Lorentz 4D-STEM: Correlative Imaging of the Magnetic/Electric Fields, Strain Fields, and Atomic Packing Structure of Materials

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Contents of the Talk

We have developed Lorentz 4-dimensional scanning transmission electron microscopy (Ltz-4D-STEM) for correlative mapping of the magnetic/electronic structure, strain fields, and relative packing density in materials using a Themis Z double-corrected TEM (Thermofisher Scientific). Figure 1a schematically shows the Lorentz 4D-STEM setup. A quasi-parallel electron probe is focused to ~10 nm diameter on an electron transparent sample in field-free Lorentz mode. Using a 4D-STEM approach, 2D images of local diffraction patterns on a 2D grid are recorded at each probe position by stepwise scanning of the probe over the area of interest. In our analysis, we consider the momentum transfer of the electron beam due to the local magnetic field, the elliptic distortion of the amorphous diffraction pattern under strain, and the area encompassed by the ring to quantify the relative atomic density and reveal their spatial-correlative variance. This enables not only a quantitative analysis of local density and strain variations with high precision in amorphous materials [1], but further allows for a direct pixel-level correlation of the magnetic/electric field and the atomic structure and thus experimentally maps the correlative energy of magnetic/electric materials.

With this presentation, we will introduce our methodology developments and illustrate them using a soft ferromagnetic Fe-based metallic glass as example, which has received attention owing to its high energy efficiency and power density during energy conversion [2]. The magnetic structure of the ferromagnet consists of domains, where the magnetic dipoles are aligned to minimize the magnetostatic energy. The resulting magnetic structure is sensitive to local variations of the atomic spacing (strain) of the materials due to the magnetoelastic coupling [3]. This can be critical for their application in magnetoelectric systems, e.g. electric motors, in which, the magnetic parts are usually stressed during application [4]. With our new method, we experimentally map the magnetoelastic energy of soft magnetic Fe_85.2Si_0.5B_9.5P₄Cu_0.8 metallic glass ribbons at different processing states (as spun, thermally annealed, and after plastic deformation). The results reveal that the plastic deformation of the material gives rise to residual strain fields and eventually induces a complicated magnetic structure to stabilize the magnetoelastic energy. We studied the magnetoelastic energy for both as-spun and annealed metallic glasses and found that the magnetoelastic contribution after plastic deformation is substantially smaller in the annealed sample compared to the as-spun sample. This observation reveals new insight into the correlative material property and shows that the relaxation treatment of Fe-based metallic glasses can improve magnetic stability against imposed stress during usage. The method opens a new door to studying the correlative properties of materials.

References

[1] S. Kang et al., Advanced Materials, 202212086 (2023)

[2] Silveyra et al., Science 362, 418 (2018)

[4] Li et al., Prog. in Mater. Sci. 103, 235-318 (2019)

[4] Shen et al., Nat. Comm. 9, 4414 (2018)

Short CV

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