Disentangling Magnetic and Grain Contrast in Polycrystalline $\mathrm{Fe}\mathrm{Ge}$ Thin Films Using Four-Dimensional Lorentz Scanning Transmission Electron Microscopy

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

Kayla X. Nguyen, Xiyue S. Zhang, Emrah Turgut, Michael C. Cao, Jack Glaser, Zhen Chen, Matthew J. Stolt, Celesta S. Chang, Yu-Tsun Shao, Song Jin, Gregory D. Fuchs, and David A. Muller

Phys. Rev. Applied 17, 034066 – Published 28 March 2022

Abstract

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 $Fe Ge$ on $Si$ , 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 $Fe Ge$ , probing the magnetism of sputtered B20 $Fe Ge$ 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 $Fe Ge$ on $Si$ , 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.

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Received 30 January 2022
Accepted 4 March 2022

DOI:https://doi.org/10.1103/PhysRevApplied.17.034066

Physics Subject Headings (PhySH)

Magnetic domains Skyrmions Spin texture

Chiral magnets Polycrystalline materials

Convergent beam electron diffraction Lorentz microscopy Scanning transmission electron microscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Kayla X. Nguyen ^1,2,*,†, Xiyue S. Zhang ^2,†, Emrah Turgut^2,3, Michael C. Cao ², Jack Glaser ², Zhen Chen², Matthew J. Stolt⁴, Celesta S. Chang⁵, Yu-Tsun Shao², Song Jin⁴, Gregory D. Fuchs ^2,6, and David A. Muller^2,6

¹Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, USA
²School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, USA
³Department of Physics, Oklahoma State University, Stillwater, Oklahoma 74078, USA
⁴Department of Chemistry, University of Wisconsin–Madison, Madison, Wisconsin 53706, USA
⁵Department of Physics, Cornell University, Ithaca, New York 14853, USA
⁶Kavli Institute at Cornell for Nanoscale Science, Ithaca, New York 14853, USA