4-Dimensional Lorentz-STEM of Chiral FeGe Thin Films

Chiral magnets require extremely low current densities to manipulate their spins have been promising for applications in power-efficient memory devices. However, it is necessary to understand the behavior of chiral magnetic texture in thin-film materials with realistic disorder, such as B20 FeGe sputtered on Si. Lorentz TEM has been successful for high-resolution studies of chiral magnetic states in single-crystal B20 FeGe, imaging FeGe thin films in plan-view remains a challenge as grain sizes are comparable to those of magnetic textures. This makes distinguishing magnetic contrast from grain contrast challenging. We present a solution to this problem by collecting the full angular distribution of scattered electrons at each scan position with the electron microscopy pixel array detector developed at Cornell in Lorentz-STEM mode. We measure the chiral magnetic textures of B20 FeGe quantitatively, and directly separate grain structure contrast from magnetic contrast. We observe that the helical phases of B20 FeGe moves between grain boundaries and that magnetic skyrmions only form in free-standing FeGe films, illuminating the relationship between the FeGe chiral phases and lattice-induced strain.