Magnetic Contrast
Many materials, such as magnetic recordings, or naturally occurring substances such as cobalt, have magnetic fields above their surfaces. A secondary electron leaving the surface will travel through this field and will be deflected by the Lorentz force that is produced. since this deflection will be normal to both the direction of travel of the electron and to the magnetic field, a leakage field into (or out of) the plane containing the specimen and the secondary detector will produce a deflection in such as sense as to add to, or subtract from, the collection efficiency of the detector.
This variation in the collection efficiency for the secondary electrons produces magnetic contrast (Joy and Jakubovics, 1968). Figure 3-16 shows an example of such contrast from a single crystal of cobalt. the pattern which is visible comes from the stray magnetic field above the surface, which in turn reflects the underlying magnetic domain structure of the material. If the crystal were to be rotated in its own plane by 180 then the fields, and hence the contrast, would reverse. In this mode the spatial resolution of the contrast will be limited by the scale of the domain structure that produced it.
On bulk materials this may be of the order of a micron or so, while for thin foils the corresponding value might be as low as 0.1 micromatre since the domain size is typically equal to or less than the sample thickness. As for other secondary electron modes the performance is best at low beam energies although on strongly magnetic materials severe astigmatism may result if too low an energy is selected. Other classes of magnetic materials do not exhibition leakage fileds above their surfaces