FAR-OUT-OF-FOCUS ELECTRON HOLOGRAPHY IN A DEDICATED FEG STEM

Flexible operation of off-axis holography modes is achieved when a bip rism is placed in the illumination system of a scanning transmission e lectron microscope, equipped with a field emission electron source. Th e separation of the two virtual sources created by the biprism can be varied by changing the voltage applied to the biprism or by simply cha nging the excitation of the condenser and/or objective lenses. Two dis tinct modes of holography are used to study the magnetic microstructur e in thin magnetic films. In the absolute mode the phase difference ch anges linearly in regions of constant magnetization and thickness and the slope determines the magnitude of magnetization. In addition, this mode permits the determination of the mean inner potential of a solid of known geometry. In the differential mode the phase difference is c onstant in regions of constant magnetization, allowing a simple and st raightforward determination of domain wall profiles. The contrast obta ined in the holography. modes is compared to well known contrast in th e Fresnel and differential phase contrast modes of Lorentz microscopy. The combination of several scanning transmission electron microscopy based techniques presents a powerful tool for the investigation of mag netic microstructure.