Process for preparing a single oxide crystal
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a process for preparing a single oxide crystal, and particularly to an improved process employing the flux method for growing a single oxide crystal of high quality. 
     2. Description of the Prior Art 
     The flux method comprises the steps of dissolving raw materials, from which a single crystal is to be grown, in an adequate flux to prepare a high temperature saturated solution, and cooling the solution slowly to precipitate and grow a single crystal. The flux method can advantageously be applied to a very wide variety of raw materials, and enables a high melting point single crystal to be grown at a temperature far lower than the melting point thereof. In addition, growth of the single crystal can be done using a simple furnace according to this method. Therefore, the flux method has been used for the growth of many kinds of single crystals. 
     Generally, in the flux method, the step of extracting a grown single crystal from the fluid or solidified flux is required after the completion of the crystallization. The extracting methods conventionally employed include: 
     (1) a method of picking up a single crystal by crushing the solidified flux with a hammer, 
     (2) a method of extracting a single crystal by dissolving away the flux with a solvent suitable for the flux used, such as hot hydrochloric or, nitric acid or hot water, and 
     (3) a method of extracting a single crystal from the flux by overflowing it by slanting a crucible before the solidification of the flux. 
     Here, it is necessary to select such a material as flux that only a sufficient amount of raw materials is dissolved at a high temperature and that a solid solution of the flux with the raw materials is not formed in the solid phase. In addition, it is also required to select such a flux as to be used at such a temperature and in such an atmosphere as to allow the raw materials to remain stable. 
     Examples of the fluxes conventionally used for the growth of single oxide crystals include PbO, PbO-PbF 2 , B 2  O 3 , Bi 2  O 3  and KF, which were empirically found to be suitable for such a flux. Of those flux materials, Bi 2  O 3 , PbO or the like is shrunk and hardened upon solidification. When a flux of such properties is used, a grown single crystal receives such a pressure applied thereto upon solidification of the flux that the single crystal is disadvantageously subject to the development of cracks in the case of the above-mentioned method (1) or (2). Further, the above-mentioned method (3) is seldom employed, because it involves such demerits that the overflown flux adheres to the furnace material and dissolves the furnace material, and that expensive raw materials are disposed every time after the production of a single crystal. 
     On the other hand, in order to grow a single crystal of bismuth-substituted gadolinium-iron garnet, it has been reported that Bi 2  O 3  is used as flux (see &#34;Faraday Rotation and Optical Absorption of a Single Crystal of Bismuth-Substituted Gadolinium Iron Garnet&#34; by H. Takeuchi, S. Ito, I. Mikami and S. Taniguchi, J. Appl. Phys., Vol. 44, No. 10 (1973), pp. 4789-4790). In this paper the, conditions of growing such a single crystal have not being clearly disclosed. If such a single crystal is grown by the usual flux method, a grown single crystal receives a pressure from the solidified flux and is subject to the development of cracks, as mentioned above. The crystal having bismuth uniformly substituted therein could be obtained. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide an improved process employing the flux method for preparing a grown single oxide crystal of high quality in which the above-mentioned disadvantages are eliminated. 
     In order to achieve this object, a grown single crystal is extracted from the flux in the molten state, according to the improved process of the present invention. 
     More specifically, in accordance with the present invention, there is provided a process for preparing a single crystal comprising the steps of mixing and fusing a mixture of raw materials for crystal growth and a flux, cooling the fused mixture slowly to grow and precipitate a single crystal, and extracting the grown single crystal from the molten flux. 
     Here, it is preferable that the single crystal is dipped up from; i.e., scooped out of the molten flux. 
     Preferably, the single crystal is of bismuth-substituted gadolinium-iron garnet derived from a mixture of 8 mol% or less, exclusive of 0 mol%, of Gd 2  O 3 , 30 to 53 mol% of Fe 2  O 3 , and 40 to 61 mol% of Bi 2  O 3 . 
     More preferably, the single crystal is of bismuth-substituted gadolinium-iron garnet derived from a mixture of Gd 2  O 3  in the range of 5-7 mol%, Fe 2  O 3  in the range of 39-44 mol% and Bi 2  O 3  in the range of 50-56 mol%. 
     The above-described mixture may further include B 2  O 3  in such an amount as to satisfy the formula: 0&lt;B 2  O 3  amount (mol)/Bi 2  O 3  amount (mol)≦0.084, more preferably 0&lt;B 2  O 3  amount (mol)/Bi 2  O 3  amount (mol)≦0.08. 
     It is preferable that the single crystal is dipped up at a temperature ranging from 1,050° C. to the solidification temperature of the fused mixture. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be illustrated in more detail by examples regarding preparation of single crystals of bismuth-substituted gadolinium-iron garnets with reference to the accompanying drawings, in which: 
     FIG. 1 is a composition diagram demonstrating a mixing ratio of Gd 2  O 3  /Fe 2  O 3  /Bi 2  O 3  allowing the growth of a single crystal of gadolinium-iron garnet; 
     FIG. 2 is a chart of characteristic curves illustrating respective relationships between the temperature of dipping up a single crystal and the substitution amount of bismuth, and between that dipping up temperature and the weight of the single crystal; 
     FIGS. 3A and 3B are photographs of Gd 3-x  Bi x  Fe 5  O 12  single crystals grown by the process in accordance with the present invention; and 
     FIG. 4 is a chart of characteristic curves illustrating the wave length dependency of the Faraday rotation angle (θ F ) for each one of Gd 3-x  Bi x  Fe 5  O 12  crystals and a Gd 3  Fe 5  O 12  crystal. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Bismuth-substituted gadolinium-iron garnets (Gd 3-x  Bi x  Fe 5  O 12 ) show a considerably large Faraday effect, so that they are under investigations for possible use thereof as isolator materials for optical communication or magneto-optic heads for reading out information recorded in magnetic recording medium. 
     In order to grow a bismuth-containing magnetic garnet crystal showing a large Faraday effect by means of the flux method, a mixture of bismuth oxide with lead oxide, lead fluoride or the like has conventionally been used as the flux. Where a lead-containing flux is used, lead is likely to be incorporated into a crystal. As a result, the Faraday rotation angle (θ F ) of the crystal becomes larger. However, the absorption coefficient (α) also becomes larger. As a result, there is a disadvantage in that a large figure of merit (θ F  /α) is not obtained. 
     A flux of Bi 2  O 3  alone is seldom used for the growth of a garnet single crystal, since a hole is likely to be opened in a platinum crucible by a reaction thereof with the flux at a high temperature in a reducing atmosphere. 
     Examples will now be given, but the following examples should not be construed as limiting the scope of the present invention. 
     EXAMPLES 1-39 
     Bi 2  O 3  alone, or a mixture of Bi 2  O 3  and B 2  O 3  were used as the flux in examples 1-39. 
     Optimum conditions were obtained as to the mixing ratio of raw materials and flux, the mixing ratio of Gd 2  O 3  and Fe 2  O 3  as the raw materials, the highest temperature to melt and keep a mixture, a slow cooling rate, the lowest temperature in the slow cooling and the like. The results were as shown in Tables 1, 2, 3, 4 and 5. In the Remarks of the Tables, values of ##EQU1## and values of Faraday rotation angle (θ F ) at a wave length of 0.8 μm are indicated. 
     
                                           TABLE 1                                 
__________________________________________________________________________
Single Crystals of Gd.sub.3-x Bi.sub.x Fe.sub.5 O.sub.12                  
Prepared by Slow Flux-Cooling Method                                      
 Example                                                                  
      (mol %)Gd.sub.2 O.sub.3 (g)                                         
           (mol %)Fe.sub.2 O.sub.3 (g)                                    
                (mol %)Bi.sub.2 O.sub.3 (g)                               
                     B.sub.2 O.sub.3 (g)                                  
                          (°C.)TemperatureHighest                  
                                 (°C.)in Slow CoolingTemperatureLow
                                est       (°C./h)RateCoolingSlow   
                                               ##STR1##                   
                                                   Remarks                
__________________________________________________________________________
1    4.87 5.36 30        1280   785      1.1  O                           
     (12.1)                                                               
          (30.1)                                                          
               (57.8)                                                     
2    4.27 6.58 40        1280   785      1.1  O                           
     (8.5)                                                                
          (29.7)                                                          
               (61.8)                                                     
3    4.06 13.42                                                           
               30        1285   758      5.8  G                           
     (7.0)                                                                
          (52.6)                                                          
               (40.3)                                                     
4    4.06 13.42                                                           
               40        1285   758      5.8  G                           
     (6.2)                                                                
          (46.4)                                                          
               (47.4)                                                     
5    8.12 30.41                                                           
               80        1273   800      2.2  G                           
     (5.8)                                                                
          (49.5)                                                          
               (44.7)                                                     
6    8.12 26.84                                                           
               80        1273   800      2.2  G                           
     (6.2)                                                                
          (46.4)                                                          
               (47.4)                                                     
7    8.12 23.26                                                           
               80        1273   800      2.2  G   θ.sub.F = 5000    
     (6.6)                                                                
          (42.9)                                                          
               (50.5)                                                     
8    8.12 21.46                                                           
               80        1273   800      2.2  G                           
     (6.8)                                                                
          (40.9)                                                          
               (52.3)                                                     
9    8.12 17.88                                                           
               80        1273   800      2.2  G &amp; O                       
     (7.3)                                                                
          (36.6)                                                          
               (56.1)                                                     
__________________________________________________________________________
 note                                                                     
  O: Orthoferrite,                                                        
  G: Garnet                                                               
 
    
     
                                           TABLE 2                                 
__________________________________________________________________________
Single Crystals of Gd.sub.3-x Bi.sub.x Fe.sub.5 O.sub.12                  
Prepared by Slow Flux-Cooling Method                                      
 Example                                                                  
      (mol %)Gd.sub.2 O.sub.3 (g)                                         
           (mol %)Fe.sub.2 O.sub.3 (g)                                    
                (mol %)Bi.sub.2 O.sub.3 (g)                               
                     B.sub.2 O.sub.3 (g)                                  
                          (°C.)TemperatureHighest                  
                                 (°C.)in Slow CoolingTemperatureLow
                                est       (°C./h)RateCoolingSlow   
                                               ##STR2##                   
                                                   Remarks                
__________________________________________________________________________
10   8.12 14.31                                                           
               80        1273   800      2.2  G &amp; O                       
     (7.9)                                                                
          (31.6)                                                          
               (60.5)                                                     
11   8.12 26.84                                                           
               80        1310   760      3.2  G                           
     (6.2)                                                                
          (46.4)                                                          
               (47.4)                                                     
12   8.12 23.26                                                           
               80        1310   760      3.2  G                           
     (6.6)                                                                
          (42.9)                                                          
               (50.5)                                                     
13   6.09 20.13                                                           
               80        1310   760      3.2  G                           
     (5.3)                                                                
          (40.1)                                                          
               (54.6)                                                     
14   8.12 21.46                                                           
               80        1310   760      3.2  G                           
     (6.8)                                                                
          (40.9)                                                          
               (52.2)                                                     
15   6.09 18.78                                                           
               80        1310   760      3.2  G                           
     (5.5)                                                                
          (38.4)                                                          
               (56.1)                                                     
16   6.09 17.45                                                           
               80        1310   760      3.2  G &amp; O                       
     (5.6)                                                                
          (36.7)                                                          
               (57.7)                                                     
17   16.24                                                                
          40.56                                                           
               140       1315   970      7.5  G &amp; O                       
     (7.5)                                                                
          (42.4)                                                          
               (51.1)                                                     
18   8.12 23.26                                                           
               100       1315   870      7.5  G &amp; O                       
     (5.9)                                                                
          (38.1)                                                          
               (56.1)                                                     
__________________________________________________________________________
 note                                                                     
  O: Orthoferrite,                                                        
  G: Garnet                                                               
 
    
     
                                           TABLE 3                                 
__________________________________________________________________________
Single Crystals of Gd.sub.3-x Bi.sub.x Fe.sub.5 O.sub.12                  
Prepared by Slow Flux-Cooling Method                                      
 Example                                                                  
      (mol %)Gd.sub.2 O.sub.3 (g)                                         
           (mol %)Fe.sub.2 O.sub.3 (g)                                    
                (mol %)Bi.sub.2 O.sub.3 (g)                               
                     B.sub.2 O.sub.3 (g)                                  
                          (°C.)TemperatureHighest                  
                                 (°C.)in Slow CoolingTemperatureLow
                                est       (°C./h)RateCoolingSlow   
                                               ##STR3##                   
                                                   Remarks                
__________________________________________________________________________
19   16.24                                                                
          40.5 140       1305   970      7.5  G &amp; O                       
     (7.5)                                                                
          (42.3)                                                          
               (50.2)                                                     
20   8.12 23.25                                                           
               90        1290   840      0.9  G                           
     (6.2)                                                                
          (40.3)                                                          
               (53.5)                                                     
21   8.12 25.1 80        1255   850      0.6  G   θ.sub.F = 7200    
     (4.6)                                                                
          (44.7)                                                          
               (48.9)                                                     
22   8.12 23.25                                                           
               40        1160   840      1    O                           
     (8.8)                                                                
          (57.4)                                                          
               (33.8)                                                     
23   8.12 23.26                                                           
               80   10   1255   850      0.6  O   R = 84                  
     (6.6)                                                                
          (30.1)                                                          
               (35.3)                                                     
                    (29.7)                                                
24   8.12 25.1 80   10   1255   850      0.6  O   R = 84                  
     (4.5)                                                                
          (31.8)                                                          
               (34.7)                                                     
                    (29.0)                                                
25   8.12 26.84                                                           
               80   10   1255   850      0.6  O   R = 84                  
     (4.4)                                                                
          (33.7)                                                          
               (34.0)                                                     
                    (28.4)                                                
26   8.12 23.25                                                           
               80   2    1150   910      0.42 O   R = 17                  
     (6.1)                                                                
          (39.5)                                                          
               (46.6)                                                     
                    (7.8)                                                 
27   8.12 23.25                                                           
               80   1    1160   840      1    G &amp; O                       
                                                   R = 8.4                
     (6.3)                                                                
          (41.1)                                                          
               (48.5)                                                     
                    (4.1)                                                 
__________________________________________________________________________
 note                                                                     
  O: Orthoferrite,                                                        
  G: Garnet                                                               
 
    
     
                                           TABLE 4                                 
__________________________________________________________________________
Single Crystals of Gd.sub.3-x Bi.sub.x Fe.sub.5 O.sub.12                  
Prepared by Slow Flux-Cooling Method                                      
 Example                                                                  
      (mol %)Gd.sub.2 O.sub.3 (g)                                         
           (mol %)Fe.sub.2 O.sub.3 (g)                                    
                (mol %)Bi.sub.2 O.sub.3 (g)                               
                     B.sub.2 O.sub.3 (g)                                  
                          (°C.)TemperatureHighest                  
                                 (°C.)in Slow CoolingTemperatureLow
                                est       (°C./h)RateCoolingSlow   
                                               ##STR4##                   
                                                   Remarks                
__________________________________________________________________________
28   3.62 5.43 20.09     1310   950      1.4  G &amp; O                       
     (10) (34) (56)                                                       
29   3.62 6.07 24.2      1310   950      1.4  G &amp; O                       
     (10) (38) (52)                                                       
30   3.62 6.71 22.37     1310   950      1.4  G &amp; α                 
     (10) (42) (48)                                                       
31   3.62 7.35 20.5      1310   950      1.4  G &amp; α                 
     (10) (46) (44)                                                       
32   3.62 7.99 18.64     1310   950      1.4  G &amp; α                 
     (10) (50) (40)                                                       
33   7.25 4.79 23.3      1310   950      1.4  G &amp; O                       
     (20) (30) (50)                                                       
34   7.25 6.39 18.64     1310   950      1.4  G &amp; O                       
     (20) (40) (40)                                                       
__________________________________________________________________________
 note                                                                     
 α: α-Fe.sub.2 O.sub.3                                        
 
    
     
                                           TABLE 5                                 
__________________________________________________________________________
Single Crystals of Gd.sub.3-x Bi.sub.x Fe.sub.5 O.sub.12                  
Prepared by Slow Flux-Cooling Method                                      
 Example                                                                  
      (mol %)Gd.sub.2 O.sub.3 (g)                                         
           (mol %)Fe.sub.2 O.sub.3 (g)                                    
                (mol %)Bi.sub.2 O.sub.3 (g)                               
                     B.sub.2 O.sub.3 (g)                                  
                          (°C.)TemperatureHighest                  
                                 (°C.)in Slow CoolingTemperatureLow
                                est       (°C./h)RateCoolingSlow   
                                               ##STR5##                   
                                                   Remarks                
__________________________________________________________________________
35   13.8 39.53                                                           
               136       1195   dipped up                                 
                                         12   G &amp; O                       
     (6.6)                                                                
          (42.9)                                                          
               (50.5)           at 1,015° C.                       
36   34.1 17.66                                                           
               350.7     1200   dipped up                                 
                                         2    G   θ.sub.F = 8000    
     (6.5)                                                                
          (41.9)                                                          
               (51.6)           at 1,000° C.                       
37   34.1 97.66                                                           
               350.7                                                      
                    0.08 1310   dipped up                                 
                                         0.4  G   θ.sub. F = 8800   
     (6.45)                                                               
          (41.91)                                                         
               (51.57)                                                    
                    (0.08)      at 978° C. R = 0.1                 
38   17.05                                                                
          48.83                                                           
               175.36                                                     
                    0.08 1300   dipped up                                 
                                         0.6  G   θ.sub.F = 9500    
     (6.44)                                                               
          (41.88)                                                         
               (51.53)                                                    
                    (0.15)      at 972° C. R = 0.3                 
39   41.0 117.4                                                           
               421.69                                                     
                    0.1  1300   dipped up                                 
                                         0.6  G   R = 0.2                 
     (6.45)                                                               
          (41.90)                                                         
               (51.58)                                                    
                    (0.08)      at 980° C.                         
__________________________________________________________________________
 note                                                                     
  O: Orthoferrite,                                                        
  G: Garnet                                                               
 
    
     As shown in the Tables, when Bi 2  O 3  alone was used as the flux (Bi 2  O 3  also serves as a component of the raw materials) (see Examples 1-22, 28-26), a stoichiometric Gd 2  O 3  /Fe 2  O 3  ratio of 3/5 in mols for garnet cannot provide garnet single crystal but a single crystal of gadolinium orthoferrite (GdFeO 3 ). The residual matter remaining after the removal of the single crystal was subjected to analysis by an X-ray diffractometer. It was found that almost all of the residual matter is composed of BiFeO 3  and Bi 2  O 3 , and the rest of that contains small amounts of Gd 2  O 3  and Bi 2  Fe 4  O 9 . 
     These experimental results indicate that BiFeO 3 , which is an undesired product, may be formed at a high temperature before the growth of a single crystal of garnet, whereby Fe 2  O 3 , which is necessary for the growth of a desired single crystal of Gd 3  Fe 5  O 12 , may become insufficient in amount, thus leading to the growth of GdFeO 3 . With an increase in amount of Fe 2  O 3  for compensating the insufficiency of Fe 2  O 3 , a single crystal of bismuth-substituted gadolinium-iron garnet was grown together with a single crystal of gadolinium orthoferrite in a single crucible (Examples 9, 10 and 16 to 19). With a further increase in amount of Fe 2  O 3  to provide a Gd 2  O 3  /Fe 2  O 3  /Bi 2  O 3  molar ratio of 6.6/42.9/50.5, only a single crystal of garnet of 5 to 6 cubic mm in size was grown (Example 12). Even if the amount of Fe 2  O 3  was further increased, however, a grown single garnet crystal was obtained with a tendency of a smaller size of the crystal. 
     FIG. 1 illustrates a composition diagram demonstrating a mixing ratio of Gd 2  O 3  /Fe 2  O 3  /Bi 2  O 3  allowing the growth of a single crystal of gadolinium-iron garnet. In FIG. 1, the symbol ○  indicates the growth of a garnet single crystal alone (&#34;G&#34; in the Tables), the symbol   the growth of both a single garnet crystal and an orthoferrite crystal (&#34;G &amp; O&#34; in the Tables), the symbol   the growth of both a single crystal of garnet and α-Fe 2  O 3  (&#34;G and α&#34; in Table 4), and the symbol X the growth of an orthoferrite crystal alone (&#34;O&#34; in the Tables). Further, it is a matter of course that a single crystal of the garnet type is not grown when the amount of gadolinium is zero. As is apparent from FIG. 1, it is preferable that the mixing ratio of raw materials for allowing the growth of a garnet single crystal is substantially in the range of 8 mol% or less (exclusive of 0 mol%) for Gd 2  O 3 , 30 to 53 mol% for Fe 2  O 3  and 40 to 61 mol% for Bi 2  O 3 . 
     In Examples 7, 8, 12-14, 20, 23, 26, 27 and 35-39, a single crystal having a large size was obtained. Thus, it is preferable that 5 (mol%)≦Gd 2  O 3  ≦7 (mol%), 39 (mol%)≦Fe 2  O 3  ≦44 (mol%) and 50 (mol%)≦Bi 2  O 3  ≦56 (mol%). 
     Bi 2  O 3  and B 2  O 3  were used as the flux in Examples 23-27 and Examples 37-39 mentioned in Tables 3 and 5. With a small amount of B 2  O 3 , a garnet single crystal alone was grown (Examples 37-39). With an increase in an amount of B 2  O 3  to 1 g as against 80 g of Bi 2  O 3 , both a single garnet crystal and a single orthoferrite crystal were grown (Example 27). With a further increase in an amount of B 2  O 3  to 10 g or 2 g as against 80 g of Bi 2  O 3  (see Examples 23-26), an orthoferrite crystal alone was grown despite the composition of the components other than B 2  O 3 , where it is expected that a single garnet crystal alone is grown. The addition of B 2  O 3  to Bi 2  O 3  as an additive component of the flux lowered the solidification temperature of the flux by about 20° to 50°. 
     Examples 27 and 37-39 show that it is preferable that a ratio of B 2  O 3  amount (mol) to Bi 2  O 3  amount (mol) is larger than zero and equal to or less than 0.084, and more preferably equal to or less than 0.08. 
     As described before, the use of Bi 2  O 3 , PbO or the like as the flux frequently results in the formation of a number of cracks in the inside of a grown single crystal because of the pressure imposed thereon by the solidified flux unlike in a case where PbF 2 , B 2  O 3  or the like is used. 
     In order to avoid the formation of such cracks in Examples 35-39 in Table 5 of the present invention, a grown single crystal alone was extracted, for example, dipped up or scooped out of the molten flux by using a platinum net or plate before solidification of the flux in the course of slow flux-cooling. In this case, a single crystal of high quality having few cracks was obtained. 
     In the method of the present invention, a size and a bismuth-substitution amount (x) of a single crystal can be measured at any temperature as it slowly cools down. 
     FIG. 2 demonstrates the temperature dependence of bismuth substitution amount and weight of a single crystal. The chart in FIG. 2 was drawn by using the data when dipping up the crystal at various temperatures in the case that the Gd 2  O 3  /Fe 2  O 3  /Bi 2  O 3  molar ratio is 6.6/42.9/50.5 (Example 35). As is apparent from FIG. 2, the size and the bismuth substitution amount of the single crystal increase non-linearly as the dipping up temperature is lowered. A single crystal of about 50 to 50 mg in weight was obtained, from which the single crystal was polished to provide a cube of 2 to 3×2 to 3 mm in size. In order to obtain a larger cube than the above-mentioned one, it is necessary that a single crystal is extracted by dipping it up after the fused solution is slowly cooled down 1050° C. or less. A single crystal dipped up from the fused solution having a temperature of 980° C. had the weight of 160 mg and a bismuth substitution amount (x) was about 1.15. 
     In general, as the temperature of the used solution lowers, the viscosity of the used solution increases to make it difficult to dip up therefrom the grown single crystal. When Bi 2  O 3  alone is used as the flux, the lowest temperature allowing the crystal to be dipped up is about 1,000° C. in the aforementioned range of the mixing ratio of raw materials and the flux in which a single garnet crystal is grown. Since the addition of B 2  O 3  to the flux can lower the solidification temperature of the fused solution or flux by several tens degrees in centigrade, the lowest temperature of the flux allowing the crystal to be dipped up lowers by substantially the same degrees. As a result, if B 2  O 3  is added as a component of the flux, a single crystal having a larger size and a larger amount of bismuth substitution can be obtained. 
     FIGS. 3A and 3B are photographs of single Gd 3-x  Bi x  Fe 5  O 12  crystals grown respectively by the process in accordance with the present invention in which the grown single crystal is not extracted from the melting mixture (Example 7) and by the process in accordance with the present invention in which the grown single crystal is dipped up (Example 37). A glimpse of the photographs, particularly the faces of the grown crystals thus photographed, shows that the crystal grown by the latter method of the present invention has better quality than that grown by the former one. 
     FIG. 4 demonstrates the wave length dependency of Faraday rotation angle (θ F ) for each one of single crystals of Gd 3-x  Bi x  Fe 5  O 12  with respective bismuth substitution amounts (x) of 1.15 (Example 38), 1.05 (Example 36) and 0.7  (Example 35), and a single crystal of Gd 3  Fe 5  O 12  with a bismuth substitution amount (x) of zero. The sign of Faraday rotation angle (θ F ) of the crystals containing bismuth (x≠0) is reversed to that of the crystal not containing bismuth (x=0) (see &#34;The Faraday Effect of Bismuth Substituted Rare-Earth Iron Garnet&#34; by H. Takeuchi, Japanese J. Appl. Phys., Vol. 14 (1975), p. 1903). The value of θ F  decreases as the wave length increases. The Faraday rotation angles (θ F ) of the Gd 3  Fe 5  O 12  crystal and the Gd 3-x  Bi x  Fe 5  O 12  crystals (x=0.7, 1.05 and 1.15) were about 300°/cm, 5,200°/cm, 7,800°/cm and 9,500°/cm, respectively, at a wavelength of 0.8 μm. It is to be noted that the value of θ F  for the crystal with bismuth substitution: x=1.15 was about 30 times larger than that for the crystal of Gd 3  Fe 5  O 12  without bismuth substitution: x=0. Further, an absorption coefficient of the crystal prepared according to the process of the present invention was about 100 cm -1  at a wave length of 0.8 μm. 
     As is apparent from the foregoing explanation, the process of the present invention provides the following advantageous effects. 
     (1) Since the substitution amount of an impurity (for instance, bismuth in Examples given herein) strongly depends on the extracting temperature of a single crystal, a substitution amount of the impurity in the grown crystal can be controlled. 
     (2) Since a grown single crystal is extracted, for example, dipped up before the solidification of the flux, the crystal obtained is of high quality with little cracks. 
     (3) While in the conventional flux method, it is impossible to examine the state of a single crystal on its way of growth. By contrast, according to the process of the present invention, the state and manner of the growth of a single crystal can be examined at various temperatures by dipping up and checking the crystal at those temperatures. Thus, the optimum condition for the growth of a desired single crystal can easily be determined. 
     (4) As described hereinbefore, in the conventional flux method, a grown single crystal is obtained by overflowing the fused flux by slanting a crucible in a furnace before solidification of the flux, by washing to dissolve away the solidified flux with hydrochloric acid, nitric acid or the like over several days, or by crushing the solidified flux. On the other hand, the process of the present invention provides a single crystal of very high quality without the above-mentioned troublesome work accompanying the conventional flux method. 
     Further, the present invention is not only applicable to the oxide mentioned above, but also to the growth of other single oxide crystal, such as orthoferrite (RFeO 3 , R:Y, La, Lu or rare earth element), orthoaluminate (RAlO 3 , R:Y, La, Lu or rare earth element), orthochromite (RCrO 3 , R:Y, La, Lu or rare earth element). As the flux, V 2  O 5  may be also used. 
     Furthermore, the process of the present invention can provide a single crystal of gadolinium-iron garnet with a bismuth substitution amount (x) of 1 or more, so that a magneto-optic material having a very large Faraday rotation angle can be obtained.