Process for producing cobalt-modified ferromagnetic iron oxide
FIELD OF THE INVENTION 
     The present invention relates to a process for producing ferromagnetic iron oxide, and more particularly, to a process for producing ferromagnetic iron oxide for magnetic recording which has superior magnetic characteristics, reduced print-through tendency, and high saturation magnetization (σs). 
     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 (σs), 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 magnetic iron oxide with cobalt. 
     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 print-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 magentization (σs). 
     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 U.S. Pat. Nos. 3,117,933 and 3,671,435, Japanese Patent Publication Nos. 6538/19566, 4264/1974, 27719/1966 (counterpart of U.S. Pat. No. 3,573,980), 15759/1973, 10994/1973, and 6113/1967, and Japanese Patent Application (OPI) No. 101599/1973 (the term &#34;OPI&#34; as used herein refers to &#34;published unexamined Japanese Patent Application&#34;). 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 Japanese Patent Application (OPI) Nos. 108599/1974, 37667/1975, 37668/1975, 108599/1974, 37667/1975, 37668/1975, 82076/1975, 5497/1977, 5498/1977, and 129894/1978; Japanese Patent Publication No. 49475/1974; and West German Patent Application (OLS) No. 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-Fe 2  O 3  which is modified with cobalt is usually produced by (1) dehydrating α-FeOOH at 300° to 700° C. to provide α-Fe 2  O 3 , (2) reducing the α-Fe 2  O 3  in a reducing gas at 300° to 400° C. to provide Fe 3  O 4 , and (3) oxidizing the Fe 3  O 4  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-Fe 2  O 3  is partly converted, irreversibly, into α-Fe 2  O 3 , which is a non-magnetic stable material, when subjected to a high temperature. This leads to a decrease in magnetism. 
     SUMMARY OF THE INVENTION 
     In order to overcome the problems of the above-mentioned relationship between the print-through property and the particle size of magnetic material, the present inventors carried out extensive research, and have now discovered the finding that the print-through tendency can be greatly reduced in the case of cobalt-modified γ-Fe 2  O 3  by heating γ-Fe 2  O 3  beforehand and then modifying it with cobalt so that the atomic ratio of Fe ++  /Fe +++  in the resulting magnetic material becomes lower than 0.10. 
     Accordingly, it is an object of this invention to provide a process for producing ferromagnetic iron oxide, which process comprises heating γ-Fe 2  O 3  at a temperature higher than the conventionally employed oxidizing temperature, and then coating or growing a cobalt ferrite layer on the surface of γ-Fe 2  O 3  so that the atomic ratio of Fe ++  /Fe +++  in the resulting magnetic material becomes lower than 0.10. 
     More particularly, the present invention involves a process for preparing cobalt-modified ferromagnetic iron oxide by modifying ferromagnetic γ-Fe 2  O 3  particles with cobalt, wherein prior to cobalt modification treatment, the ferromagnetgic γ-Fe 2  O 3  is heat treated at a temperature in the range of from about -200° C. to +30° C. of the critical temperature at which partial transition of γ-Fe 2  O 3  into α-Fe 2  O 3  occurs and which reduces the saturation magnetization value to 99% of the saturation magnetization value measured before the heat treatment, and the atomic ratio of Fe ++  /Fe +++  after the cobalt modification is 0.00 to 0.10. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a graph showing the relationship between the particle size of γ-Fe 2  O 3  crystals and the print-through property. The circles indicate the property of γ-Fe 2  O 3  produced according to the conentional process; and the triangles indicate the property of γ-Fe 2  O 3  produced according to the process of this invention. 
     FIG. 2 shows graphical representation showing the relationship between the heat treatment temperature for γ-Fe 2  O 3  and the saturation magnetization (σs) of the heat-treated γ-Fe 2  O 3 , 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-Fe 2  O 3  was subjected to differential thermal analysis (DTA). 
     FIG. 4 is a schematic representation of DTA curve obtained before and after γ-Fe 2  O 3  undergoes heat treatment according to the process of this invention. &#34;A&#34; is the DTA curve before heat treatment, and &#34;B&#34; is the DTA curve after heat treatment. 
     FIG. 5 is a graph showing how the Fe ++  /Fe +++  ratio after cobalt modification is related to Hc, σs, 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-Fe 2  O 3  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-Fe 2  O 3  into α-Fe 2  O 3  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-Fe 2  O 3  and the saturation magnetization (σs) of the heat-treated γ-Fe 2  O 3 , 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-Fe 2  O 3  with cobalt. The saturation magnetization (σs) increases as the temperature 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-Fe 2  O 3  to α-Fe 2  O 3  takes place. Since γ-Fe 2  O 3  is a metastable substance, its transition temperature varies depending on its purity and the raw material and production method employed. Even the same γ-Fe 2  O 3  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-Fe 2  O 3  undergoes transition to α-Fe.sub. 2 O 3 . Since the transition from γ-Fe 2  O 3  to α-Fe 2  O 3  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. Ordinarily, a tubular furnace, e.g., one manufactured by Hayashi Electric Co., Ltd. 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-Fe 2  O 3  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-Fe 2  O 3  to α-Fe 2  O 3  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-Fe 2  O 3  to reduce the print-through tendency and to improve the saturation magnetization (σs). 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-Fe 2  O 3 , pH, α-transition temperature, and the number of peaks in DTA. For example, γ-Fe 2  O 3  described in U.S. Pat. Nos. 3,931,025, 3,272,595, 3,974,079, 3,904,540, etc. can be used in the present invention. The same effects as mentioned above can be produced by performing the oxidation of Fe 3  O 4  into γ-Fe 2  O 3  at the above-mentioned temperature range instead of performing the heat treatment for γ-Fe 2  O 3 . In this case, however, the transition temperature is obtained from γ-Fe 2  O 3   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-Fe 2  O 3  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-Fe 2  O 3  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-Fe 2  O 3  becomes less effective in reducing the print-through tendency. 
     The coercive force of γ-Fe 2  O 3  can be improved by adding cobalt or cobalt and Fe ++  to γ-Fe 2  O 3  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. 
     
                                           TABLE 1                                 
__________________________________________________________________________
Methods for Modification with Cobalt and Ferrous Ions                     
           Reaction                                                       
Method                                                                    
     Additive                                                             
           Atmosphere                                                     
                  Outline of Method                                       
                                   Literature                             
__________________________________________________________________________
A    Co.sup.++                                                            
           Non-oxidative                                                  
                  A solution containing Co.sup.++ is                      
                                   Japanese Patent                        
                  added to γ-Fe.sub.2 O.sub.3 slurry.               
                                   Application(OPI)                       
                  slurry is heated, while being                           
                                   No. 74399/1984 and                     
                  kept alkaline. The order of                             
                                   others                                 
                  addition may be changed.                                
B    Co.sup.++  &amp;                                                         
           Non-oxidative                                                  
                  A solution containing Co.sup.++  and                    
                                   Japanese Patent                        
     Fe.sup.++    Fe.sup.++  is added to γ-Fe.sub.2 O.sub.3         
                                   Publication                            
                  The slurry is heated, while being                       
                                   No. 36751/1977 and                     
                  kept alkaline.   others                                 
           Oxidative               Japanese Patent                        
                                   Publication                            
                                   No. 49475/1984 and                     
                                   others                                 
C    Co.sup.++, or                                                        
           Non-oxidative                                                  
                  The product obtained by method A,                       
                                   Japanese Patent                        
     Co.sup.++  &amp;                                                         
           or oxidative                                                   
                  B, or C is heated in an inert gas.                      
                                   Application(OPI)                       
     Fe.sup.++    The product obtained by method A,                       
                                   No. 74399/1984 and                     
                  B, or C is heated in a reducing or                      
                                   others                                 
                  oxidizing atmosphere, so that the                       
                  Fe.sup.++ /Fe.sup.+++  ratio is changed.                
__________________________________________________________________________
 
    
     In FIG. 5, the solid lines indicate the results obtained with the conventional method (in which γ-Fe 2  O 3  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-Fe 2  O 3  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. (&#34;Parts&#34; denotes &#34;parts by weight&#34;.) 
     
         ______________________________________                                    
Magnetic Materials                                                        
______________________________________                                    
Magnetic Powder          100 parts                                        
Vinyl Chloride-Vinyl Acetate Copolymer                                    
                          10 parts                                        
(VAGH, a product of Union Carbide Corp.)                                  
Polyurethane (Nipporan 2301,                                              
                          10 parts                                        
a product of Nippon Polyurethane Co.)                                     
Polyisocyanate (Colonate L,                                               
                          3 parts                                         
a product of Nippon Polyurethane Co.)                                     
Soya lecithin             1 part                                          
Methyl Ethyl Ketone      180 parts                                        
Methyl Isobutyl Ketone   120 parts                                        
______________________________________                                    
 
    
     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 5 kOe by using a flux-meter USM-III made by Toei Kogyo Company. 
     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-Fe 2  O 3 . 
     The transition temperature of this γ-Fe 2  O 3  was 675° C. when measured by DTA at the heating rate of 10° C./min. 50 g of γ-Fe 2  O 3  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-Fe 2  O 3  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-Fe 2  O 3  was dispersed in 2 liters of water, and to the dispersion was added 31.6 g of cobalt sulfate (CoSO 4 .7H 2  O) 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. 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-Fe 2  O 3  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-Fe 2  O 3  used as the raw material had Hc 392 Oe, σs 69.6 emu/g, pH 8.8, crystal particle size 300 Å, and specific surface area 30.4 m 2  /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. 
     
                       TABLE 2                                                     
______________________________________                                    
       Treat-                                                             
       ment                               Print-                          
       Temp.   Treatment Hc        σ s                              
                                          Through                         
Sample (°C.)                                                       
               Time (hr) (Oe) SQ   (emu/g)                                
                                          (dB)                            
______________________________________                                    
1-1    400     1         665  0.81 70.0   54.5                            
1-2    500     1         650  0.82 71.7   55.8                            
1-3    550     1/4       642  0.82 71.3   58.0                            
1-4    550     4         641  0.83 71.0   58.2                            
1-5    600     1         607  0.82 67.4   58.8                            
1-6    615     1         603  0.81 49.2   58.9                            
Compar-                                                                   
       --      --        670  0.80 68.5   53.1                            
ative                                                                     
Sample 1                                                                  
______________________________________                                    
 
    
     EXMPLE 2 
     Goethite as used in Example 1 was converted into γ-Fe 2  O 3  under the same conditions as in Example 1. The resulting γ-Fe 2  O 3  underwent heat treatment at 590° C. for 1 hour. The heat-treated γ-Fe 2  O 3  had Hc 406 Oe, σs 69.7 emu/g, specific surface area 30.5 m 2  /g, and crystal particle size 310 Å. 
     This γ-Fe 2  O 3  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-Fe 2  O 3 . 
     The γ-Fe 2  O 3  thus obtained had Hc 423 Oe, σs 73.2 emu/g, a pH of 8.8, and a crystal particle size of 320 Å. 
     This γ-Fe 2  O 3  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. 
     
                       TABLE 3                                                     
______________________________________                                    
                                 Print-Through                            
       Hc(Oe) SQ      σ s (emu/g)                                   
                                 (dB)                                     
______________________________________                                    
Sample 2 664      0.83    68.5     58.1                                   
Sample 3 659      0.82    68.9     56.7                                   
Comparative                                                               
         670      0.80    68.5     53.1                                   
Sample 1                                                                  
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     It is noted that the print-through property and σs are improved by either performing a heat treatment subsequent to the calcination of γ-Fe 2  O 3  or raising the oxidation temperature. 
     EXAMPLE 4 
     γ-Fe 2  O 3  was obtained from lepidocrosite. It had Hc 316 Oe, σs 74.7 emu/g, specific surface area 23.8 m 2  /g, a crystal particle size of 400 Å, 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-Fe 2  O 3  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-Fe 2  O 3  was obtained from lepidocrosite. It had Hc 315 Oe, σs 72.9 emu/g, specific surface area 26.7 m 2  /g, a pH of 3.69, and a crystal particle size of 420 Å. The transition temperature and critical temperature obtained in the same way as in Example 1 were 486° C. and 400° C., respectively. 
     This γ-Fe 2  O 3  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-Fe 2  O 3  was obtained by calcination in the conventional method. It had a pH of 5.5, σs 73.9 emu/g, Hc 321 Oe, specific surface area of 24.9 m 2  /g, and a crystal particle size of 480 Å. 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-Fe 2  O 3  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 characteristic properties are shown in Table 4. 
     It is noted from Table 4 that if γ-Fe 2  O 3  undergoes heat-treatment, the print-through property of the cobalt-modified magnetic powder is greatly improved regardless of the properties of γ-Fe 2  O 3  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-Fe 2  O 3  was obtained from goethite which had been formed in an acid solution. It had Hc 327 Oe, σs 73.0 emu/g, specific surface area of 23.3 m 2  /g, a pH of 3.8, and a crystal particle size of 490 Å. The transition temperature and critical temperature determined in the same way as in Example 1 were 643° C. and 565° C., respectively. 
     This γ-Fe 2  O 3  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. 
     
                       TABLE 4                                                     
______________________________________                                    
       Comparative          σ s                                     
                                   Print-through                          
Sample Sample     Hc(Oe)    (emu/g)                                       
                                   (dB)                                   
______________________________________                                    
4-1               642       73.0   59.2                                   
4-2               635       73.8   59.0                                   
       2          610       73.1   56.5                                   
5-1               598       69.0   57.7                                   
5-2               605       72.6   56.1                                   
       3          602       72.0   54.9                                   
6-1               635       72.7   60.1                                   
6-2               642       73.3   57.0                                   
       4          660       72.7   53.8                                   
7-1               633       69.2   59.8                                   
7-2               625       72.2   58.3                                   
       5          630       71.5   57.5                                   
______________________________________                                    
 
    
     EXAMPLE 8 
     The same γ-Fe 2  O 3  as used in Example 1 and the one which had been annealed at 600° C. for 1 hour underwent coblt modification according to the following method. 
     300 g of γ-Fe 2  O 3  was dispersed in an aqueous solution of 31.6 g of cobalt sulfate (CoSO 4 .7H 2  O) 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 (Fe 2  (SO 4 ) 3 .7H 2  O) dissolved in 1 liter of water was added over 1 hour at a rate of 16 ml per/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-Fe 2  O 3  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-Fe 2  O 3  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 (CoSO 4 .7H 2  O) and 81 g of ferrous sulfate (FeSO 4 .7H 2  O) 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 as 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 as shown in Table 5. 
     
                       TABLE 5                                                     
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            Comparative                                                   
                       Print-through                                      
Sample      Sample     (dB)                                               
______________________________________                                    
 8                     59.3                                               
            6          54.0                                               
9                      55.5                                               
            7          51.2                                               
10                     55.5                                               
            8          51.3                                               
11                     57.3                                               
            9          53.8                                               
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     As Table 5 shows, the print-through property of the magnetic material is improved when cobalt modification is carried out after γ-Fe 2  O 3  has undergone heat treatment. This holds true regardless of the method of cobalt modification. 
     EXAMPLE 12 
     The same γ-Fe 2  O 3  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-Fe 2  O 3  was dispersed in 2 liters of water. To the dispersion were added 31.6 g of cobalt sulfate (CoSO 4 .7H 2  O) and 31.3 g of ferrous sulfate (FeSO 4 .7H 2  O) 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. 
     498 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-Fe 2  O 3  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 smples 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-Fe 2  O 3  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 the SQ in the width direction is as great as 0.85. 
     
                       TABLE 6                                                     
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                                      Print-                              
                      σ s                                           
                               Fe.sup.++ /                                
                                      through                             
       Hc(Oe) SQ      (emu/g)  Fe.sup.+++                                 
                                      (dB)                                
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Sample                                                                    
1-4      641      0.83    71.0   &lt;0.01  58.2                              
12       624      0.81    73.5   0.032  55.0                              
13       657      0.79    76.8   0.088  47.6                              
14       671      0.76    78.7   0.124  43.5                              
15       630      0.82    75.0   0.052  51.6                              
Comparative                                                               
Samples                                                                   
 1       670      0.80    68.5   &lt;0.01  53.1                              
10       652      0.80    70.9   0.031  50.4                              
11       679      0.78    74.0   0.089  46.2                              
12       691      0.75    76.0   0.125  43.6                              
13       661      0.78    71.0   0.045  50.2                              
14       635      0.86    69.8   &lt;0.01  51.1                              
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     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. 
     While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.