Characterizing embedded nanomagnets using fast pixelated detectors in STEM

Session

PSA.9 - Magnetic and Spintronic Materials

Authors

Dr. Magnus Nord (2, 3), Dr. Anna Semisalova (1), Dr. Attila Kákay (1), Dr. Gregor Hlawacek (1), Dr. Ian MacLaren (2), Dr. Vico Liersch (1), Dr. Oleksii M. Volkov (1), Dr. Denys Makarov (1), Dr. Gary W. Paterson (2), Dr. Kay Potzger (1), Dr. Jürgen Lindner (1), Prof. Dr. Jürgen Fassbender (1), Dr. Rantej Bali (1), Dr. Damien McGrouther (2)

Affiliations

1. Institute of Ion Beam Physics and Materials Research, Helmholtz‐Zentrum Dresden‐Rossendorf
2. SUPA, School of Physics and Astronomy, University of Glasgow
3. Electron Microscopy for Materials Science, University of Antwerp

Keywords

Nanomagnets, STEM-DPC, 4-D STEM

Abstract text

The development of fast 2-dimensional direct electron detectors has enabled high acquisition rate imaging in transmission electron microscopy (TEM). This has been especially impactful in Scanning TEM (STEM), where a nanoscale electron probe is scanned across the sample material in a raster pattern. Conventionally, the information contained in this rich diffraction pattern has been integrated into a single value for each position in the raster scan. However, by utilizing these new direct electron detectors, a large fraction of the diffraction pattern can be captured for every probe position. One use for this is STEM differential phase contrast (STEM-DPC), which enables imaging of magnetisation by determining the shift of the primary beam due to magnetic deflection [1]. In addition, by carefully selecting the appropriate microscope settings, structural information can also be acquired in the same datasets as the ones used for magnetic imaging from the lowest angle diffraction spots.

This presentation will show the results of such a study [2] on irradiated Fe₆₀Al₄₀ films. As-deposited Fe₆₀Al₄₀ is paramagnetic and has an ordered B2 structure. Through irradiation with Ne⁺ ions, the structure changes to a ferromagnetic and disordered A2 BCC structure with larger unit cell [3]. Utilizing a Ne⁺ focused ion beam (FIB), stripe shaped ferromagnetic regions with lateral sizes from 0.03 to 4 µm and length of 10 µm were written. These nanoscale magnets were studied using an aberration corrected JEOL ARM200cF fitted with a MERLIN 1R (Medipix3) fast pixelated detector, where switching the objective lens into a field-free mode allowed for the magnetic structure to be imaged using STEM-DPC. This revealed an unexpected domain structure in the stripes, with a large fraction of the magnetization deviating from the long-axis direction (Fig. 1), contrary to the normal expectations from the shape anisotropy energy. Utilizing the low angle 110 diffraction spots present in the same datasets, the in-plane lattice parameter was found to be anisotropic in the stripes, with a larger lattice parameter along the short-axis direction. When this is studied with micromagnetic simulations, it is found that the competing effects of the lengthways shape anisotropy and the strain-induced lateral easy axis stabilized the periodic ferromagnetic domain structure seen in Fig. 1. These effects also hold for non-rectilinear high aspect ratio patterns, such as the spiral structure in Fig. 1e. [4]

References

[1] J. N. Chapman, I. R. McFadyen, S. McVitie, IEEE Trans. Magn. 1990, 26, 1506.

[2] M. Nord et al, Small (2019), 15, 1904738.

[3] Rantej Bali, et al., Nano Letters, 14 (2014) 435-441.

[4] Financial support from the Deutsche Forschungsgemeinschaft (DFG) through the Grants No. BA5656/1-1 and EPSRC via the project “Fast Pixel Detectors: a paradigm shift in STEM imaging” (Grant reference EP/ M009963/1) is acknowledged. M.N. received additional support from the European  Union’s  Horizon  2020  research  and  innovation programme under the Marie Skłodowska-Curie grant agreement No 838001.