Far-out-of-focus electron holography in a dedicated FEG STEM

Abstract

Flexible operation of off-axis holography modes is achieved when a biprism is placed in the illumination system of a scanning transmission electron microscope, equipped with a field emission electron source. The separation of the two virtual sources created by the biprism can be varied by changing the voltage applied to the biprism or by simply changing the excitation of the condensor and/or objective lenses. Two distinct modes of holography are used to study the magnetic microstructure in thin magnetic films. In the absolute mode the phase difference changes 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 constant in regions of constant magnetization, allowing a simple and straightforward determination of domain wall profiles. The contrast obtained in the holography modes is compared to well known contrast in the 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 magnetic microstructure.