BARIUM FERRITE PARTICLES FOR MAGNETIC RECORDING MEDIA

Abstract: Plate-like barium ferrite particles which have a BET specific surface area of 20 to 70 m²/g, an average particle diameter of 0.05 to 0.3 µm, a magnetization of higher than 30 emu/g in a field of 7.96 × 10⁵ A m⁻¹ (10 kOe) and a coercive force of 2.39 × 10⁴ to 7.96 × 10⁴ A m⁻¹ (300 to 1,000 Oe) and which are represented by the formula BaCo x Ti y Fe 12-x-y O₁₉ wherein x and y are independently numbers from 0.1 to 0.5, are useful as the magnetic material for magnetic recording media.

Description: The present invention relates to the preparation of barium ferrite particles and to magnetic recording media containing the particles. In recent years, accompanying with the growth of VTR, audio component, word processers and computers, non-acicular ferromagnetic particles of a favourable dispersibility provided with a suitable coercive force (Hc) have been required as the magnetic material for recording, particularly as the magnetic material for perpendicular magnetic recording. In general, as the ferromagnetic non-acicular particles barium ferrite particles have been well known. However, the coercive force of barium ferrite particules obtained by the dry method is ordinarily larger than 2.39 × 10⁵A m _1 (3,000 Oe), and because of such a high coercive force, barium ferrite particules are not favourable as magnetic material for magnetic recording. Further, conventional barium ferrite particles obtained by sintering are composed of polycrystals of an average diameter of a few micrometers. Even after pulverizing, the average diameter of the particles is around one micrometer. Accordingly, such barium ferrite particles are poor in dispersibility in paints and are not favourable as a magnetic-material for magnetic recording. Namely, as a magnetic material for magnetic recording, those which are as fine as possible, particularly those of an average particle diameter of 0.05 to 0.3 µm, have been demanded. For instance, Japanese Patent Application Laying Open No. 53-20596 (1978) discloses that there is a difficulty in uniformly dispersing the particles of diameter over 0.5 µm. Japanese Patent Application Laying Open No. 56-125219 (1981) says: "In the range of recording wavelength of shorter than Iµm, perpendicular magnetic recording is more useful as compared to longitudinal magnetic recording, and in order to effect sufficient recording and regeneration in the abovementioned wavelength region, the particle diameter of ferrite is favorably less than about 0.3 µm. Since the particles of about 0.01 µm in diameter do not show the desired ferromagnetism, as an preferable diameter of particle, the range of 0.01 to 0.3 µm is demanded." Japanese Patent Application Laying Open No. 57-212623 (1982) discloses that the average particle diameter of magnetic barium ferrite particle is favorably below 0.3 µm, and particularly, those fine particles of average particle diameter of 0.03 to 0.3 µm are favorably preferable, because the particles of the particle diameter of below 0.03 µm do not exhibit the sufficient ferromagnetic property necessary for magnetic recording, and on the other hand, the particles of the average particle diameter of over 0.3 µm cannot advantageously carry out the magnetic recording as the high density recording. Further, it is necessary that the value of magnetization of the magnetic particles is as large as possible, and its necessity is clearly seen in the description in Japanese Patent Application Laying Open No. 56-149328 (1981) that it is required that the saturation magnetization of magnetoplumbite-ferrite used as a material for the medium of magnetic recording is as large as possible. On the other hand, as a method for producing barium ferrite particles, a method of treating an aqueous alkaline suspension containing Ba-ions and Fe(III) in an autoclave as a reactor (the method is hereinafter referred to as "the autoclaving method") has been hitherto known, and by selecting the reaction conditions in the autoclaving method, barium ferrite particles precipitate. The thus precipitated particles are hexagonal plate-like particles, and the distribution of the particle size and the average size of the particles differ according to the reaction conditions resulting in the difference of the magnetic properties of the particles. In the technical field of producing plate-like barrium ferrite particles for use in magnetic recording, not only the method of forming the product from an aqueous solution but also the method for forming the product from a fluid has been hitherto tried. On the other hand, a technique by which non-ferromagnetic barium ferrite particles are formed from an aqueous solution according to the autoclaving method and the thus obtained non-ferromagnetic barium ferrite particles are sintered at a high temperature to obtain ferromagnetic barium ferrite particles has been tried. Further, as is seen in Japanese Patent Application Laying Open No. 56-149328 (1981), in order to reduce the coercive force of the barium ferrite particles, it is proposed that a part of Fe(III) in the ferrite is substituted by Co(II) and Ti(IV). In this case, as is clearly seen in Example of Japanese Patent Application Laying Open No. 56-149328 (1981), unless the large amount of Co(II) and Ti(IV) is added so that in the formula BaCo x Ti x Fe 12-2x O₁₉, x becomes larger than 0.8, the coercive force cannot be reduced to the extent which is approrriate as, the magnetic material for magnetic recording. However, such a large amount of Co(II) and Ti(IV) causes the reduction of the purity of the thus formed barium ferrite particles resulting in a large reduction of the magnetization although the effect of reducing the coercive force is larger. The above-mentioned fact is clearly seen in Comparative Example 1 of Japanese Patent Application Laying Open No. 56-149328 (1981) wherein the extent of saturation magnetization is as small as 25 emu/g. The present inventors, in order to reduce the coercive force effectively without noticeably reducing the magnetization by adding Co(II) and Ti(IV) in an amount of as small as possible, have repeated a systematic investigation of the relationship between the conditions for formation of barium ferrite particles from an aqueous solution and the average particle diameter, the particle size distribution and the magnetic properties of the thus formed barium ferrite particles. As a result of the present inventors' study, it has been found that in the case where an aqueous alkaline suspension containing Fe(III), Co(II), Ti(IV) and Ba ions is autoclaved at a temperature of from 250 to 320°C,wherein the total amount of Co(II) and Ti(IV) is 0.017 to 0.09 to one atomic amount of Fe(III), the atomic ratio of Ba ions to the sum of Fe(III), Co(II) and Ti(IV) ions is 1: 7 to 1: 9 and preferably the molar amount of Co(II) is the same as that of Ti(IV), fine plate-like particles are formed represented by the formula BaCo x Ti y Fe 12-x-y O₁₉ wherein independently 0.1 ≦ x ≦ 0.5 and 0.1 ≦ y ≦ 0.5, having the properties of 20 to 70m²/g of BET specific surface area and 0.05 to 0.3 µm of average particle diameter, and showing more than 30 emu/g of value of magnetization in a magnetic field of 7.96 × 10⁵ A m _1 (10 kOe) and 2.39 × 10⁴ to 7.96 × 10⁴ A m _1 (300 to 1,000 Oe) of coercive force. Accordingly, the present invention provides a process for producing plate-like barium ferrite particles represented by the formula: BaCo x Ti y Fe 12-x-y O₁₉ wherein x and y independently are numbers from 0.1 to 0.5; which process comprises autoclaving at a temperature of from 250 to 320°C an aqueous alkaline suspension containing Fe(III), Co(II), Ti(IV) and Ba ions and a molar excess with respect to the total amount of Fe(III), Co(II), Ti(IV) and Ba ions of an alkali metal hydroxide and in which suspension the atomic ratio of the total amount of Co(II) and Ti(IV) to the amount of Fe(III) is from 0.017: 1 to 0.09: 1 and the atomic ratio of Ba ions to the total sum of Fe(III), Co(II) and Ti(IV) is from 1: 7 to 1: 9; thereby obtaining plate-like barium ferrite particles of the said formula which have a BET specific surface area of from 20 to 70 m²/g, an average particle diameter of from 0.05 to 0.3 µm, a magnetization of larger than 30 emu/g in a magnetic field of 7.96 × 10⁵ A m _1 (10 kOe) and a coercive force of from 2.39 × 10⁴ to 7.96 × 10⁴ A m _1 (300 to 1,000 Oe). In the attached drawings: Figs. 1 to 3 respectively show the diagrams obtained by plotting the values of magnetization (M), coercive force (Hc) and specific surface area (S) of the barium ferrite particles represented by the formula, BaCo x Ti x Fe 12-2x O₁₉, i.e. in which x = y, formed according to the present invention, against x which shows the composition of the barium ferrite; Fig. 4 is an electronmicrograph (×40,000) of the barium ferrite particles of x = y = 0.15 obtained in Example 1; Fig. 5 is an electronmicrograph (×100,000) of the barium ferrite particles of x = y = 0.40 obtained in Example 2; and Fig. 6 is an electronmicrograph (×20,000) of the barium ferrite particles obtained in Comparative Example 1. In order to obtain the barium ferrite particles of an effectively reduced coercive force with the addition of Co(II) and Ti(IV) in an amount as small as possible without remarkable reduction of magnetization, the present inventors have prepared various kinds of plate-like barium ferrite particles represented by the formula BaCo x Ti y Fe 12-x-y O₁₉ while changing the conditions for formation thereof such as the ratio of Co(II) and Ti(IV) to Fe(III), the ratio of Ba ions to Fe(III), Co(II) and Ti(IV), the concentration of alkali in the aqueous alkaline solution, the temperature and time of the reaction and the conditions of agitation of the aqueous suspension. As a result of the study, the present inventors have found that in the case where an aqueous alkaline suspension containing Fe(III), Co(II), Ti(IV) and Ba ions wherein the total atomic ratio of Co(II) and Ti(IV) to Fe(III) is 0.017: 1 to 0.09: 1, the atomic ratio of Ba ion to the sum of Fe(III), Co(II) and Ti(IV) is 1: 7 to 1: 9 and preferably the molar amount of Co(II) is the same as that of Ti(IV), is autoclaved at a temperature of 250 to 320°C, it is possible to obtain from the aqueous suspension the fine plate-like barium ferrite particles represented by the formula, BaCo x Ti y Fe₁₂ -x-y 0₁₉ wherein 0.1 ≦ x ≦ 0.5 and independently 0.1 ≦ y ≦ 0.5, having a BET specific surface area of 20 to 70 m²/g and average particle diameter of 0.05 to 0.3 µm, and showing magnetization of larger than 30 emu/g in a magnetic field of 7.96 × 10⁵ A m _1 (10 kOe) and coercive force of 2.39 × 10⁴ to 7.96 × 10⁴ A m _1 (300 to 1,000 Oe) and that there is a close relationship among the composition of the thus obtained fine plate-like barium ferrite particles represented by the above-mentioned formula and the shape, the size, the properties such as the coercive force (Hc) and the magnetization (M). The following is the explanation of the representative examples of the numerous experiments carried out by the present inventors. Figs. 1 to 3 respectively show the diagrams obtained by plotting the magnetization (M), the coercive force (Hc) and the specific surface area (S) of the thus formed fine plate-like barium ferrite particles of an average particle diameter of 0.1 µm and of a degree of packing of 1.6 g/cm³ against x when x = y which represents the composition of the barium ferrite when the plate-like particles are magnetized at 7.96 × 10⁵ A m _1 (10 kOe). As is clearly seen in Fig. 1, there is a tendency of a slight reduction of the magnetization (M) with the increase of the value of x, and as are clearly seen in Figs. 2 and 3, there are the tendencies of an increase in the BET specific surface area (S) and of a reduction in the coersive force (Hc) with the increase of the value of x. Accordingly, the coercive,force (Hc) can be controlled at a value of less than 7.96 × 10⁴ A m _1 (1,000 Oe) mainly by changing x which represents the composition of barium ferrite. As a result, the plate-like barium ferrite particles of the formula BaCo x Ti y Fe 12-x-y O₁₉ wherein 0.1 ≦ x ≦ 0.5 and independently 0.1 ≦ y ≦ 0.5 and having a BET specific surface area of 20 to 70 m²/g according to the present invention show a small value of coercive force. Preferably in the invention x is the same as y. The conditions for the production of the plate-like barium ferrite particles according to the invention are described as follows: As the source of Fe(III) for the present invention, ferric chloride, ferric nitrate and powdery ferric oxyhydroxide may be used. As the source of Ba ions for the present invention, barium hydroxide and barium oxide may be used. As the source of Co(II) for the present invention, cobalt nitrate and cobalt chloride may be used, and as the source of Ti(IV) for the present invention, titanium chloride and alkali titanate may be used. The autoclaving treatment of the present invention may be carried out at any temperature which is lower than the critical temperature of the aqueous solution, and particularly the reaction temperature of 250 to 320°C is suitable for an economic production of the ferrite. The atomic ratio of the total amount of Co(II) and Ti(IV) to the amount of Fe(III) in the present invention is 0.017: 1 to 0.09: 1, preferably 0.024: 1 to 0.057: 1. In the case where the atomic ratio is below 0.017: 1 or in the case where the atomic ratio is over 0.09: 1, it is unable to obtain barium ferrite particles represented by the formula, BaCo x Ti y Fe₁₂ -x-y O₁₉ wherein 0.1 ≦ x ≦ 0.5 and independently 0.1 ≦ y ≦ 0.5. The atomic ratio of Ba ions to the total amount of Fe(III), Co(II) and Ti(IV) is 1: 7 to 1: 9. When the pH becomes higher, reaction temperature becomes higher or the feed ratio of Co(II) and Ti(IV)to Fe(III) becomes larger, the atomic ratio of Ba ions to the total amount of Fe(III), Co(II) and Ti(IV) becomes up to 1: 9. The plate-like barium ferrite particles produced by the procedures according to the present invention exhibit the following properties: Namely, according to the process of the present invention, plate-like barium ferrite particles are obtained which are favourable in dispersibility and show a BET specific surface area of 20 to 70 m²/g, preferably 45 to 65 m²/g, average particle diameter of 0.05 to 0.3 µm, preferably 0.05 to 0.25 µm magnetization of larger than 30 emu/g in a magnetic field of 7.96 × 10⁵ A m _1 (10 kOe) and a coercive force of 2.39 × 10⁴ to 7.96 × 10⁴ A m _1 (300 to 1,000 Oe), preferably 3.18 × 10⁴ to 6.36 × 10⁴ A m _1 (400 to 800 Oe). Such plate-like barium ferrite particles are suitable as the magnetic material for magnetic recording now being demanded, particularly as the magnetic material for perpendicular magnetic recording. Accordingly the present invention further provides a magnetic recording medium in which the magnetic material comprises plate-like barium ferrite particles produced according to the invention. The following Examples 1 and 2 illustrate the invention. The value of specific surface area means that the value is obtained by the BET method, the value of magnetization is measured in a magnetic field of 7.96 × 10⁵ A m _1 (10 kOe), and the coercive force is measured at a packing density of 1.6 g/cm³. EXAMPLE 1 Into 20 litres of decarbonized water in an autoclave, 14 mol of Fe(N₃)₃, each 0.179 mol of Co(NO₃)₂ and TiCl₄, 1.76 mol of Ba(OH)₂ · 8H₂O and 162 mol of NaOH were added, and the mixture was heated to 270°C. Thereafter, the mixture was kept at the temperature for 5 hours under mechanical agitation to form a ferromagnetic precipitate of brown in colour. After cooling the content of the autoclave to room temperature, the ferromagnetic brown precipitate was collected by filtration, washed with water and dried at 80°C. The thus obtained ferromagnetic brown particles were identified as BaCo 0.15 Ti 0.15 Fe₁ 1.7 O₁₉ as a result of fluorescent X-ray analysis. Fig. 4 shows the electronmicrograph (×40,000) of the thus obtained particles, and as is seen in Fig. 4, the product was hexagonal plate-like particles having specific surface area of 28 m²/g, uniform in particle size and the particle diameter of 0.1 to 0.2 µm. According to vibrating sample magnetometer, magnetization was 38 emu/g in a field of 10 kOe and the coercive force of the product was 850 Oe at a packing density of 1.6 g/cm³. EXEMPLE 2 Into 20 litres of decarbonized water in an autoclave, 14 mol of Fe(NO₃)₃, each 0.499 mol of TiCl₄ and Co(NO₃)₂, 1.75 mol of Ba(OH) · 8H₂O and 167 mol of NaOH were added, and after heating the mixture to 300°C, the content of the autoclave was kept at the same temperature for 3 hours while mechanically agitating the mixture to obtain a ferromagnetic precipitate brown in colour. By treating the product of the reaction in the same procedures as in Example 1, ferromagnetic brown particles was obtained, and identified as BaCo 0.40 Ti 0.40 Fe 11.20 O₁₉ as a result of fluorescent X-ray analysis. The specific surface area (S) of the thus obtained particles was 58 m²/g. Fig. 5 is the electronmicrograph (×100,000). As are clearly seen in Fig. 5, the product of Example 2 was hexagonal plate-like particles of uniform in particle size, having the particle diameter of 0.05 to 0.1 µm. The magnetization of the product was 32 emu/g in a magnetic field of 10 kOe and the coercive force(Hc) was 415 Oe at a packing density of 1.6 g/cm³. Further, in barium ferrite, the direction of C axis is the direction of easy magnetization, and also the thus obtained fine plate-like particles of BaCo x Ti x Fe₁₂ -2x O₁₉ according to the present invention are easily orientable by pressing, thereby the value of coercive force (Hc) becomes larger. COMPARATICE EXAMPLE 1 In the same procedures as in Example 1 except for using no Co(II) and Ti(IV), ferromagnetic brown particles were obtained, which showed the specific surface area(S) of 6.5 m²/g. Fig. 6 is an electronmicrograph of the product (×20,000). As are clearly seen in Fig. 6, the product was composed of hexagonal plate-like particles of the particle diameter of 1.0 µm. The product showed the magnetization(M) of 49.6 emu/g in a magnetic field of 10 kOe and the coercive force of 1,250 Oe at a packing density of 1.6 g/cm³. COMPARATIVE EXAMPLE 2 In the same procedures as in Example 1 except for using each 0.999 mol of Co(II) and Ti(IV), a ferromagnetic brown precipitate was obtained. The ferromagnetic brown particles obtained from the precipitate was identified as BaCo O.75 Ti₀ .75 Fe 10.5 O₁₉ as a result of fluorescent X-ray analysis. The value of magnetization (M) was 15 emu/g in a magnetic field of 10 kOe.

Claims: 1. A process for producing plate-like barium ferrite particles represented by the formula: BaCo x Ti y Fe 12-x-y O₁₉ wherein x and y independently are numbers from 0.1 to 0.5; which process comprises autoclaving at a temperature of from 250 to 320°C an aqueous alkaline suspension containing Fe(III), Co(II), Ti(IV) and Ba ions and a molar excess with respect to the total amount of Fe(III), Co(II), Ti(IV) and Ba ions of an alkali metal hydroxide and in which suspension the atomic ratio of the total amount of Co(II) and Ti(IV) to the amount of Fe(III) is from 0.017: 1 to 0.09: 1 and the atomic ratio of Ba ions to the total sum of Fe(III), Co(II) and Ti(IV) is from 1: 7 to 1: 9; thereby obtaining plate-like barium ferrite particles of the said formula which have a BET specific surface area of from 20 to 70 m²/g, an average particle diameter of from 0.05 to 0.3 µm, a magnetization of larger than 30 emu/g in a magnetic field of 7.96 × 10⁵ A m _1 (10 kOe) and a coercive force of from 2.39 × 10⁴ to 7.96 × 10⁴ A m _1 (300 to 1,000 Oe). 2. A process according to claim 1, wherein the source of Fe(III) is ferric chloride, ferric nitrate or powdery ferric oxyhydroxide. 3. A process according to claim 1 or 2, wherein the source of Ba ions is barium hydroxide or barium oxide. 4. A process according to any one of the preceding claims wherein the source of Co(II) is cobalt nitrate or cobalt chloride. 5. A process according to any one of the preceding claims wherein the source of Ti(IV) is titanium chloride or alkali titanate. 6. A process according to any one of the preceding claims wherein the molar amount of Co(II) in the aqueous alkaline suspension is the same as the molar amount of Ti(IV). 7. A magnetic recording medium in which the magnetic material comprises plate-like barium ferrite particles which are represented by the formula: BaCo x Ti y Fe 12-x-y O₁₉ wherein x and y independently are numbers from 0.1 to 0.5; which have a BET specific surface area of from 20 to 70 m²/g, an average particle diameter of from 0.05 to 0.3 µm, a magnetisation of larger than 30 emu/g in a magnetic field of 7.96 × 10⁵ A m _1 (10 kOe) and a coercive force of from 2.39 × 10⁴ to 7.96 × 10⁴ A m _1 (300 to 1000 Oe); and which have been prepared by a process as claimed in any one of the preceding claims.

Inventors: HORIISHI, NANAO, KIYAMA, MASAO, NAGAI, NORIMICHI, TAKADA, TOSHIO
Applicants: TODA KOGYO CORP.
Application year: 1984
Main classification: H01F1
IPC codes: G11B5, C01G49, H01F1