Improving Magnetic STEM-Differential Phase Contrast Imaging using Precession

Differential phase contrast in scanning transmission electron microscopy (STEM-DPC) is a technique used to image electromagnetic fields in materials. STEM-DPC is based on tracking the minute changes in the position of the bright-field disk, so any effects which cause inhomogeneities in the intensity or geometry of the disk can lead to the contrast from the electromagnetic fields to be obscured. Structural changes, like grain boundaries, thickness variations, or local crystallographic orientation, are a major cause of these inhomogeneities. In this paper, we present how precession of the STEM probe with the objective lens turned off, providing a near field-free environment for magnetic imaging, can average out nonsystematic inhomogeneities in the electron beam. The methodology was tested on a polycrystalline Fe60Al40 thin film with embedded ferromagnetic structures. The effect of precession was assessed on magnetic induction maps created by three different processing algorithms. Results demonstrate that precessed STEM-DPC with the objective lens turned off shows an improvement in the form of smoothing of the variations found in the DPC signal arising from the underlying polycrystalline background.