Schlieren method as applied to magnetic recording heads in the scanning electron microscope

The fringing field above the surface of a magnetic recording head or magnetic tape can be studied in transmission in the scanning electron microscope (SEM) by either the ‘‘image distortion’’ or the ‘‘Schlieren’’ method. In the image distortion method, the magnetic field is calculated from the distortion caused by the fringing field to the image of either the specimen itself, or of a reference mesh. The Schlieren image shows a contour of equal field-times-distance integral (either for the total field or for a specified component of the field) directly. Methods used previously to show the contours of constant values of this integral in the SEM can be improved by the use of the rocking scanning mode about the plane of the aperture stop. The Schlieren method can also give a two-part image in which one-half of the micrograph shows the surface of the head viewed at a glancing angle, while the other half of the image shows the fringing field. This method has the potential for stroboscopic operation.