JPH025690B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing magnetic powder for magnetic recording, and is a method for producing powder consisting of fine particles, which is particularly suitable for high-density magnetic recording media, and is short in production time and highly practical. Regarding the method.

Although residual magnetization in the in-plane longitudinal direction of a recording medium is conventionally used for magnetic recording and reproduction, there is a limit to the high recording density in the method using this in-plane longitudinal direction. That is, in a recording/reproducing system using residual magnetization in the in-plane longitudinal direction, the demagnetizing field within the magnetic recording medium increases as the recording density increases. In order to overcome this demagnetizing field and perform high-density recording, it is necessary to increase the coercive force of the recording medium (recording layer) and to select a thin recording medium layer. However, increasing the coercive force of the recording medium layer requires a magnetic head with a high magnetic flux density, which is difficult to expect at present, and thinning the recording medium layer also poses problems such as a reduction in reproduction signals. Therefore, in response to the demand for higher density recording of magnetization, it has been proposed to use residual magnetization in a direction perpendicular to the plane of the recording medium layer.

In such perpendicular magnetization recording, the magnetic recording medium needs to have an axis of easy magnetization in a direction perpendicular to the surface. Therefore, attempts have been made to use ferrite having a hexagonal system and uniaxial anisotropy, such as BaFe ₁₂ O ₁₉ , as the magnetic powder. These BaFe ₁₂ O ₁₉ particles are plate-shaped and the axis of easy magnetization is perpendicular to the flat surface, so they are mixed with a solvent, dispersant, binder, etc. and coated on the supporting substrate surface, and then applied using a magnetic field. A magnetic recording medium having an axis of easy magnetization in the perpendicular direction can be manufactured by orienting it in a direction perpendicular to the surface and drying it. However, the coercive force ( _I H _C ) of BaFe ₁₂ O ₁₉ was over 5000 Oe, which was too large for magnetic recording, but by replacing some of the Fe with Co etc., _I H _C could be used for magnetic recording. suitable for 200
~2000Oe was realized, and the above problem was solved for the time being.

The present invention relates to a method for producing Co-substituted hexagonal ferrite powder suitable for high-density magnetic recording, particularly high-density perpendicular magnetic recording, as described above.

Co-substituted hexagonal ferrite for magnetic recording has the general formula: AFe _12-XY Co _X M _Y O ₁₉ ...(1) (where A is
It is a comprehensive concept of Ba, Sr and Ca, and M is for example
Substitute metal elements other than Co, such as Ti, Zn, Nb, V, Sb, and Ta. X is a number between 0.5 and 1.1, and Y is between 0.5 and 1.1.
The number is 1.1. ). As a method for producing such Co-substituted hexagonal ferrite powder, Fe
ion, Co ion, and one of Ba, Sr, and Ca
An aqueous solution containing ions of a species or two or more elements, and in some cases also containing ions of M (same meaning as above), is PH-treated with an alkali such as NaOH.
A carbonate solution adjusted to 9 or more is brought into contact to obtain a coprecipitate, then this coprecipitate is washed and dried, and then approximately
A method of applying heat treatment at 700 to 1000°C is known.
The ratio of each metal ion in the aqueous solution is adjusted to a stoichiometric ratio depending on the composition of the target ferrite represented by the above formula (1).

By the way, when Co-substituted hexagonal ferrite is used for magnetic recording media, especially high-density magnetic recording media, etc., due to the characteristics obtained, the maximum particle size of the ferrite powder is 0.3 μm or less, and the particle size distribution is narrow.
It is strongly desired that the size and shape of each particle be uniform. However, the above-mentioned conventional method has disadvantages in that it tends to produce powder in an agglomerated state, including particles with a particle size of about 0.5 μm, and the uniformity of the shape is low. This is thought to be because ferrite particles aggregate when a coprecipitate is formed, and sintering occurs while the particles remain aggregated during the subsequent heat treatment. Co-substituted hexagonal ferrite powder that has agglomerated in this way has poor dispersibility even when mixed with a solvent or binder, and it is difficult to obtain a uniform magnetic recording layer even when coated on a supporting substrate. It has been difficult to obtain a recording medium having these characteristics.

Another drawback of the conventional method is that it takes a very long time to wash the coprecipitate. This washing is performed to remove alkaline components adsorbed or encapsulated in the coprecipitate by washing with water in order to generate the coprecipitate at a pH of 9 or higher, preferably 10 or higher, but conventional methods However, it took more than 1000 hours to sufficiently remove the alkaline content, which was a drawback that reduced its practicality. There is a relationship between the length of the required washing time and the degree of agglomeration of the powder particles, and the higher the agglomeration, the longer the time required for washing, which was a major drawback in any case. In addition, if the washing is insufficient, the alkali will remain and will coagulate during drying, and will coagulate and sinter when heated, so it is important from an industrial perspective to quickly remove this alkali.

An object of the present invention is to provide a method for producing Co-substituted hexagonal ferrite powder, which improves the above-mentioned production method and makes it difficult for agglomeration to occur between the particles produced, and which requires only a short time to clean the coprecipitate. There is a particular thing.

The present invention produces a coprecipitate by bringing an aqueous carbonate solution with a pH of 9 or more into contact with an aqueous solution containing at least Fe ions; Co ions; and ions of one or more elements among Ba, Ca, and Sr. This is a method for producing magnetic powder for magnetic recording, which comprises freezing the coprecipitate slurry, thawing it to a temperature of 20° C. or lower, washing it, drying it, and then subjecting it to a heat treatment.

The main features of the present invention are cleaning the generated coprecipitate,
It includes a step of once freezing and then thawing to a temperature of 20°C or less before drying. By adding this step, we succeeded in significantly shortening the washing time following this step, and the resulting powder is highly uniform with no agglomeration and a grain size of 0.3 μm or less. I was able to do that.

During thawing, the temperature of the coprecipitate must be kept below 20°C. When the temperature rises to 20° C., the required washing time becomes significantly longer and at the same time agglomerated particles are obtained.

To prepare the aqueous solution used in the present invention,
Fe, CO and alkaline earth metals (Ba, Ca, Sr)
What is necessary is to dissolve an appropriate water-soluble salt in water. As is well known, such salts include, for example, chlorides, sulfur oxides, sulfates, etc. for Fe;
For Co, examples include chloride, sulfur oxide, and sulfate; and for Ba, etc., examples include chloride.

Further, the carbonate used in the carbonate aqueous solution may be water-soluble, such as Na ₂ CO ₃ , K ₂ CO ₃ , (NH ₄ ) ₂ CO ₃
can be given. This carbonate aqueous solution is, for example,
Use NaOH, KOH, NH ₄ OH, etc. to adjust the pH to 9 or higher, preferably 10 or higher. If the pH is less than 9, coprecipitates are difficult to form.

In order to bring the carbonate aqueous solution into contact with the aqueous solution containing Fe ions and the like, it is possible to drop the former into the latter as appropriate, and at this time, the solution may be added dropwise while stirring.

After the generated coprecipitate is collected, it is subjected to the freezing and thawing steps described above, but at this time, the coprecipitate is in the state of a wet slurry.

Washing after thawing is generally done with water. Drying may be carried out naturally in the air or by hot air using an air bath.

The heat treatment is carried out in air at a temperature of approximately 700-1000°C.

This will be explained in detail below using examples.

Example BaCl ₂・2H ₂ O 0.014 mol, FeCl ₃・6H ₂ O 0.13
A metal salt solution was prepared by dissolving 0.01 mol of TiO and 0.01 mol of CoCl ₂ .6H ₂ O in 200 ml of pure water.
Add this metal salt aqueous solution to 400 ml of pure water, 50 g of NaOH,
It was mixed with an alkali solution prepared by dissolving 12.5 g of Na ₂ CO ₃ and stirred to obtain a coprecipitate. Six types were prepared under the same conditions as above, and the resulting coprecipitates were collected and frozen in liquid nitrogen. This coprecipitate was naturally thawed10
Five kinds of coprecipitates heated to 15°C, 15°C, 20°C, 25°C and 30°C were transferred to a glass filter and washed with pure water.
The results shown in Figures 1 and 2 were obtained.

Figure 1 shows the time course of the pH of each coprecipitate during washing, and Figure 2 shows the relationship between the thawing temperature and the time it takes for the pH to reach 8.5.

The washing time for each coprecipitate until pH 8.5 was 53 hours when thawed at 10℃, 55 hours when thawed at 15℃, and 55 hours when thawed at 20℃.
The coprecipitate thawed at 25°C and 30°C required extremely long cleaning times of 435 and 470 hours, compared to 80 hours for those thawed at 25°C and 30°C. Furthermore, when these five types of coprecipitates were air-dried, the coprecipitates thawed at 10°C, 15°C, and 20°C were almost the same as each other, and were easily loosened and solid powders, but at 25°C, The coprecipitate thawed at 25°C became a powder containing many hard-to-disintegrate lumps.

As is clear from the above specific example, after freezing the coprecipitate
By thawing at a temperature below 20°C, the washing time can be shortened, and the coprecipitate does not solidify even after drying, making it possible to obtain a powder without agglomeration. The coprecipitate thus obtained was heat treated in air at 950° C. for 2 hours, and the powder obtained was a magnetic powder with good dispersibility and no secondary agglomeration.

[Brief explanation of drawings]

Figure 1 shows the changes in cleaning time and pH of coprecipitates thawed at various temperatures, and Figure 2 shows the changes in PH and thawing temperature.
It is a figure which shows the relationship of the washing time until it becomes 8.5.

Claims (1)

[Claims] 1. At least Fe ions; Co ions; and
A carbonate aqueous solution with a pH of 9 or more is brought into contact with an aqueous solution containing ions of one or more elements among Ba, Ca, and Sr to obtain a coprecipitate, and after freezing the slurry of the coprecipitate, 1. A method for producing magnetic powder for magnetic recording, which comprises thawing the powder to a temperature of 0.degree. C. or lower, washing, drying, and subjecting it to heat treatment.