SEM with polarization analyzer (SEMPA)


The SEMPA technique was first introduced by Koike et al in 1984. It uses the spin polarization of secondary electrons which are emitted from the samples surface when hit by a primary electron beam to generate a magnetic image (compare figure). Our setup is based on a Zeiss Gemini UHV SEM (Scanning Electron Microscope) which has a resolution of 3nm. For the spin detection a SPLEED (Spin Polarized Low Energy Electron Diffraction) is incorporated. It uses the spin-dependent diffraction at a W(100) single crystal surface. For magnetic imaging a resolution of about 20nm is specified.




SEMPA is a highly surface sensitive technique and measures an absolute magnetic signal: the polarization of the secondary electrons. Furthermore, by counting the electrons in all 4 main LEED spots, one obtains simulatenously images of the topography and the two polarization components parallel to the crystal surface.

example of SEMPA imaging: 10µm Py Pad

The technique has a surface sensitivity of less than 1nm, which allows the selective imaging of surface magnetization. To provide the required clean surfaces, our setup is equipped with a dedicated preparation chamber where we have the facility of in-situ sample preparation via an Ar sputter gun & three evaporators. For more complex samples a vacuum transfer chamber can be used to minimize surface contamination between external growth chambers, such as the pulsed laser deposition system, and the SEMPA.
The system is further equipped with 4 electrical contacts to allow the steady-state imaging of magnetization following current excitation and a liquid helium flow cryostat which provides a temperature range between 30 and 450 K for the measurements. Since permanent magnetic fields cannot be applied during imaging, a magnetization stage provides the ability to initialize the magnetic state of a sample with magnetic fields up to ~80 mT.