  We report a method for acquiring Foucault images and small-angle electron
diffraction patterns in external magnetic fields using a conventional
transmission electron microscope without any modification. In the electron
optical system that we have constructed, external magnetic fields parallel to
the optical axis can be controlled using the objective lens pole piece under
weak excitation conditions in the Foucault mode and the diffraction mode. We
observe two ferromagnetic perovskite-type manganese oxides,
La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ and Nd$_{0.5}$Sr$_{0.5}$MnO$_{3}$, in order to
visualize magnetic domains and their magnetic responses to external magnetic
fields. In rhombohedral-structured La$_{0.7}$Sr$_{0.3}$MnO$_{3}$, pinning of
magnetic domain walls at crystallographic twin boundaries was found to have a
strong influence on the generation of new magnetic domains in external applied
magnetic fields.
