LIGHT (OPTICAL) MICROSCOPY 



from the c-axis and udvuiiciiifi; through posi- 

 tions to 20 degrees, 49 degrees and 70 de- 

 grees, illustrate the change of domain pat- 

 terns from rosettes to elongated areas. On 

 the surface perpendicular to the c-axis (Fig- 

 ure 6) the magnetization forms small regions 

 having poles of opposed polarity. This gives 

 rather complex domain patterns on the sur- 

 face, as can be seen in the comparison pho- 

 tomicrograph. Figure 7. Photomicrograph 

 Figure 7, left, shows a collection of magnetite 





Fig. 6. Domain structure in a cobalt crystal 

 cut in the form of a half disc. Arrows show direc- 

 tions of magnetization in the domains. No mag- 

 netic field applied. 



particles (darkfield illumination) and Figure 

 7, right, is a direct view (polarized light) of 

 the polished surface. The latter shows many 

 fine details that cannot be seen in the colloi- 

 dal magnetite picture. 



In the microscopy of the magnetic do- 

 mains of cobalt it has been found that the 

 contrast is very low and, at times, has caused 

 considerable difficulty in visual study. Pho- 

 tomicrography Avith high contrast emulsion 

 films and special development has further 

 aided this study. 



Recently, B. M. Roberts and C. P. Bean 

 of the General Electric Research Labora- 

 tories have studied the ferromagnetic inter- 

 metallic compound manganese-bismuth. 

 This also has a hexagonal crystal structure 

 with the direction of easy magnetization 

 along the c-axis. In this compound, a much 

 greater contrast is achieved with the do- 

 main patterns appearing virtually black and 

 white. A study of this compound is currently 

 in progress at Bell Telephone Laboratories ; a 

 domain pattern showing rosettes that indi- 

 cate a surface normal to the c-axis is shown 

 in the reference on page 440. Such patterns are 

 an aid in determining the orientations of the 

 crystallites in a polycrystalline specimen. 



The importance of microscopy as an aid 

 in the fundamental studies of magnetic do- 



•*^-^^Vf 



■hm^ 



«■ 



(Old) Pattern obtained using colloidal 

 magnetite 



Fig. 7. Comparison of techniques. 



(New) Polarized light pattern showing greater 

 detail on same area of the cobalt crystal. 



444 



