conditioned under the same conditions as in Example 1 was prepared except that the stationary holding time after the completion of moisture vaporization in the humidity conditioning step was 1 hour, and a storage experiment was conducted for one week. The magnetic powder was measured and evaluated. The results are shown in Table 1 together with the humidity control treatment conditions.

(Comparative Example 3)
A Ba ferrite magnetic powder was prepared under the same conditions as in Example 1 except that the moisture content was reduced in the drying step and no humidity conditioning was performed. A one week storage experiment was conducted to measure and evaluate the magnetic powder. Was done. The results are shown in Table 1 together with the humidity control treatment conditions.

(Comparative Example 4)
A Ba ferrite magnetic powder was prepared under the same conditions as in Example 1, the humidity was adjusted, and a vinyl bag was filled with the magnetic powder in a water vapor transmission rate of 3.8 g / m ² / day. They were stored together in a storage room for 1 week. Measurement and evaluation of the magnetic powder after storage were performed, and the results are shown in Table 1.

  FIG. 2 shows a plot of the relationship between the incubation holding time after vaporization and the variation coefficient of variation in water content for the results shown in Table 1. In the above Examples 1, 3, 4 and 7, the holding time after vaporization is changed, and other conditions are set. Also, Comparative Example 2 has almost the same conditions as these except that the device holding time after vaporization is set short. From the comparison of these results, the moisture content variation coefficient of variation is within 5% at the time of the retention time of 3 hours after the moisture is vaporized, and the variation coefficient becomes smaller as the incubation time becomes longer. I understand that. Further, from comparison between Examples 1, 4 and 7, it can be seen that a sufficient reduction in the coefficient of variation has already been obtained by holding the device for 12 hours.

  In Comparative Example 1, since the vaporization temperature was increased, the water was rapidly evaporated, which became water droplets on the surface of the magnetic powder, causing the magnetic powder to aggregate, and the aggregation was not eliminated even after holding for 12 hours.

  In Comparative Example 2, the device holding time is as short as 1 hour, and this short time device holding shows that the coefficient of variation in moisture content variation is large.

  In Comparative Example 3, the moisture content was adjusted by drying with a conventional hot air dryer, and the variation coefficient of variation in moisture content was as large as 14.0. This seems to indicate that the difference in moisture content between the surface and the inside of the magnetic powder lot during drying cannot be resolved.

  An undispersed lump was not found in each of Examples 1 to 7, and the results of the comparative example showed that the undispersed lump had a large correlation with the variation coefficient of moisture content variation.

  The amount of moisture adsorbed after storage does not change when the sealed metal container is filled, and increases when the vinyl bag having a high water vapor transmission rate of Comparative Example 4 is used, and is suitable for binder adsorption. I found that the amount was exceeded.

  According to the present invention, it has become possible to produce a magnetic powder suitable for a coating type ultra-high density magnetic recording medium, which has been difficult to manufacture. By using the magnetic powder according to the present invention, a coating type ultra-high density magnetic recording medium can be realized. For this reason, the industrial applicability of the present invention is great.

It is the figure which showed typically the inside of the humidity control apparatus of the magnetic powder used in one Embodiment of this invention. It is the figure which plotted the relationship between the apparatus holding time after vaporization in an Example and a comparative example, and a content moisture dispersion | variation variation coefficient.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Humidity control apparatus, 2 ... Humidity control container (large desiccator), 3 ... Water vaporization apparatus (hot plate), 4 ... Stirring mechanism (electric fan), 5 ... Flat powder container, 6 ... Container, 7 ... Water, 8: Magnetic powder.

Claims (5)

A magnetic powder for magnetic recording media having a specific surface area of 40 to 100 m ² / g, having a water content of 0.9 to 4% by mass and a coefficient of variation of the water content of 5% or less. Magnetic powder for recording media.   2. The magnetic recording medium according to claim 1, wherein the magnetic powder for magnetic recording medium is a hexagonal ferrite magnetic powder having an average particle diameter of 15 to 30 nm and a coercive force of 2,000 to 5,000 Oe. Magnetic powder. 3. The magnetic powder for a magnetic recording medium according to claim 1, wherein the magnetic powder for a magnetic recording medium is filled and sealed in a moisture-proof package having a water vapor transmission rate of 0.2 g / m ² / day or less. package.   The magnetic powder for the magnetic recording medium is accommodated in a humidity controller, and after a predetermined amount of water is vaporized at 30 to 80 ° C. in the humidity controller, the moisture is maintained in the humidity controller for 2 hours or more. A method for producing a magnetic powder for a magnetic recording medium, comprising adjusting the water content of the magnetic powder and the coefficient of variation of the water content by containing the magnetic powder in the magnetic powder.   The magnetic powder for magnetic recording medium is a hexagonal ferrite magnetic powder having an average particle diameter of 15 to 30 nm. The moisture content of the magnetic powder is adjusted to 0.9 to 4% by mass and the coefficient of variation of the moisture content is adjusted. The method for producing a magnetic powder for a high magnetic recording medium according to claim 4, wherein the content is adjusted to 5% or less.

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