Imaging skyrmions in polycrystalline FeGe thin films

Visualizing magnetic skyrmions is an essential step in the process of correlating parameters like skyrmion density with other material properties. Lorentz Transmission Electron Microscopy (LTEM) is a well-established technique for imaging magnetic textures like skyrmions, but this approach is challenging to apply to samples in which contrast comes from features besides the magnetic texture, such as grain boundaries in polycrystalline materials, especially if the grains and skyrmions are similar in size. Here, we use Magnetic Force Microscopy (MFM) and LTEM to examine a polycrystalline FeGe thin film grown epitaxially on a Si(111) substrate via molecular beam epitaxy (MBE). Combining MFM and LTEM allows for imaging skyrmions and determining their density as a function of temperature and applied field. Magnetic texture contrast in LTEM is enhanced through use of an image subtraction method to reduce the grain contrast. We also characterize the microstructure and quality of the film in plan view and cross section specimens using Transmission Kikuchi Diffraction (TKD) in a scanning and electron microscope (SEM) and high angle annular dark field (HAADF) scanning (S)TEM imaging. We conclude by showing how the observed skyrmion density correlates with topological Hall measurements.