Phase imaging using 2-path interferometric 4D-STEM

Session

PST.1 - Phase Microscopy

Authors

Benjamin McMorran (1), Alice Greenberg (1), Tyler Harvey (1, 3), Cameron Johnson (1), Amy Turner (1), Fehmi Yasin (1, 2)

Affiliations

1. University of Oregon
2. Center for Emergent Matter Science, RIKEN
3. Georg-August-Universität Göttingen

Keywords

4D-STEM, holography, interferometry, magnetism, phase imaging

Abstract text

While TEMs are the most versatile instrument for directly imaging materials at sub-nanomater lengthscales, many features of interest are very transparent to electrons and thus difficult to image. For example, materials composed of light elements such as biological specimens do not efficiently scatter electrons. Likewise, nanoscale magnetic field textures inside samples do not scatter electrons. However, these features do introduce phase shifts to electrons, and several phase-sensitive TEM techniques are used to provide contrast. Defocused-based techniques can be employed in any TEM to provide phase contrast, albeit at the cost of spatial resolution. Differential phase contrast (DPC) microscopy and 4D-STEM can measure phase gradients, from which phases themselves can be reconstructed. Off-axis electron holography can measure phase shifts directly relative to a vacuum reference, but requires a highly coherent electron beam and electrostatic biprism wires.

 

We are developing an interferometric STEM technique that uses coherent separate probe beams [1–4], also known as STEM holography. We have begun to apply this technique to image carbon materials [5] and nanoscale magnetic texture [6,7]. This arrangement only requires a modified, passive condenser aperture and a direct electron imaging detector. The aperture features an efficient nanoscale phase grating that serves as an amplitude-dividing beam splitter in the probe-forming STEM optics of the instrument. Each probe electron traverses a superposition of two paths – one path forms a probe beam and the other provides a reference. The relative phase shift between these paths is measured by recording an image of the far-field interference pattern using the imaging detector. This a 4D-STEM interferometry technique where we measure the phase of the bright field disc. 

 

We have begun to apply this technique to study nanoscale magnetization. We examine magnetic flux closure domain in patterned squares of permalloy thin film. We also perform Lorentz TEM studies of magnetic skyrmions  in FeGd thin films [8–10] and apply the interferometric STEM technique to these materials, as well. 

 

Direct phase contrast provided by interferometric STEM yield information similar to off-axis TEM holography. However, in principle other signals can also be provided simultaneously in a single scan, such as HAADF, EDS, and DPC, providing even more information. Furthermore, interferometric STEM has reduced spatial coherence requirements when using a nanoscale grating as a beamsplitter. We will discuss the challenges of this technique and ways to improve it. These results represent a further development of interferometric STEM as applied to magnetic imaging. 

References

[1]     F. S. Yasin, T. R. Harvey, J. J. Chess, J. S. Pierce, and B. J. McMorran, "Development of STEM-Holography," Microsc. Microanal. 22, 506–507 (2016).

[2]     F. S. Yasin, T. R. Harvey, J. J. Chess, J. S. Pierce, and B. J. McMorran, "Path-separated electron interferometry in a scanning transmission electron microscope," J. Phys. Appl. Phys. 51, 205104 (2018).

[3]     T. R. Harvey, F. S. Yasin, J. J. Chess, J. S. Pierce, R. M. S. dos Reis, V. B. Özdöl, P. Ercius, J. Ciston, W. Feng, N. A. Kotov, B. J. McMorran, and C. Ophus, "Interpretable and Efficient Interferometric Contrast in Scanning Transmission Electron Microscopy with a Diffraction-Grating Beam Splitter," Phys. Rev. Appl. 10, 061001 (2018).

[4]     B. J. McMorran, T. R. Harvey, C. Ophus, J. Pierce, and F. Yasin, "Demonstration of STEM Holography Using Diffraction Gratings," Microsc. Microanal. 24, 200–201 (2018).

[5]     F. S. Yasin, T. R. Harvey, J. J. Chess, J. S. Pierce, C. Ophus, P. Ercius, and B. J. McMorran, "Probing Light Atoms at Subnanometer Resolution: Realization of Scanning Transmission Electron Microscope Holography," Nano Lett. 18, 7118–7123 (2018).

[6]     A. Greenberg, F. Yasin, C. Johnson, and B. McMorran, "Lorentz Implementation of STEM Holography," Microsc. Microanal. 25, 96–97 (2019).

[7]     B. J. McMorran, "Seeing with Phase: Interferometric Electron Microscopy for Magnetic Materials and Biological Specimens," Microsc. Microanal. 25, 1210–1211 (2019).

[8]     S. A. Montoya, S. Couture, J. J. Chess, J. C. T. Lee, N. Kent, D. Henze, S. K. Sinha, M.-Y. Im, S. D. Kevan, P. Fischer, B. J. McMorran, V. Lomakin, S. Roy, and E. E. Fullerton, "Tailoring magnetic energies to form dipole skyrmions and skyrmion lattices," Phys. Rev. B 95, 024415 (2017).

[9]     J. Chess, S. Montoya, J. Lee, S. Roy, S. Kevan, E. Fullerton, and B. McMorran, "Observation of Skyrmions at Room-temperature in Amorphous Fe/Gd Films," Microsc. Microanal. 21, 1649–1650 (2015).

[10]   J. J. Chess, S. A. Montoya, T. R. Harvey, C. Ophus, S. Couture, V. Lomakin, E. E. Fullerton, and B. J. McMorran, "Streamlined approach to mapping the magnetic induction of skyrmionic materials," Ultramicroscopy 177, 78–83 (2017).

[11] This work was supported by the National Science Foundation under Grant No. 1607733.