Lorentz Ptychography for high-resolution, high sensitivity magnetic imaging

David A. Muller (Cornell University)

Figure 1: a, The 4D-LSTEM setup. The deflection angle (black-dashed lines) caused by the lateral magnetic induction field (B) in the sample via the Lorentz force is measured by a 2D pixel array detector, such as an EMPAD. b, Measured magnetic phase shift of a skyrmion lattice in FeGe reconstructed from LEP using a 4D-LSTEM dataset. Out-of-plane component of B goes inwards (labelled as a circle with a cross). c, Lateral magnetic field map calculated via the first-order gradient of the phase recorded in b. The color shows the direction, and the brightness shows the magnitude, as given by the color wheel in the lower left inset in c. d, Magnetic-field vector map from the enlarged region marked by a white box in c. The arrows represent the magnetic field vectors with the length representing the lateral magnetic field magnitude. The false color shows only the magnetization direction. The dataset was acquired at a temperature of 94 K with an external magnetic field of 130 mT pointing into the page. Scale bars, 50 nm.

PARADIM Highlight #65

Nanoscale spin textures, especially magnetic skyrmions, have attracted intense interest as candidate high-density and power-efficient information carriers for spintronic devices. Facilitating a deeper understanding of nanometer to atomic-scale spin textures requires more advanced magnetic imaging techniques.

Electron microscopes use magnetic lenses to focus the electron beam, but the lens fields are usually large enough to disrupt most magnetic structures of interest. Here, members of PARADIM’s in-house research team demonstrate a Lorentz electron ptychography method that can enable high-resolution, high-sensitivity magnetic field imaging for widely available electron microscopes. The resolution of Lorentz electron ptychography is not limited by the usual diffraction limit of lens optics, allowing the main lens to be turned off, but instead is determined by the maximum scattering angle at which a statistically meaningful dose can still be recorded. The researchers used FeGe as the model system to realize a more accurate magnetic field measurement of skyrmions with an improved spatial resolution and sensitivity. The new approach resolves subtle internal structures of magnetic skyrmions near the skyrmion cores, boundaries, and dislocations in an FeGe single crystal.

The study establishes a quantitative, high-resolution magnetic microscopy technique that can reveal nanoscale spin textures, especially magnetization discontinuities and topological defects in nanomagnets. The technique’s high-dose efficiency should also make it well suited for the exploration of magnetic textures in electron radiation-sensitive materials such as organic or molecular magnets.

What has been achieved:

Demonstration of high resolution and high-sensitivity magnetic imaging, showing that magnetic imaging beyond the diffraction limit of the lens is possible, giving a roughly five-fold improvement in resolution, and allowing sub-nanometer features to be resolved.

Importance of the Achievement:

Electron lenses use strong magnetic fields to focus the electron beam, which would also disrupt most magnetic textures in the sample. Thus in conventional electron microscopes the main lens has to be turned off to avoid disturbing the sample, resulting in a much lower spatial resolution.  Here the team showed how to greatly improve the resolution without disturbing the sample.

Unique Feature(s) of the MIP that Enabled this Achievement:

The unique electron detector capabilities with sufficient speed and dynamic range to reach high signal to noise ratios, as well as the ptychographic reconstruction algorithms developed in the MIP.

Full reference:

Z. Chen, E. Turgut, Y. Jiang, K.X. Nguyen, M.J. Stolt, S. Jin, D.C. Ralph, G.D. Fuchs, and D.A. Muller, "Lorentz Electron Ptychography for Imaging Magnetic Textures beyond the Diffraction Limit," Nat. Nanotech.17, 1165–1170 (2022). DOI: 10.1038/s41565-022-01224-y

Acknowledgments:

This work is supported by the DARPA TEE-D18AC00009. Z.C. was partly supported by the PARADIM Materials Innovation Platform in-house program by NSF grant DMR-2039380. M.J.S. and S.J. were supported by NSF ECCS-1609585. This work made use of the Cornell Center for Materials Research facility supported by NSF grant DMR-1719875.