Revealing nanoscale chiral magnetic ordering in polycrystalline FeGe thin films using 4-D Lorentz Scanning Transmission Electron Microscopy*

We study nanoscale chiral magnetic ordering in sputtered polycrystalline B20 FeGe thin film on [111] Si substrate with Lorentz Scanning transmission electron microscopy. Chiral magnetism is present in bulk B20 FeGe but far less is known about the impact of grain boundaries on the magnetic order. We use 4D-STEM enabled by a new design of pixel-array detector to disentangle contrast from the local magnetic and electrostatic fields and thus image magnetic structures despite varying grain contrast. Atomic resolution images of the grain boundaries show how the crystal chirality correlates with the local magnetic phase. Across a mirror twin boundary where crystal chirality is preserved, our magnetic induction mapping shows the magnetic helicity is preserved in both the helical and skyrmion states. Across grain boundaries where the crystal chirality is reversed, the magnetic induction for helical and skyrmion states show an inversion of magnetic helicity. Both examples indicate the crystal chirality is coupled to magnetic helicity in the polycrystalline FeGe thin film.