Disentangling magnetic and grain contrast in polycrystalline FeGe thin films using 4-D Lorentz Scanning Transmission Electron Microscopy.

The study of nanoscale chiral magnetic order in polycrystalline materials with a strong Dzyaloshinkii-Moriya interaction (DMI) is interesting for the observation of new magnetic phenomena at grain boundaries and interfaces. Although conventional Lorentz electron microscopy provides the requisite spatial resolution to probe chiral magnetic textures in single-crystal FeGe, probing the magnetism of sputtered B20 FeGe is more challenging because the sub-micron 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 4-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. From our experimental results on sputtered B20 FeGe on Si, we observe phase shifts of the magnetic helices as they thread through neighboring grains. We reproduce this effect using micromagnetic simulations by assuming that the grains have distinct orientation and magnetic chirality. We then compare spin textures of strained FeGe on a Si substrate to those on free-standing FeGe by performing electron energy loss spectroscopy and find that the helical phases are only present in free-standing FeGe. 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.