Off-axis electron holography (EH) is a TEM technique that records the phase information of an electron wave transmitted through a thin specimen in an electron hologram. By reconstructing this phase information, it enables electrostatic and magnetic potentials to be mapped quantitatively with high spatial resolution and, when combined with tomography to electron holographic tomography (EHT), in three dimensions (3D) [1,2]. Tomograms obtained by EHT provide the 3D mean inner potential (MIP) distribution of nanoscale materials from which the 3D morphology and the chemical composition can be inferred [3]. Moreover, functional potentials, e.g., introduced by doping of impurities in semiconductors, have been successfully revealed in 3D [4]. Recently, we succeeded in the 3D reconstruction of the axial component of the B-field prevailing in magnetic nanowires [5,6].

EHT as applied on magnetic samples proceeds as follows (see Fig. 1): (1) an electron hologram tilt series (ideally covering a range of 360°) is acquired, (2) the phase image tilt series is reconstructed from the holograms, (3) electric and magnetic phase shifts are separated by computing half of the sum/difference between opposite (180° tilted) projections, and (4) both the electric potential and the B-field component parallel to the tilt axis are reconstructed with tomographic techniques. Here, we report EHT studies achieved by means of tomography-dedicated TEM sample holders, in combination with advanced in-house developed software packages for acquisition, alignment and tomographic reconstruction.

Fig. 2 shows the 3D electric potential reconstruction of a GaAs/AlGaAs core-multishell nanowire (NW) grown by metalorganic vapour phase epitaxy (MOVPE) using an Au nanoparticle (NP) as catalyst. Such NWs may serve as novel coherent nanoscale light sources (lasers), because they provide an effective gain medium, low-loss optical waveguiding, and strong optical confinement for axially guided optical modes. The difference in the MIP allows discriminating between GaAs and AlGaAs within the NW. Longitudinal (Fig. 2b) and cross-sectional (Fig. 2e) 2D slices averaged over a well-defined thickness reveal not only the GaAs core and the AlGaAs shell, but also a 5nm thin GaAs shell within the AlGaAs, which acts as a quantum well.

Fig. 3 comprises two recent EHT studies revealing the B-field within a Co nanowire (NW) [5] and a Co₂FeGa Heusler alloy NW [6] both with spatial resolution higher than 10 nm. The reconstructions of the dominant axial component of the magnetic induction exhibit a small inversion domain at the apex of the Co NW, whereas at the Co₂FeGa NW, a magnetic dead layer of 10 nm width could be revealed.

The powerful approach presented here is widely applicable to a broad range of 3D electric and magnetic nanostructures and may trigger the progress of novel nanodevices.

[1] P A Midgley and R E Dunin-Borkowski, Nat. Mater. 8 (2009) p. 271.

[2] D Wolf, A Lubk, F Röder and H Lichte, Curr. Opin. Solid State and Mater. Sci. 17 (2013) p. 126.

[3] A Lubk, D Wolf, P Prete, N Lovergine, T Niermann, S Sturm and H Lichte, Phys. Rev. B 90 (2014) p. 125404.

[4] D Wolf, A Lubk, A Lenk, S Sturm and H Lichte, Appl. Phys. Lett. 103 (2013) p. 264104.

[5] D Wolf et al., Chem. Mater. 27 (2015) p. 6771.

[6] P Simon, D Wolf, C Wang, A A Levin, A Lubk, S Sturm, H Lichte, G H Fecher and C Felser, Nano letters 16 (2016) p. 114.

[7] We thank N Lovergine of University of Salento, Lecce for provision of the GaAs/AlGaAs core-multishell nanowire samples.

[8] This work was supported by the European Union under the Seventh Framework Program under a contract for an Integrated Infrastructure Initiative Reference 312483-ESTEEM2.

Figures:

Figure 1: Principle of electron holographic tomography. (1) Holographic acquisition; (2) Holographic reconstruction; (3) Separation of electric and magnetic phase shifts; (4) Tomographic reconstruction.

Figure 2. 3D potential of a GaAs/AlGaAs core-multishell NW. (a) 3D volume rendering; (b) 2D longitudinal slice averaged over the region indicated in (a); (c) 1D line profile along the arrow in (b); (d) 8V iso-surface rendering; (e) Cross-sections averaged over region indicated in (d).

Figure 3: 3D reconstruction of magnetic nanowires (NWs). 3D Volume rendering of electric potential (a,b) and axial (predominant) B-field component (c,d) inside the NWs. Colors correspond to the potential/B-field values. (e) Axial B-field component inside the Co NW obtained from micromagnetic simulation. The arrow plots visualize the out-of-plane components showing the twist of magnetic induction. (f) Line scans in axial direction through the center of the Co2FeGa NW from the tip to the back. This Figure is adapted from Refs. [5,6].

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