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The study of nanoscale chiral magnetic order in polycrystalline materialswith a strong Dzyaloshinkii-Moriya interaction (DMI) is interesting for theobservation of magnetic phenomena at grain boundaries and interfaces. One suchmaterial is sputter-deposited B20 FeGe on Si, which has been activelyinvestigated as the basis for low-power, high-density magnetic memorytechnology in a scalable material platform. Although conventional Lorentzelectron microscopy provides the requisite spatial resolution to probe chiralmagnetic textures in single-crystal FeGe, probing the magnetism of sputteredB20 FeGe is more challenging because the sub-micron crystal grains addconfounding contrast. We address the challenge of disentangling magnetic andgrain contrast by applying 4-dimensional Lorentz scanning transmission electronmicroscopy using an electron microscope pixel array detector. Supported byanalytical and numerical models, we find that the most important parameter forimaging magnetic materials with polycrystalline grains is the ability for thedetector to sustain large electron doses, where having a high-dynamic rangedetector becomes extremely important. Despite the small grain size in sputteredB20 FeGe on Si, using this approach we are still able to observe helicityswitching of skyrmions and magnetic helices across two adjacent grains as theythread through neighboring grains. We reproduce this effect using micromagneticsimulations by assuming that the grains have distinct orientation and magneticchirality and find that magnetic helicity couples to crystal chirality. Ourmethodology for imaging magnetic textures is applicable to other thin-filmmagnets used for spintronics and memory applications, where an understanding ofhow magnetic order is accommodated in polycrystalline materials is important.

Disentangling magnetic and grain contrast in polycrystalline FeGe thin  films using four-dimensional Lorentz scanning transmission electron  microscopy