OBSERVATION OF MAGNETIC DOMAINS USING THE MAGNETO-OPTICAL EFFECT*)

As is known, magnetization of a reflecting surface leads to rotation of the plane of polarization of the reflected light, and the angle of rotation depends on the degree of magnetization, while the direction of rotation depends on the direction of magnetization. The authors of the paper reviewed, carried out with the consultative participation of Kittel and Shockley, successfully applied this phenomenon to the direct observation of domain structure.

The observations were carried out with hexagonal crystals of cobalt. Since the direction of easy magnetization of cobalt coincides with the $c$ axis of the crystal, the domains are magnetized (positively or negatively) along this axis. Therefore, if a section is made perpendicular to the $c$ axis, then on the surface of the section the north or south poles of the domains will appear. Plane-polarized light incident normally on the surface $(0001)$ of the crystal will undergo, upon reflection, a rotation of the plane of polarization (approximately by a quarter of a degree) in one direction or the other, depending on the direction of magnetization of the domain. By selecting a suitable compensator placed between crossed polarizer and analyzer, it is then not difficult to extinguish light reflected by domains with one direction of magnetization, while light reflected by domains with the opposite magnetization remains unextinguished, which makes it possible to observe directly the domain structure of the surface.

The observations were carried out with the aid of a metallographic microscope equipped with appropriate polarization devices.

Below are photographs obtained in this way. Figure 1 shows the domain structure of a cobalt single crystal (a section approximately perpendicular to the $c$ axis), revealed in polarized light. $a$—the compensator is oriented so that the rotation of the plane of polarization upon reflection by $1/4^\circ$ is compensated; $b$—the compensator is oriented so that the rotation of the plane of polarization upon reflection by $-1/4^\circ$ is compensated; $c$—the compensator does not rotate the plane of polarization, and therefore the intensity of the light reflected by oppositely magnetized domains is the same, and the domain structure is not visible. Figure 2 shows the change in the domain structure of a cobalt single crystal with changing strength of the external magnetic field, revealed in polarized light. $a$—the field is weaker; $b$—the field is stronger. Figure 3 gives the change in the domain structure of polycrystalline cobalt with changing strength of the external magnetic field. Photographs $a$ (weak field) and $b$ (strong field) were taken in polarized light. Photograph $c$ was obtained from the same surface somewhat later by means of the powder method (colloidal magnetite).

The photographs clearly reveal the domain structure of the surface and have obvious advantages over photographs of the domain

*) H. J. Williams, F. G. Foster and E. A. Wood, Phys. Rev. 82, 119 (1951).

Fig. 1.

Fig. 3.

Fig. 2.

structures obtained by other methods (for example, by means of powder techniques). A significant drawback of the method is the enormous loss of light associated with the extremely small value of the angle of rotation of the plane of polarization. As the authors note, this makes visual observation of the domain structure very difficult, but does not constitute a serious obstacle to photographing it with contrast.

G. R.