Lorentz-Electronmicroscopy of Ferromagnetic Specimens at High Voltages

The paper deals with the calculation of the image contrast of a divergent wall as a function of the accelerating voltage in the defocussed mode of Lorentz microscopy. The influence of the following three quantities on the image contrast is considered: (i) the magnetic scattering angle phi(m), (ii) the finite source illumination angle alpha(0),, and (iii) an angle alpha(s) which describes the beam broadening due to inelastic scattering of the electrons in the sample. It is shown that phi(m) and alpha(s) depend in essentially different ways on the accelerating voltage. As a consequence the contrast increases with accelerating voltages and - under normal operational conditions - reaches a flat maximum between 400 and 700 kV. The theoretical predictions are in reasonable agreement with experimental results obtained on permalloy films of 1500 and 2000 angstrom thicknesses.