Electron holography (EH) enable a sensitivity to electromagnetic fields at the nanoscale in a Transmission Electron Microscope (TEM) through the Aharonov-Bohm effect (which was by far experimentally demonstrated with EH [1]).  Beside EH we consider here the Moirés pattern than can arise with the fringe network of a hologram. Moirés originate from a superposition of two networks  and their use in electron microscopy enable to reveal hidden details within crystal stacks (from stacking faults in crystals to graphene misalignments).

The technique presented in this contribution propose to reach one step beyond the sensitivity that is actually achieved in EH by combining it with Moirés. The basic idea of what we call Dynamical Holographic Moirés (DHM) is to superpose various and controlled states of the sample under observation within the same out-of-axis hologram giving rise to the apparition of Moirés in a holographic interferogram. Holographic interferometry was initially designed in optics [2] and then implemented in its double exposure at the beginning of electron holography development. Our approach can be described as the implementation of time-average holographic interferometry by an application of in situ experiment with EH.

The system in use here is a Hard Disk Drive writing head that has been mounted to be characterized in operando by EH [3]. We can easily switch the writing pole state between two different states and thus reverting the magnetic flux sense. The DHM method [4] was thus used to fully characterize the magnetic flux escaping from such a magnetic pole in a quantitative way. By far, the obtained interferogram are directly quantized in term of magnetic flux quantum () linked to phase shift via the Aharonov-Bohm effect. A typical phase image of the system along with an interferogram are displayed in Fig 2. The main advantages of this new TEM technique, namely frequential analysis, electron optic distorsion and disturbed reference problem or direct quantification, will be discussed.

References :

[1]            Tonomura A.  Rev. Mod. Phys. 59, 639–669 (1987)
[2]            Heflinger, L. O.  et al.  Journal of Applied Physics 37, 642–649 (1966)
[3]            Einsle, J. F. et al.   Nano Res. 1–9 (2014)
[4]            Gatel, C. et al.   Under review

Figures:

Figure 1. Interferogram of a bullet (speed of Mach 1,7) within Ar gas. Black lines in the interferogram show optical indice variation within gaz due to bullet trajectory. Image taken from [3]. Figure 2 : Left image : phase shift obtain in vacuum on the front of the writing pole (located just at the top of the image) with 60 mA injected into the coil. The lines (2π humps) are describing the magnetic flux. Right image : Interferogram obtain on the same location for the writing pole working between to opposite states (± 30 mA) operated @ 1kHz. One can see the absence of the distorsions that appear in the phase image.

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