Nonmagnetic contrast in scanning Lorentz electron microscopy of polycrystalline magnetic films

The effect of the probe beam defocus on scanning Lorentz electron microscopy images has been investigated. Wave optical analyses show that, given a finite defocus, the variation of the transmission amplitude across the illuminated area on the specimen causes nonmagnetic contrast in the differential phase contrast (DPC) images. This nonmagnetic contrast depends on the sign and the amount of defocus. Since this contrast is virtually proportional to the gradient of the transmission amplitude, it should appear similar to the differentiated contrast of the bright-field scanning transmission electron microscope (STEM) image. Therefore, the nonmagnetic noise appearing in the defocused DPC image can be substantially reduced by subtracting a suitably weighted differentiated bright-field STEM image of the same area. This noise reduction procedure has been demonstrated by applying it to the unintentionally defocused DPC image of a recorded longitudinal magnetic storage medium.