Magnetic domain wall dynamics studied by in-situ Lorentz microscopy with aid of custom-made Hall-effect sensor holder

We built a custom-made holder with a Hall-effect sensor to measure the single point magnetic flux density inside a transmission electron microscope (TEM, JEM-F200, JEOL). The measurement point is at the same place as the sample inside the TEM. We utilized information collected with the Hall-effect sensor holder to study magnetic domain wall (DW) dynamics by in-situ Lorentz microscopy. We generated an external magnetic field to the sample using the objective lens (OL) of the TEM. Based on our measurements with the Hall-effect sensor holder, the OL has nearly linear response, and when it is switched off, the strength of the magnetic field in the sample region is very close to 0 mT. A ferritic-pearlitic sample studied has globular and lamellar cementite (Fe₃C) carbides in the ferrite matrix. Based on the in-situ Lorentz microscopy experiments, DWs in the ferritic matrix perpendicular to the lamellar carbides start to move first at ∼10 mT. At 160 mT, DWs inside the globular carbide start to disappear, and the saturation occurs at ∼210 mT. At 288 mT, the DWs parallel to the lamellar carbides still exist. Thus, these lamellar carbides are very strong pinning sites for DWs. We also run dynamical micromagnetic simulations to reproduce the DW disappearance in the globular carbide. As in the in-situ experiments, the DWs stay stable until the external field reaches the magnitude of 160 mT, and the DWs disappear before the field is 214 mT. In general, the micromagnetic simulations supported very well the interpretation of the experimental findings.