MAGNETOGRAPHY 



intensity or direction. As a result, the domain 

 patterns could be photographed and re- 

 corded in still or motion pictvu'es. (The lat- 

 ter were taken by F. Tylee in collaboration 

 with H. J. Williams and J. G. Walker.) 



Although much was gained in the study 

 of magnetic effects with colloidal methods, 

 which are still used to great advantage, there 

 are certain inherent restrictions. Among 

 these limitations are the time recjuired for 

 the particles to collect along the domain 

 walls and the failure to delineate the fine 

 structure that the microscope can resolve. 



During the latter part of the nineteenth 

 century, a useful effect was observed by the 

 Scottish scientist, John Kerr. He found that, 

 when a beam of polarized light is reflected 

 perpendicularly from a surface that is mag- 

 netized normal to itself, the plane of polari- 

 zation of the light is rotated. The direction 

 of rotation depends upon the polarity of the 

 reflecting surface. Thus, if the north pole of a 

 material rotates the plane in a clockwise di- 

 rection, the south pole will rotate the plane 

 in a counter-clockwise direction. It was 

 thought that a study of the domains in a 

 magnetic material might be possible using 

 this effect. The phenomenon, known as the 

 Kerr magneto-optic effect, was in 1950 ap- 

 plied to the microscopical study of magnetic 

 domain structure in cobalt by H. J. Williams, 

 E. A. Wood and the author. 



In 1938, L. V. Foster of the Bausch and 

 Lomb Optical Company described an illumi- 

 nating system for a metallograph, using a 

 cut and cemented calcite prism as a polariz- 

 ing illuminator. Later, a polarized light com- 

 pensator was developed to be used in con- 

 junction with this metallograph for the study 

 of opaciue minerals. The combination of these 

 two illuminating devices is shown in the dia- 

 gram of Figure 5. 



With this device, either the north or south 

 poles in the surface of the specimen can be 

 made to appear bright while the other set re- 

 mains dark. This is done by adjusting the 

 elliptical and rotation compensators, shown 



in Figure 5, to extinguish the light from one 

 set of domains. When there is no compensa- 

 tion, both sets of poles or domains have the 

 same intensity and they cannot be distin- 

 guished. 



By means of this polarizing attachment 

 for the metallograph, it was believed that 

 the Kerr magneto-optic effect could be used 

 to study the domain structures in magnetic 

 materials. In the initial studies, using this 

 effect, a single crystal of cobalt was selected. 

 Subsequently, a cobalt crystal was made in 

 the form of a half disc. With this shape crys- 

 tal it was possible to observe domains be- 

 tween positions parallel to the c-axis, or easy 

 direction of magnetization, to positions per- 

 pendicular to this axis. To minimize the pos- 

 sibility of a disturbed sm'face of cold worked 

 material, the crystal was carefully electro- 

 polished. 



Cobalt has a hexagonal structure. In this 

 element, the domains tend to lie with the 

 direction of magnetization in either a posi- 

 tive or negative sense along the c-axis. The 

 photomicrographs that are shown in Figure 

 6, starting with a displacement of 4 degrees 



SPECIMEN 



BLACKENED 



OBSERVATION 



_./ elliptical > 

 '^'-IcompensatorJ 



/ rotation ^ 

 ^compensator; 



EXTRAORDINARY RAY 



CALCITE PRISM 



LIGHT BEAM 



Fig. 5. Adaptation of elliptical vibration com- 

 pensator used in conjunction with metallograph 

 for studies of domains in magnetic materials. 



443 



