Direct Visualization of Magnetic Skyrmion by Aberration‐Corrected Differential Phase Contrast Scanning Transmission Electron Microscopy

Abstract

Magnetic skyrmion is a topologically protected quantum spin texture. It is so stable and expected to be utilized for future memory devices featuring ultralow energy consumption. However, influences of structural defects in real materials remain to be elucidated in such practical applications. Since magnetic skyrmion is a nano‐scale magnetic structure, visualization techniques with very high spatial resolution are essential to investigate such influences of nano‐scale structural defects, such as edges, dislocations and grain boundaries on magnetic skyrmion. Here, we present the direct visualization of magnetic skyrmion lattice in a thin film specimen of FeGe 1‐x Si x by aberration‐corrected differential phase‐contrast scanning transmission electron microscopy (DPC‐STEM) taking advantages of a segmented annular all‐field (SAAF) detector [1] connected via photomultiplier tubes with a high‐speed numerical processer. Polycrystalline FeGe 1‐x Si x was grown from FeGe 0.8 Si 0.2 ingot by conventional solid‐state reaction annealed at 900 °C for 100 hours. A thin film specimen was fabricated from a bulk crystal by using an Ion Slicer (EM‐9100IS, JEOL, Ltd.). For DPC STEM observations, we used a STEM (JEM‐2100F, JEOL, Ltd.) equipped with a probe‐forming aberration corrector (CEOS, GmbH) and a Schottky field emission gun operated at 200 kV. This microscope was equipped with a SAAF detector. We used a double‐tilt liquid‐nitrogen cooling specimen holder (Model 636, Gatan, Inc.). Analysis of DPC STEM images was done either online by using a direct reconstruction system [2] implemented as an application of LabView software (National Instruments, Inc.) running on Windows or offline by using a program written in Digital Micrograph Scripting language (Gatan, Inc.). Figure 1a shows a schematic diagram of electron‐optical system of DPC STEM used in the present study. In‐plane magnetization can be mapped by analyzing the Lorentz deflection of electron beam using segmented annular detector as schematically shown in Fig. 1b. As shown in Fig. 2a, a set of four images is selected from sixteen images obtained by the segmented detector. Such images are first converted into two images corresponding to a horizontal and a vertical component image of Lorentz deflection (Fig. 2b). Finally, the two Lorentz deflection images are analyzed to show a in‐plane magnetization vector, intensity, and magnetic helicity images as shown in Fig. 2c. Figure 3 demonstrates the live reconstruction of in‐plane magnetic field and intensity of a magnetic skyrmion lattice. A unique structural relaxation mechanism in a magnetic Skyrmion domain boundary core, revealed by the technique for the first time [3], will be presented in detail.