PROCESS FOR PRODUCING COBALT-MODIFIED FERROMAGNETIC IRON OXIDE

Abstract: A process for preparing cobalt-modified ferromagnetic iron oxide by modifying ferromagnetic γ-Fe₂O₃ particles with cobalt, wherein prior to cobalt modification treatment, the ferromagnetic γ-Fe₂O₃ is heat treated at a temperature in the range of from about -200°C to +30°C of the critical tempera­ ture at which partial transition of γ-Fe₂O₃ into α-Fe₂O₃ occurs and which reduces the saturation magnetization to 99% of the saturation magnetization measured before the heat treat­ ment, and the atomic ratio of Fe⁺⁺ /Fe⁺⁺⁺after the cobalt modification is 0.00 to 0.10.

Description: FIELD OF THE INVENTION The present invention relates to a process for preparing cobalt-modified ferromagnetic iron oxide, and more particularly, to a process for preparing cobalt-modified ferromagnetic iron oxide by heat-treating and modifying ferromagnetic γ-Fe₂O₃ particles with cobalt for use in magnetic recording which has superior magnetic characteristics, reduced print-through tendency and high saturation magnetization (σ s ). BACKGROUND OF THE INVENTION Recently there has been a demand for magnetic recording media capable of high-density recording. A magnetic material that meets this demand should have high coercive force (Hc), high saturation magnetization (σ s ), and low-noise performance. In order to fulfill these requirements, several methods have been proposed. For instance, high coercive force (Hc) is achieved by increasing the length/diameter ratio of needle crystals of α-FeOOH (which is a raw material of magnetic iron oxide) and calcining α-FeOOH without breaking the needle shape of the crystals, or by incorporating cobalt into magnetic iron oxide. The most effective way of reducing noise of a magnetic material itself is to make the particles of a magnetic material fine (i.e., small particle size). This method, however, has a disadvantage, in that, as the particles are made finer, the unit of magnetization becomes small and the magnetization becomes thermally unstable. Magnetic tape made of such a magnetic material is poor in pring-through property (S/P). The relationship between the crystalline particle size and the print-through property is graphically shown in Fig. 1. According to the conventional technology, cobalt-modified iron oxide (marked with circles) becomes poor in print-through property (S/P) as the crystalline particle size is made finer, and it has been impossible to improve it beyond the limit indicated by the broken line. In addition, making the particles of a magnetic material finer has another disadvantage, viz., decreasing the saturation magnetization (σ s ). There are several known processes for producing cobalt-modified ferromagnetic iron oxide. According to a first category of processes, cobalt-modified ferromagnetic iron oxide is produced by causing iron oxide to form a solid solution with cobalt. Such processes are described in US-A-3,117,933 and 3,671,435, JP-B-6538/1966, 4264/1974, 27719/1966 (counterpart of US-A-3,573,980), 15759/1973, 10994/1973, and 6,113/1967, and JP-A-101599/1973. The cobalt containing iron oxide produced according to these processes, however, suffers from some disadvantages when applied to magnetic tape or magnetic recording media. That is, it is unstable to pressure and heat and the magnetic signals recorded thereon become weak with time and undergo considerable print-through. According to a second category of processes, cobalt-modified ferromagnetic iron oxide is produced by coating or growing a cobalt compound layer or a cobalt-ferrite layer on the surface of magnetic iron oxide powder containing no cobalt (in the form of solid solution). Such processes are described in JP-A-108599/1974, 37667/1975, 37668/1975, 82076/1975, 5497/1977, 5498/1977, and 129894/1978; JP-B-49475/1974; and DE-A-2,905,352. The magnetic iron oxide powder produced according to these processes is more stable to pressure and heat and improved in print-through property as compared with the powder produced according to the first category of processes. Nevertheless, it is impossible for these processes to provide a magnetic iron oxide powder which exhibits performance beyond the broken line in Fig. 1. According to the conventional technology, γ-Fe₂O₃ which is modified with cobalt is usually produced by (1) dehydrating α-FeOOH at 300 to 700°C to provide γ -Fe₂O₃, (2) reducing the γ-Fe₂O₃ in a reducing gas at 300 to 400°C to provide Fe₃O₄, and (3) oxidizing the Fe₃O₄ at a low temperature of from 200 to 300°C. The final oxidizing step is accomplished at a comparatively low temperature because it has been considered that γ-Fe₂O₃ is partly converted, irreversibly, into α-Fe₂O₃, which is a non-magnetic stable material, when subjected to a high temperature. This leads to a decrease in magnetism. The same applies to EP-A-O 075 991 wherein a process for preparing cobalt-modified ferromagnetic iron oxide is described which comprises the steps of dispersing seeds of acicular magnetic iron oxide particles in an aqueous solution containing a cobalt compound; adding thereto an alkali to form a layer of cobalt-containing iron oxide onto the surface of the seed particles; separating by filtration, washing with water and drying the particles; heating the particles in a non-oxidizing atmosphere at a temperature of 300°C or higher; oxidizing the particles by heating at a temperature of 200°C or higher in an oxidizing atmosphere to give acicular magnetic iron oxide particles having crystalline magnetic anisotropy wherein cobalt ion is uniformly doped; and preparing a magnetic paint composition by dispersing the resulting magnetic particles in a binder to be applied onto a base film. Therefore, it is the object of the present invention to provide cobalt-modified ferromagnetic iron oxide particles having superior magnetic characteristics, reduces print-through tendency and high saturation magnetization. After having carried out extensive research in order to overcome the problems of the above mentioned relationship between the print-through property and the particle size of ferromagnetic material it has been found that the print-through tendency can be greatly reduced and the magnetic characteristics can be substantially improved by heat-treating γ-Fe₂O₃ in a specific manner prior to cobalt modification and then modifying it with cobalt so that the atomic ratio of Fe² + /Fe³ + in the resulting magnetic material becomes lower than 0,10. Subject-matter of the present invention is a process for preparing cobalt-modified ferromagnetic iron oxide by heat-treating and modifying ferromagnetic γ-Fe₂O₃ particles with cobalt, which is characterized in that prior to cobalt-modification treatment, the ferromagnetic γ-Fe₂O₃ is heat-treated at a temperature in the range of from -200°C to +30°C of the critical temperature at which partial transition of γ-Fe₂O₃ into α-Fe₂O₃ occurs and which reduces the saturation magnetization to 99 % of the saturation magnetization measured before the heat-treatment, and the atomic ratio of Fe 2+ /Fe 3+ after the cobalt-modification is from 0,00 to 0,10. According to the present invention cobalt-modified ferromagnetic iron oxide for magnetic high density recording is obtained which has superior magnetic characteristics, in particular high coercive force (Hc), high saturation magnetization (σ s ), reduced print-through dendency (S/P) and low-noise performance. According to a preferred embodiment of the present invention the ferromagnetic γ-Fe₂O₃ is heat-treated at a temperature in the range of from -100°C to O ° , most preferably in the range of from -50°C to 0°C, of the critical temperature at which partial transition of γ -Fe₂O₃ into α-Fe₂O₃ occurs and which reduces the saturation magnetization to 99 % of the saturation magnetization measured before the heat-treatment. According to another preferred embodiment of the present invention the modification of ferromagnetic γ-Fe₂O₃ particles with cobalt is continued until the atomic ratio of Fe ²+ /Fe ³+ is from 0,00 to 0,05, most preferably from 0,00 to 0,02. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a graph showing the relationship between the particle size of γ-Fe₂O₃ crystals and the print-through property. The circles indicate the property of γ-Fe₂O₃ produced according to the conventional process; and the triangles indicate the property of γ-Fe₂O₃ produced according to the process of this invention. Fig. 2 is a graphical representation showing the relationship between the heat-treatment temperature for γ-Fe₂O₃ and the saturation magnetization (σ s ) of the heat-treated γ-Fe₂O₃, and the relationship between the heat-treatment temperature and the print-through property (S/P) of the cobalt-modified magnetic material. Fig. 3 is a schematic representation showing the relationship between the temperature and the heat value which was obtained when γ-Fe₂O₃ was subjected to differential thermal analysis (DTA). Fig. 4 is a schematic representation of DTA curve obtained before and after γ-Fe₂O₃ undergoes heat-treatment according to the process of this invention. "A" is the DTA curve before heat-treatment, and "B" is the DTA curve after heat-treatment. Fig. 5 is a graph showing how the Fe² + /Fe³ + ratio after cobalt-modification is related to Hc, σ s , and print-through. The solid lines indicate the results in the case where the heat-treatment of this invention was not carried out; and the broken lines indicate the results in the case where the heat-treatment of this invention was carried out. Fig. 6 is a graph showing the change of Hc with temperature. The broken line indicates the results in the case of this invention; the solid line indicates the results in the case where the heat-treatment of this invention was not carried out; and the chain line indicates the results in the case where the heat-treatment was carried out after cobalt-modification. DETAILED DESCRIPTION OF THE INVENTION The heat treatment of γ-Fe₂O₃ is accomplished at a temperature in the range of about -200°C to +30°C, preferably from -100°C to 0°C, of the critical temperature (i.e., the temperature at which partial transition of γ-Fe₂O₃ into α-Fe₂O₃ occurs and the saturation magnetization (σs) becomes 99% of that measured prior to the heat treatment). If the heat treatment temperature is lower than that specified above, the desired effect is hardly produced; and in the reverse case, the resulting magnetic material does not decrease in print- through tendency any longer but greatly decreases in saturation magnetization (σs). Fig. 2 shows the relationship between the heat treatment temperature for γ-Fe₂O₃ and the saturation magnetization (σs) of the heat-treated γ-Fe₂O₃, and the relationship between the heat treatment temperature and print-through property (S/P) of the ferromagnetic iron oxide obtained by modifying the heat-treated γ-Fe₂O₃ with cobalt. The saturation magnetization (σs) increases as the temperture goes up. It reaches a peak at a temperature 50°C to 100°C lower than the critical temperature; and it begins to decline as the treatment temperature is raised further. At the critical temperature, the saturation magnetization (σs) is lower than that measured before the heat treatment. On the other hand, the print-through property is improved as the treatment temperature is raised; and it becomes conspicuous at a temperature about 100°C lower than the critical temperature. It levels off at the critical temperature and beyond it. The critical temperature is closely related to the temperature at which the transition of γ-Fe₂O₃ to α-Fe₂O₃ takes place. Since γ-Fe₂O₃ is a metastable substance, its transition temperature varies depending on its purity and the raw material and production method employed. Even the same Y-Fe₂O₃ undergoes transition at different temperatures depending on the heating rate and holding time. In the working of this invention, the critical temperature should be established according to the heating rate and holding time of the heat treatment and other conditions such as charge amount, the capacity of a container used, and the speed of rotation or agitation of the container. Heating rate is from 1 to 10°C/min., preferably from 2 to 5°C/min., and holding time is from 1 minute to 4 hours, preferably from 10 minutes to 1 hour. Moreover, the critical temperature is closely related to the temperature at which γ-Fe₂O₃ undergoes transition to α-Fe₂O₃. Since the transition from γ-Fe₂O₃ to α-Fe₂O₃ is an exothermic reaction, the critical temperature can be easily estimated by differential thermal analysis (DTA) or differential scanning calorimetry (DSC). Various types of furnaces can be used for calcining iron oxide powders, for example, non-continuous type furnaces such as a tubular furnace, a box furnace, a flactar furnace, a crucible type electric furnace, a pit furnace and a hot air circulating incubator, continuous type furnaces such as a pusher furnace, a mesh conveyer furnace, an infra-heater drier, atmosphere furnaces such as a non-oxidative atmosphere furnace, a vacuum furnace and an ammonia decomposing apparatus can be used. Ordinarily, a tubular furnace, or its variants are used. Those having an enlarged central portion are particularly preferred. Calcining tube introduced in the furnaces typically is of rotary kiln type. As Fig. 3 shows, γ-Fe₂O₃ gives one or two exothermic peaks in DTA or DSC. The transition temperature measured by DAT (or DSC) is indicated by T tr in the figure. The critical temperature at which the transition from γ - Fe₂O₃ to α-Fe₂O₃ takes place in the ordinary kiln is 70 to 75°C lower than the transition temperature T tr which is measured in DTA or DSC with a heating rate of 10°C/min. The heat treatment of this invention can be applied to any γ-Fe₂O₃ to reduce the print-through tendency and to improve the saturation magnetization (σ s ). In other words, the heat treatment produces the effects regardless of the starting material (α-FeOOH or γ-FeOOH), method of calcination, specific surface area of γ-Fe₂O₃, pH, α-transition temperature, and the number of peaks in DTA. For example, γ-Fe₂O₃ described in U.S. -A- 3,931, 025, 3,272,595, 3,974,079, 3,904,540, can be used in the present invention. The same effects as mentioned above can be produced by performing the oxidation of Fe₃O₄ into γ-Fe₂O₃ at the above-mentioned temperature range instead of performing the heat treatment for γ-Fe₂O₃. In this case, however, the transition temperature is obtained from γ-Fe₂O₃ which is obtained by oxidation at 250°C. It is not fully elucidated yet why the magnetic characteristics are improved by the heat-treatment as mentioned above. It is considered that the heat-treatment makes the crystal structure more complete and increases the magnetic bonding strength, in view of the fact that the DTA curve of γ-Fe₂O₃ becomes sharp as a result of the heat-treatment as shown in Fig. 4. It is undesirable to perform the heat-treatment of this invention on magnetic iron oxide which has been modified with cobalt, because the heat-treatment will cause cobalt ions to diffuse into the particles, resulting in the so-called doping-type cobalt modification. Such cobalt modification causes the magnetic characteristics to greatly vary depending on temperature, aggravates the print-through tendency, and reduces the coercive force. The cobalt modification accomplished by forming a solid solution is not preferable. The coercive force and magnetization of the resulting cobalt-modified magnetic iron oxide are unstable to pressure and heat. The cobalt-modification should be achieved by coating or growing a cobalt compound layer or a cobalt ferrite layer on the surface of magnetic iron oxide powder. This method permits the heat-treatment of γ-Fe₂O₃ to fully exhibit its effect. If Fe ++ is added to cobalt, the resulting magnetic material is improved in coercive force and saturation magnetization. On the other hand, as the quantity of Fe ++ is increased, the heat-treatment of γ-Fe₂O₃ becomes less effective in reducing the print-through tendency. The coercive force of γ -Fe₂O₃ can be improved by adding cobalt or cobalt and Fe ++ toy γ-Fe₂O₃ according to the methods shown in Table 1. Where methods B to D (shown in Table 1) are employed, the atomic ratio of Fe ++ /Fe +++ in the product should be lower than 0.10 so that the print-through tendency is reduced. Fig. 5 shows how the Fe ++ /Fe +++ ratio is related with the coercive force (Hc), saturation magnetization (σs), and print-through tendency of the magnetic iron oxide produced according to method B in Table 1. The magnetic iron oxide having the Fe ++ /Fe +++ ratio = 0 in Fig. 5 corresponds to the one obtained according to method A in Table 1. In Fig. 5, the solid lines indicate the results obtained with the conventional method (in which γ-Fe₂O₃ is cobalt-modified without a preliminary heat treatment), and the broken lines indicate the results obtained with the method of this invention (in which γ-Fe₂O₃ is cobalt-modified after preliminary heat treatment). It is noted that a great improvement in the reduction of print-through tendency is obtained in the latter case. However, the improvement becomes less significant as the Fe ++ /Fe +++ ratio increases, and virtually no improvement is made when it is greater than 0.10. The invention is now described in more detail with reference to the following examples. The specific surface area of the powder was measured by means of the adsorption of nitrogen. pH was measured according to JIS method A. The transition temperataure was determined by DTA, with the heating rate of 10°C/min. For the purpose of measuring the print-through of the magnetic powder produced according to the process of this invention, magnetic tape was prepared as follows. ("Parts" denotes "parts by weight".) The above-mentioned raw materials, excluding polyisocyanate, were mixed thoroughly by using a ball mill to make a dispersion, and then the polyisocynate was added to the dispersion. After filtration, the dispersion was applied to a smooth 15-µm thick PET backing, followed by orientation in the magnetic field and drying. The coated backing film underwent calendering and slitting into 3.8-mm widths. The print-through property (S/P) was measured as follows: The 3.8-mm wide tape was wound on the reel of a compact cassette. After bulk erasure, 1 kHz signals were recorded at a prescribed input level (according to the standard MTS-102 of the Magnetic Tape Industry Association). The cassette was allowed to stand at 50°C for 48 hours, and the output signals of playback were measured through a filter having the center frequency of 1 kHz and the band width of 6 dB per 1/3 octave. Thus the signal level and the print-through level were compared. The crystal particle size was determined by averaging the particle size calculated from the half-width of the face (220) of X-ray diffraction peak and the particle size calculated from the half-width of the face (440) of X-ray diffraction peak. The magnetic characteristic properties were measured at an external magnetic field of 396825 (5 kOe) A/m by using a commercially available flux-meter. EXAMPLE 1 Goethite formed in an alkaline solution was dehydrated at 600°C in the air, followed by reduction at 320°C in hydrogen, and oxidation at 250°C in air, to provide γ-Fe₂O₃. The transition temperature of this γ-Fe₂O₃ was 675°C when measured by DTA at the heating rate of 10°C/min. 50 g of γ-Fe₂O₃ powder was placed in a 2-liter silica glass retort having one open end. The saturation magnetization (σs) of the powder was measured after it was kept at 500°C, 550°C, 600°C, and 620°C for 10 minutes each. The saturation magnetization increased as the powder was heated up to 550°C. It decreased by 1 emu/g as compared with that measured before heat treatment. The heat treatment at 620°C caused the saturation magnetization to decrease by 50% as compared with that measured before heat treatment. These results indicate that the critical temperature is 595°C. The γ-Fe₂O₃ was heated at 400°C, 500°C, 550°C, 600°C and 615°C for from 15 minutes to 4 hours (see Table 2 below). Each batch (300 g) of the heat-treated γ-Fe₂O₃ was dispersed in 2 liters of water, and to the dispersion was added 31.6 g of cobalt sulfate (CoSO₄·7H₂O) dissolved in 1 liter of water. The dispersion was vigorously stirred for 10 minutes by blowing nitrogen at a rate of 0.5 liters/min.and 489 g of NaOH dissolved in 1 liter of water was added. With continued stirring by blowing nitrogen gas, the dispersion was heated to 95°C and kept at this temperature for 4 hours to carry out the reaction. The resulting magnetic materials were designated as samples 1-1, 1-2, 1-3, 1-4, 1-5, and 1-6. On the other hand, the cobalt modification was carried out in the same way as mentioned above for γ-Fe₂O₃ which did not undergo heat-treatment. The resulting magnetic material was designated as comparative sample 1. The characteristic properties of these magnetic materials are shown in Table 2. Incidentally, the γ-Fe₂O₃ used as the raw material had Ho 30317 A/m (392 Oe), σs 69.6 emu/g, pH 8.8, crystal particle size 300 ° / A, and specific surface area 30.4 m²/g. It is noted that as the heat treatment temperature is raised, the print-through property is improved, and the improvement levels off when the heat treatment temperature exceeds the critical point. On the other hand, σs increases as the heat treatment temperature exceeds 550°C. EXMPLE 2 Goethite as used in Example 1 was converted into γ-Fe₂O₃ under the same conditions as in Example 1. The resulting γ-Fe₂O₃ underwent heat treatment at 590°C for 1 hour. The heat-treated γ-Fe₂O₃ had Ho 32222 A/m (406 Oe), σs 69.7 emu/g, specific surface area 30.5 m²/g, and crystal particle size 31 nm (310 ° / A.) This γ-Fe₂O₃ underwent cobalt modification in the same way as in Example 1. The resulting magnetic powder was designated as sample 2. EXAMPLE 3 Goethite as used in Example 1 was dehydrated and reduced under the same conditions as in Example 1. The resulting product was oxidized at 550°C to produce γ-Fe₂O₃. The γ-Fe₂O₃ thus obtained had Hc 33571 A/m, (423 OE), σs 73.2 emu/g, a pH of 8.8, and a crystal particle size of 32 nm (320 A.) This γ-Fe₂O₃ underwent cobalt modification in the same way as in Example 1. The resulting magnetic powder was designated as sample 3. The characteristic properties of samples 2 and 3 and comprative sample 1 are shown in Table 3. It is noted that the print-through property and σs are improved by either performing a heat-treatment subsequent to the calcination of γ-Fe₂O₃ or raising the oxidation temperature. EXAMPLE 4 γ-Fe₂O₃ was obtained from lepidocrosite. It had Hc 25079 A/m (316 Oe), σs 74.7 emu/g, specific surface area 23.8 m²/g, a crystal particle size of 40 nm (400 ° / A), and a pH of 3.34. The transition temperature and critical temperature determined in the same way as in Example 1 were 484°C and 410°C, respectively. This γ-Fe₂O₃ and the ones which had been heat-treated at 410°C and 380°C for 1 hour were cobalt-modified in the same way as in Example 1. The resulting magnetic powders were designated as comparative sample 2, Sample 4-1, and sample 4-2, respectively. Their characteristic properties are shown in Table 4. EXAMPLE 5 γ-Fe₂O₅ was obtained from lepidocrosite. It had Hc 24500 A/m (315 Oe), σs 72.9 emu/g specific surface area 26.7 m²/g, a pH of 3.69, and a crystal particle size of 42 nm (420 ° / A). The transition temperature and critical temperature obtained in the same way as in Example 1 were 486°C and 400°C, respectively. This γ-Fe₂O₃ and the others which had been heat-treated at 370°C and 400°C for 1 hour were cobalt-modified in the same way as in Example 1. The resulting magnetic powders were designated as comparative sample 3, sample 5-1, and sample 5-2, respectively. Their characteristic properties are shown in Table 4. EXAMPLE 6 γ-Fe₂O₃ was obtained by calcination in the conventional method. It had a pH of 5.5, σs 73.9 emu/g, Hc 25476 A/m (321 Oe) specific surface area of 24.9 m²/g, and a crystal particle size of 48nm (480 ° / Ã) The α-transition temperature and critical temperature determined in the same way as in Example 1 were 642°C and 575°C, respectively. This γ-Fe₂O₃ and the ones which had been heat-treated at 570°C and 530°C for 1 hour were cobalt-modified in the same way as in Example 1. The resulting magnetic powders are designated as comparative sample 4, sample 6-1, and sample 6-2, respectively. Their characteristis properties are shown in Table 4. It is noted from Table 4 that if γ-Fe₂O₃ undergoes heat-treatment, the print-through property of the cobalt-modified magnetic powder is greatly improved regardless of the properties of γ-Fe₂O₃ such as specific surface area, crystal particle size, pH, transition temperature, and critical temperature, and the crystal form of iron oxide hydrate and the conditions (acidic or alkaline) under which iron oxide hydrate is formed. EXAMPLE 7 γ-Fe₂O₃ was obtained from goethite which had been formed in an acid solution. It had Hc 25952 A/m (327 Oe) σs 73.0 emu/g, specific surface area of 23.3 m²/g, a pH of 3.8, and a crystal particle size of 49 nm (490 ° / A.) The transition temperature and critical temperature determined in the same way as in Example 1 were 643°C and 565°C, respectively. This γ-Fe₂O₃ and the ones which had been heat-treated at 560°C and 510°C for 1 hour were cobalt-modified in the same way as in Example 1. The resulting magnetic powders were designated as comparative sample 5, sample 7-1, and sample 7-2, respectively. Their characteristic properties are shown in Table 4. EXMPLE 8 The same γ-Fe₂O₂ as used in Example 1 and the one which had been annealed at 600°C for 1 hour underwent cobalt modification according to the following method. 300 g of γ-Fe₂O₂ was dispersed in an aqueous solution of 31.6 g of cobalt sulfate (CoSO₄·7H₂O) in 2 liters of water. With vigorous stirring, 176 g of NaOH dissolved in 1 liter of water was added. The dispersion was heated to 95°C. 15.5 g of ferric sulfate (Fe₂(SO₄)₃·7H₂O) dissolved in 1 liter of water was added over 1 hour at a rate of 16 ml/min. Heating and stirring were continued for 3 hours to carry out reaction for 4 hours in total. The reation product was washed with water, dehydrated, and dried. The resulting magnetic powders were designated as comparative sample 6 and sample 8. Their characteristic properties are shown in Table 5. EXAMPLE 9 The same γ-Fe₂O₃ as used in Example 1 and the one which had been annealed at 600°6 for 1 hour underwent cobalt-modification according to the following method. 300 g of γ-Fe₂O₃ was dispersed in 2 liters of water. With vigorous stirring by blowing nitrogen gas at a rate of 0.5 liter/min, an aqueous solution of 31.6 g of cobalt sulfate (CoSO₄·7H₂O) and 81 g of ferrous sulfate (FeSO₄·7H₂O) in 0.5 liter of water was added. 10 minutes later, 153 g of NaOH dissolved in 0.5 liter of water was added. The dispersion was kept to 95°C for 4 hours. The reaction product was washed with water, dehydrated, and dried. The resulting magnetic powders were designated as comparative sample 7 and sample 9. Their characteristic properties are shown in Table 5. EXAMPLE 10 The magnetic materials prepared under the same conditions as in Example 9 underwent heat treatment in nitrogen at 150°C for 2 hours. The resulting magnetic powders were designated as comparative sample 8 and sample 10. Their characteristic properties are shown in Table 5. EXAMPLE 11 The magnetic materials prepared under the same conditions as in Example 9 were kept at 100°C in the air to oxidize Fe ++ so that the atomic ratio of Fe ++ /Fe +++ became nearly 0.03. The resulting-magnetic powders were designated as comparative sample 9 and sample 11. Their characteristic properties are shown in Table 5. As Table 5 shows, the print-through property of the magnetic material is improved when cobalt modification is carried out after γ-Fe₂O₃ has undergone heat treatment. This holds true regardless of the method of cobalt modification. EXAMPLE 12 The same γ-Fe₂O₃ as used in Example 1 was heated at 550°C for 4 hours and then underwent the same heat treatment as employed for cobalt modification of sample 1-4. 300 g of the heat-treated γ-Fe₂O₃ was dispersed in 2 liters of water. To the dispersion were added 31.6 g of cobalt sulfate (CoSO₄·7H₂O) and 31.3 g of ferrous sulfate (FeSO₄·7H₂O) dissolved in 0.5 liter of water, followed by vigorous stirring for 10 minutes by blowing nitrogen gas at a rate of 0.5 liter/min. 138 g of NaOH dissolved in 1 liter of water was added. The dispersion was heated, with stirring by blowing nitrogen gas. Reaction was carried out at 95°C for 4 hours The resulting magnetic material was designated as sample 12. EXAMPLE 13 Example 12 was repeated except that the quantities of ferrous sulfate and sodium hydroxide were changed to 93.8 g and 516 g, respectively. The resulting magnetic material was designated as sample 13. EXAMPLE 14 Example 12 was repeated except that the quantities of ferrous sulfate and sodium hydroxide were changed to 124.2 g and 525 g, respectively. The resulting magnetic material was designated as sample 14. EXAMPLE 15 Sample 13 was moderately oxidized at 150°C for 1 hour under a slow air stream in a retort. The resulting magnetic material was designated as sample 15. COMPARATIVE EXAMPLES Magnetic materials were prepared from comparative sample 1, which is γ -Fe₂O₃ which did not undergo heat treatment, according to the same method as used in Examples 12, 13, 14, and 15. The resulting magnetic materials were designated as comparative samples 10, 11, 12, and 13. Table 6 shows the characteristic properties of samples 1-4, 12, 13, 14, and 15, and comparative samples 1, 10, 11, 12, and 13. It is noted as the Fe ++ /Fe +++ ratio is increased, the coercive force and σs are improved. However, the improvement of print-through which is achieved by preliminarily heating γ-Fe₂O₃ is decreased. Fig. 5 shows that the improvement of print-through is hardly made when the Fe ++ /Fe +++ ratio exceeds 0.10. Comparative sample 1 was heated at 550°C for 4 hours. The resulting magnetic material was designated as comparative sample 14. Its characteristic properties are shown in Table 6. The heat-treatment for comparative sample 14 is the same as that used before cobalt modification of sample 1-4. It is noted that the heat treatment slightly improves σs, but aggravates Hc and print-through. A great improvement of squareness ratio (SQ) is noticed, but it is due to the improvement of crystal anisotropy achieved by the doping of cobalt ion, but not due to the improvement of orientation. This is apparent from the fact that SQ in the width direction is as great as 0.85. Fig. 6 shows the relationship between the temperature and Hc of comparative samples 1 and 14 and sample 1-4. It is noted that where heat treatment is carried out after cobalt modification, Hc greatly varies depending on the temperature. This suggests that the heat treatment of this invention should be carried out before cobalt modification. In Fig. 6 (Table 6), the solid line (comparative sample 1) indicates the results obtained by the conventional technology (no heat treatment), the broken line (sample 1-4) indicates the result obtained by cobalt-modification after heat-treatment, and the chain line (comparative sample 14) indicates the results obtained by heat-treatment after cobalt-modification. It is noted that where heat-treatment is carried out after cobalt-modification, Hc greatly varies depending on the temperature and the print-through property is rather deteriorated.

Claims: A process for preparing cobalt-modified ferromagnetic iron oxide by heat-treating and modifying ferromagnetic γ-Fe₂O₃ particles with cobalt, cha racterized in that prior to cobalt-modification treatment, the ferromagnetic γ-Fe₂O₃ is heat-treated at a temperature in the range of from -200°C to +30°C of the critical temperature at which partial transition of γ-Fe₂O₃ into occurs and which reduces the saturation magnetization to 99 % of the saturation magnetization measured before the heat-treatment, and the atomic ratio of Fe ²+ /Fe ³+ after the cobalt-modification is from 0,00 to 0,10. The process according to claim 1, wherein the ferromagnetic γ-Fe₂O₃ is heat-treated at a temperature in the range of from -100°C to 0°C of the critical temperature at which partial transition of the γ-Fe₂O₃ into γ-Fe₂O₃ occurs which reduces the saturation magnetization to 99 % of the saturation magnetization measured before the heat-treatment. The process according to claim 2, wherein the heating temperature is in the range of from -50°C to 0°C of the critical temperature at which partial transition of γ-Fe₂O₃ into α-Fe₂O₃ occurs. The process according to any of claims 1 to 3, wherein the atomic ratio of Fe ²+ /Fe ³+ after cobalt-modification is from 0,00 to 0,05. The process according to claim 4, wherein the atomic ratio of Fe ²+ /Fe ³+ after the cobalt-modification is from 0,00 to 0,02.

Inventors: DEZAWA, SHIN-ICHIRO, KITAMOTO, TATSUJI, KOSHA, HIDEAKI
Applicants: FUJI PHOTO FILM CO., LTD.
Application year: 1984
Main classification: H01F1
IPC codes: H01F1, C01G49, G11B5