Disentangling Magnetic and Grain Contrast in Polycrystalline FeGe Thin Films Using Four-Dimensional Lorentz Scanning Transmission Electron Microscopy

The study of nanoscale chiral magnetic order in polycrystalline materials with a strong Dzyaloshinskii-Moriya interaction is interesting for the observation of magnetic phenomena at grain boundaries and interfaces. This is especially true for polycrystalline materials, which can be grown using scalable techniques, the scalability of which is promising for future device applications. One such material is sputter-deposited B20 $\mathrm{Fe}\mathrm{Ge}$ on $\mathrm{Si}$, which is actively investigated as the basis for low-power high-density magnetic memory technology in a scalable material platform. Although conventional Lorentz electron microscopy provides the requisite spatial resolution to probe chiral magnetic textures in single-crystal $\mathrm{Fe}\mathrm{Ge}$, probing the magnetism of sputtered B20 $\mathrm{Fe}\mathrm{Ge}$ is more challenging because the submicron crystal grains add confounding contrast. This is a more general problem for polycrystalline magnetic devices, where scattering from grain boundaries tends to hide comparably weaker signals from magnetism. We address the challenge of disentangling magnetic and grain contrast by applying four-dimensional Lorentz scanning transmission electron microscopy using an electron-microscope pixel-array detector. Supported by analytical and numerical models, we find that the most important parameter for imaging magnetic materials with polycrystalline grains is the ability for the detector to sustain large electron doses, where having a high-dynamic-range detector becomes extremely important. Despite the small grain size in sputtered B20 $\mathrm{Fe}\mathrm{Ge}$ on $\mathrm{Si}$, using this approach, we are still able to observe helicity switching of skyrmions and magnetic helices across two adjacent grains, as they thread through neighboring grains. We reproduce this effect using micromagnetic simulations by assuming that the grains have distinct orientation and magnetic chirality and find that magnetic helicity couples to crystal chirality. Our methodology for imaging magnetic textures is applicable to other thin-film magnets used for spintronics and memory applications, where an understanding of how magnetic order is accommodated in polycrystalline materials is important.