TABLE OF CONTENTS Very different techniques are used to investigate the magnetic microstructures in ferromagnets. All of these techniques have their strengths and drawbacks that make them best suited for the investigation of different aspects of magnetism. In the first section, we will give some examples that show the strengths of spin-SEM, concentrating on ultrathin film systems. The second section will discuss apparent drawbacks of the technique and possible ways around them.
One advantage of SEMPA is its high surface sensitivity, which gives the signal, i.e., polarization, as an average over only a few atomic layers. This fact makes it feasible to investigate systems with thicknesses in the nanometer and sub-nanometer range, i.e., of a few atomic layers only. The technique works in reflection, which puts no limitation on the dimensions of the support of the ultrathin ferromagnet. In particular, that means spin-SEM allows the investigation of ideal systems like single crystal ferromagnets or, utilizing its surface sensitivity, the best thin films one can fabricate by growing them epitaxially on single crystal surfaces. Those well characterized ultrathin films allow for a better and easier interpretation, as the magnetic properties can be correlated with structure and morphology. Another very special feature of ultrathin films is that the magnetization orientation is identical throughout the thickness of the film as long as the thickness is smaller than the characteristic magnetic length ?. The characteristic length is either 
, with A the exchange stiffness, K the anisotropy constant, and
