Lorentz Transmission Electron Microscopy
Fresnel Lorentz Transmission Electron Microscopy (LTEM) is a tecnique to image the magnetic structure of a specimen. In LTEM mode, the specimen is illuminated with a parallel electron beam. The in-plane component of the internal magnetic fields create a phase shift and deflect the beam. Changes in the magnetic induction of the specimen can be imaged by focusing under (underfocus) or over (overfocus) the specimen.
Overfocused LTEM micrograph of two permalloy disks where magnetic vortices are formed. The vortex core on the left is oscillating at 140 MHz whereas the vortex core on the right stays static. The image is taken at zero magnetic field.
Micrograph of a hexagonal skyrmion lattice in Cu2OSeO3 crystal. Skyrmions are magnetic quasi particels which are formed in a few materials when applying a certain field and temperature. The spin structure of a skyrmion has a rotational in-plane component which acts as a lens for electrons. Therefore we observe bright contrast for skyrmions in overfocused LTEM measurements.
Differential Phase Contrast Microscopy
Differential Phase Contrast Microscopy (DPC) is a measuremt method performed in scanning TEM (STEM) mode. There, the electron beam is focused on the specimen and scanned pixelwise over the sample. For magnetic or electric fields inside the sample, the beam gets tilted due to Lorentz or Coulomb force. With a special four-segmented detector we are able to measure the strength and direction of the beam tilt at each pixel and calculate the corresponding fields in the sample. With our microscopes we can map electric and magnetic field distributions in specimens from the nm to the µm range.
Differential phase contrast micrograph of the magnetic structure in a 50 nm thick nickel film. The sample was tilted in a magnetic field to form magnetic domains. The measurement shows a 'magnetic centepede', a magnetic domain with zigzag shaped domain walls. By tilting the sample forwards and backwards we are able to record spacially resolved hystereses loops.
DPC image of a magnetic vortex formed in a permalloy disk at zero magnetic field. The in-plane component of the magnetic induction performs a rotation around the vortex core and points out of the plane at the center. With DPC we are able to map the rotation and investigate the behaviour of magnetic vortices to external influences.