JPH0123924B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for making ferromagnetic powders, particularly improved CrO ₂ magnetic powders.

The coercive force of ferromagnetic fine powder for magnetic recording is gradually increasing to enable high-density recording not only in video tapes and computer tapes but also in cassette tapes that record at low speeds.
For example, audio tapes mainly use iron oxide magnetic powder, which used to have a coercive force of 230~
It used to be 250 oersted, but recent cassette tapes are about 330 oersted. Also, for VTR tapes, monochrome tapes cost 450 oersted, while color tapes cost 550 oersted. This increase in coercive force is all due to higher density recording, but recently, ferromagnetic fine particles using fine powder of a single metal or alloy with a coercive force of 1000 oersted or more have been developed. Powder appears. However, since these are metals, they inherently have a drawback in terms of chemical stability such as oxidation in air. In other words, there is a possibility that magnetization and coercive force may decrease over time. In contrast, oxide ferromagnetic materials do not suffer from such chemical instability because they are oxides. However, iron oxide alone
It is not possible to exhibit a coercive force as high as 1000 Oe. Therefore, it has been known for a long time that the coercive force can be increased by adding Co to iron oxide, but the so-called Co-added iron oxide ferromagnetic fine powder synthesized by this method has temperature characteristics similar to that of iron oxide. It's far worse than that. If the amount of Co added is reduced in order to reduce this drawback, the coercive force will not increase, making it unsuitable for the purpose of increasing recording density. To improve this, Co-adhered iron oxide was invented in which a Co compound was adhered to the surface of iron oxide γFe ₂ O ₃ or Fe ₃ O ₄ particles, and the temperature characteristics were improved. . However, iron oxide fine particles are said to be skeletal particles of the mother salt, goethite αFeOOH, and although their shape is acicular, they are extremely irregular crystals. This is a major reason why the magnetic field distribution in the tape manufacturing process, which is essential for magnetic recording tapes, is not effective. In particular, when the coercive force is high, the above-mentioned magnetic field alignment effect is further reduced.

The present invention uses Co sulfate on the surface of chromium dioxide (CrO ₂ ) particles, which have an excellent needle-like shape.
The purpose is to synthesize high coercive force ferromagnetic oxide fine powder that exhibits high coercive force by depositing or adsorbing Co or a Co compound and has excellent orientation effects, thereby obtaining a chemically stable high-density magnetic recording material.

In the present invention, a ferromagnetic fine powder synthesized by any conventionally well-known method can be used as the CrO ₂ fine powder. FIG. 1 shows the method of the invention as a block diagram. To briefly explain the figure, the CrO ₂ fine powder undergoes surface roughening treatment, and the subsequent treatment creates a state in which it easily adheres to the Co surface. In the following description, adhesion or adsorption will be referred to using the term adhesion. In addition, in the method according to the present invention, Co
is applied to CrO ₂ powder particles by electroless plating,
Due to the limitations of electroless plating, a small amount of Co compound that inevitably precipitates along with Co is often unavoidable. The present invention also includes a case where the electroless plating deposit is mostly composed of Co and inevitably contains a small amount of Co compound. For example, 90 for surface roughening.
This is done by adding CrO ₂ to a NaOH aqueous solution (concentration 0.4M/) at ℃ and stirring for 1 hour. In the same way, a specific example of the work will be shown on the right side of the block diagram in FIG. Prior to surface roughening, it is desirable to sufficiently demagnetize the raw material by a method such as thermal demagnetization, since magnetization of the raw material is harmful to the treatment of the present invention. To give a specific example, the raw material CrO ₂ is kept at a temperature above the Curie temperature, for example 130°C, for several minutes or more, and then allowed to cool. At this time, the composition of CrO ₂ must not change. CrO ₂ whose surface has been roughened (this is A-
CrO ₂ ) is washed with acetone etc. and then sensitized. For sensitization, A-CrO ₂ is immersed in an aqueous solution of stannous chloride in hydrochloric acid (5×10 ^-2 mol/) at room temperature for 10 minutes. The sample after this treatment is designated as B-CrO ₂ . B-CrO ₂ is then treated with an aqueous solution of palladium chloride in hydrochloric acid (3×10 ^-3 mol/).
Activate the surface of _CrO2 particles. The sample after this treatment is referred to as C-CrO ₂ . What should be noted here is that the coercive force increases by about 50 oersteds in C-- _CrO2 . After the activation treatment, C-CrO ₂ is washed with water at room temperature, and then
Dry at 50℃ for 1 hour. This sample is called D-CrO ₂ . Although it is possible to move on to the subsequent work without drying, drying is generally very effective in obtaining good results in the pretreatment in the electroless plating work as described above. D-CrO ₂ is immersed in a 0.33 mol sodium hypophosphite/aqueous solution at room temperature for 10 minutes to remove unnecessary material in the next work.
Prevents precipitation of Co. Thus, E-CrO ₂ was obtained.
For example, E-CrO ₂ is electrolessly plated in a plating bath having the composition shown in Table 1 at a temperature of 80°C for a predetermined time while stirring, and then taken out.
After washing with water, it is dried to obtain a high coercive force ferromagnetic fine powder for recording.

Table 1 Cobalt sulfate CoSO ₄・7H ₂ O 0.1mol/ Sodium hypophosphite NaH ₂ PO ₂ 0.2mol/ Sodium citrate Na ₃ C ₆ H ₅ O ₇ 0.2mol/ Boric acid H ₃ BO ₃ 0.5mol/ pH 7.9 The magnetic properties of the acicular CrO ₂ (hereinafter simply referred to as CrO ₂ ) used for electroless plating are as follows. Figure 2 shows the plating time, coercive force, and magnetization σm at an applied magnetic field of 10,000 Oe. The untreated CrO ₂ used has the properties shown in Table 2.

Table 2 Saturation magnetization σs Coercive force He Squareness ratio σr/σs Curie temperature Tc 92emu/g 490Oe 0.46 116℃ Shape ratio Specific surface area 15 2.05m ² /g In Figure 2, the coercive force shows 540Oe at plating time 0. There is. This is due to the activation treatment, which increases the coercive force by about 50 Oe.
The coercive force increases in proportion to the plating time until the plating time is 2 minutes, and after 2 minutes reaching the maximum of 970 Oe, the coercive force decreases, and after 4 minutes it shows a constant value of about 800 Oe. As can be seen from this figure, in this example, the coercive force can be set to any value by controlling the plating time up to 2 minutes. The magnetization in Figure 3 shows a tendency to be inversely proportional to the increase in coercive force, and this fact shows that CrO ₂ with increased coercive force after the above treatment
This shows that magnetization reaching saturation cannot be obtained at 10,000 Oe. In other words, in addition to the inherent magnetic anisotropy of untreated _CrO2 , there is another new anisotropy,
This clearly shows that this is due to surface anisotropy caused by the bond between Cr and Co ions. Anisotropy based on such bonds has never been found before.

FIG. 4 shows the magnetization curves of untreated CrO ₂ and CrO ₂ subjected to the electroless plating described above. For convenience, the figure shows the maximum magnetization M normalized to an applied magnetic field of 10,000 Oe. As can be seen from the figure, the treated CrO ₂ has not yet fully reached saturation in a magnetic field of 10,000 Oe. Therefore, when a larger applied magnetic field is applied, the residual magnetization Mr′ of treated CrO ₂ becomes approximately the same magnitude as the residual magnetization Mr of untreated CrO ₂ . Although the results are not shown, the magnetic energy product of treated CrO ₂ is much larger than that of untreated CrO ₂ , resulting in a CrO ₂ magnetic powder that can easily achieve high-density recording.

Next, other characteristics of the treated CrO ₂ are shown in FIGS. 5 and 6. The figure shows the temperature characteristics of coercive force and magnetization, respectively. The temperature characteristics of CrO ₂ shown in the figure are already well known and need not be specifically mentioned here. Now, as shown in these figures, in treated CrO _{2 ,} both the coercive force and magnetization become 0 at around its Curie temperature of 116°C, whereas the coercive force and magnetization decrease little even at much higher temperatures. show. Such a physical phenomenon has never been reported before. It exhibits favorable properties as an industrial material.

As mentioned above, treated CrO ₂ completely improves the major drawbacks of untreated or normal CrO ₂ , which are the large temperature dependence of coercive force and magnetization at relatively low temperatures. It is possible to provide a new type of improved _CrO2 magnetic powder that has magnetic anisotropy of .

Although the specific example above has been described with respect to acicular CrO ₂ magnetic powder, the same effect can be produced even if the particle shape is granular. The reason for this is clear from the fact that the effect of the present invention is anisotropy on the surface of CrO ₂ particles based on the magnetic anisotropy between Cr and Co.

[Brief explanation of drawings]

Figure 1 is a block diagram showing the sequential steps of the method of the present invention, Figure 2 is a graph showing the relationship between electroless plating time and coercive force, Figure 3 is a graph showing the relationship between electroless plating time and magnetization, and Figure 3 is a graph showing the relationship between electroless plating time and magnetization. Figure 4 is a graph showing the magnetization curves of the magnetic powder of the present invention and the conventional one;
Figure 5 is a graph showing the relationship between coercive force and temperature of electroless plated magnetic powder and conventional magnetic powder, and
The figure is also a graph showing the relationship between magnetization and temperature.

Claims (1)

[Claims] 1. Ferromagnetic Co on the surface of acicular ferromagnetic _CrO2 powder particles
A method for producing high coercive force CrO ₂ magnetic powder, characterized by electroless plating of metal or a small amount of Co compound. 2 Roughen the surface of the acicular ferromagnetic _CrO2 powder particles,
2. The method for producing a high coercive force CrO ₂ magnetic powder according to item 1 above, which comprises sensitizing, activating, and then electroless plating using a solution containing Co ions.