The Kerr images were obtained using both the longitudinal and transverse Kerr effects. The polar effect had no influence on the two effects, since the magnetisation was confined to the plane of the film because of the large demagnetising field. The longitudinal effect is only sensitive to the components of magnetisation, which lie parallel to the plane of incidence. This meant that any components of magnetisation perpendicular to the plane of incidence did not show any appreciable magnetic contrast. This is shown if Figure 2.25a where a Kerr image was taken from a sample which possessed a well defined uniaxial anisotropy. The easy axis was oriented perpendicular to the plane of incidence and the image was taken using the optical conditions which favoured the longitudinal effect. The image indicates no magnetic contrast in the horizontal axis. A close examination reveals that there is very faint magnetic contrast in the vertical axis (transverse effect). The positions of the domain walls have been marked in order to help the eye to locate the faint contrast. In order to make the Longitudinal Kerr effect sensitive to the vertical component of magnetisation in the film, the plane of incidence of the light needs to be rotated through 900 with respect to the sample. This was not feasible in this case. The
other possibility was the rotation of the sample through 900 so that the longitudinal Kerr effect was sensitive to the magnetisation in this direction. This was also not possible in the current experimental arrangement without physically removing the sample from the holder. This led to two problems; the magnetisation, and therefore domain structure, changed on handling the film because of its magnetostrictive nature, and secondly, it was difficult to image the exact same area of the film upon rotation. Both of these problems made it difficult to correlate the two images precisely. These problems are overcome by use of the transverse Kerr effect, where it is not necessary to disturb the sample. Figure 2.25b is the identical Kerr image obtained of the domain pattern in Figure 2.25a, but using the transverse Kerr effect. The Kerr image shows a domain structure one would expect from a thin film possessing a uniaxial anisotropy. On rotating the sample, so that the easy axis is aligned parallel to the plane of incidence, the opposite effect occurs; the domain structure is only revealed by the longitudinal effect and not the transverse effect. The importance of the two effects is high-lighted by Figure 2.26,
(a)
(b)
Figure 2.25: Kerr images obtained from a FeSiBC film possessing a well defined uniaxial anisotropy. (a) Kerr image obtained using the longitudinal effect - sensitive to the magnetisation in the horizontal axis (b) Kerr image obtained using the transverse Kerr effect -sensitive to the magnetisation in the vertical axis.
where Kerr images were obtained using both effects to reveal the two components of magnetisation. Figure 2.26a shows Kerr images obtained from a FeSiBC film which has been demagnetised and Figure 2.26b are the domain images obtained where an applied field is applied in the direction indicated on the figure. The magneto optical sensitivity has been high-lighted by the arrows for each image. The contrast of the magnetic domains is clearest for the respective magneto optic sensitivity. Together the two images provide a more clear indication of the domain structure, since the individual images do not display all the domain structure. The final image was obtained by the combination of the two images by averaging them. This was a very crude method, but it provided an overall view of the domain structure, even though the grey scale of the averaged image is not quantitative. Quantitative methods have been developed [Rave et al (1987)] to map out the vector magnetisation using the grey (or colour) scale by combination of the two images, but this is a fairly difficult procedure, and requires a very stable system, along with the ability to apply fields in many directions for the purposes of calibration; it is not implemented here.
The domain images obtained using the longitudinal and transverse Kerr effects, provide only an overall view of the domain structure. They do not provide any direct information on the direction of the magnetisation within the domains themselves. For example, the Kerr images obtained in the longitudinal mode, only reveal dark and light regions, which correspond to components of the domain magnetisation magnetised in opposite directions, as shown if Figure 2.26. Any domains which are magnetised in the transverse sense will generally appear equally grey. Thus, we cannot infer directly the direction of the domain magnetisation. Even for simple uniaxial domain structures as shown in Figure 2.22 the dark regions correspond to the magnetisation pointing either to the right or to the left, and therefore the domain images need to be carefully interpreted. For magnetic materials which have a well
MO Sensitivity Average MO Sensitivity
MO Sensitivity
(a)
(b)
35 A/m Demagnetised
Figure 2.26: Domain images obtained from a FeSiBC film possessing a radial anisotropy (Chapter 5). Kerr images obtained using the longitudinal effect (sensitive to the magnetisation in the horizontal axis), and the transverse Kerr effect (sensitive to the magnetisation in the vertical axis). (a) Demagnetised sample. (b) Sample under the influence of an applied field.
defined crystalline structure, the crystalline anisotropy generally dominates the magnetic anisotropy. This simplifies the interpretation of the magnetisation of the domains, since prior knowledge of the magnetisation in such materials allows the determination of the direction of the domain magnetisation for all domains. In materials which do not possess a well defined crystalline structure, such as the amorphous FeSiBC thin films studied here, the interpretation of the resulting domains can be quite complex. In these types of materials the magnetisation is generally determined by random stresses, which tend to vary in magnitude and direction. These stresses are commonly induced during the fabrication process of the material, and therefore the resulting domain structures can be quite complex to interpret. Identification of 1800 domain walls, or closure domains at the edges of these samples, can
help to piece the domain structure together, but this is not always the case. To establish the direction of magnetisation in a uniaxial domain structure as shown in Figure 2.22, is relatively straightforward. Here, a small magnetic field of known direction is applied along the direction of the domain walls. The favourable domains will grow in size, from which one can then establish if the dark regions correspond to domain magnetisation pointing to the left or right. In more complicated domain structures such as the radial domain structures shown in Figure 2.23 and discussed in Chapter 5, the domain magnetisation no longer points in the x or y directions. Here, MOKE loops are taken at various points on the sample, from which the direction of the domain magnetisation (easy axis) can be determined accurately. Using this information, one can then apply a small magnetic field whose direction is known, along the easy axis for the domain we are interested in, to determine the direction of the domain magnetisation. By this procedure it is possible to map out the domain magnetisation.