Imaging of Magnetic Domain Structure of Fe-Based Nanocrystalline Alloys with Off-Axis Electron Holography and Lorentz Microscopy

Fe-based soft magnetic nanocrystalline alloys such as Vitroperm and Finemet are pioneering materials for high-frequency power applications, ranging from inductive cooking to automotive use. Depending on the desired use, the magnetic properties of the alloys can also be personalized during annealing by applying a magnetic field or a tensile stress, both of which yield uniform uniaxial anisotropy.

Here, the outcome of annealing on the microstructure and magnetic properties of Fe73.5Si15.5B7Nb3Cu1 alloys are considered with high spatial resolution in the transmission electron microscope (TEM) using both the Fresnel mode of Lorentz TEM and off-axis electron holography.

Materials and Methods

Defocus series of Fresnel images and off-axis electron holograms were obtained at 300 kV in Lorentz mode (magnetic field free mode) with a K2 IS camera in Summit configuration using the linear, counting and super-resolution approaches. The K2 IS camera is the quickest and highest performance in-situ camera to resolve dynamic details in heating, catalysis, mechanical deformation, STEM diffraction, electrical testing, and chemical reaction experiments.

The magnetic state of the focused ion beam prepared specimen was altered by slanting the sample and rotating the objective lens to apply a magnetic field of 1.5 T to the sample, either perpendicular or parallel to the path of the electron beam.

a) Fresnel image of domain walls in annealed FeSiBNbCu at 695 ºC for 4 s. (b) Off-axis electron hologram. The enlarged region shows the bending of the holographic interference fringes on the domain wall. (c) Magnetic induction map of the region indicated in (a) showing the local changes in magnetization direction. The phase contour spacing is 2π.

Figure 1. a) Fresnel image of domain walls in annealed FeSiBNbCu at 695 ºC for 4 s. (b) Off-axis electron hologram. The enlarged region shows the bending of the holographic interference fringes on the domain wall. (c) Magnetic induction map of the region indicated in (a) showing the local changes in magnetization direction. The phase contour spacing is 2π.

Summary

Magnetic structure of soft magnetic polycrystalline alloy was considered in this research. Using the K2 IS camera, high contrast holographic interference fringes and very clear magnetic induction maps could be documented. The Fresnel images and phase maps were used to determine the domain wall width with high spatial resolution.

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