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@INPROCEEDINGS{DuninBorkowski:827187,
      author       = {Dunin-Borkowski, Rafal and Almeida, Trevor P. and Caron,
                      Jan and Kovács, András and Diehle, Patrick and Winkler,
                      Florian and Zheng, Fengshan and Tavabi, Amir H. and Migunov,
                      Vadim and Li, Zi-An},
      title        = {{L}imitations and challenges in off-axis electron
                      holography of electromagnetic fields in nanoscale materials},
      address      = {Weinheim, Germany},
      publisher    = {Wiley-VCH Verlag GmbH $\&$ Co. KGaA},
      reportid     = {FZJ-2017-01385},
      pages        = {677 - 678},
      year         = {2016},
      comment      = {European Microscopy Congress 2016: Proceedings},
      booktitle     = {European Microscopy Congress 2016:
                       Proceedings},
      abstract     = {In contrast to most conventional transmission electron
                      microscopy (TEM) techniques, which only allow the spatial
                      distribution of image intensity to be recorded, off-axis
                      electron holography allows the phase shift of the electron
                      wave that has passed through an electron-transparent
                      specimen to be measured. The phase shift can, in turn, be
                      used to provide information about local variations in
                      magnetic induction and electrostatic potential within and
                      around the specimen. Recent developments in the technique
                      include the reconstruction of electrostatic potentials and
                      magnetic fields in three dimensions, the use of advanced
                      specimen holders with multiple electrical contacts to study
                      nanoscale working devices, improvements in the stability of
                      transmission electron microscopes to optimise phase
                      sensitivity and the development of new approaches for
                      improving temporal resolution using both direct electron
                      detectors and double exposure electron holography. We are
                      currently using the technique to characterize electrostatic
                      potentials and magnetic fields in a wide variety of
                      nanoparticles, nanostructures and thin films that are
                      subjected to electrical biases and externally applied
                      magnetic fields, as well as to elevated and reduced
                      temperatures. Figure 1 shows representative results obtained
                      from a study of the thermomagnetic behaviour of nanoscale
                      grains of magnetite during heating in situ in the TEM. The
                      magnetic induction maps show first a horseshoe-like magnetic
                      state and then magnetic phase contours that flow from the
                      bottom to top of the grain at higher temperature.An
                      important limitation of backprojection-based algorithms for
                      reconstructing magnetic fields in three dimensions is the
                      presence of artefacts resulting from incomplete tilt series
                      of phase images and the inability to include additional
                      constraints and known physical laws. Accordingly, one of our
                      aims is the development of a robust model-based approach
                      that can be used to reconstruct the three-dimensional
                      magnetization distribution in a specimen from phase images
                      recorded as a function of specimen tilt angle using off-axis
                      electron holography. In order to perform each
                      reconstruction, we generate simulated magnetic induction
                      maps by projecting best guesses for the three-dimensional
                      magnetization distribution in the specimen onto
                      two-dimensional Cartesian grids. Our simulations make use of
                      known analytical solutions for the phase shifts of simple
                      geometrical objects, with numerical discretization performed
                      in real space to avoid artefacts generated by discretization
                      in Fourier space, without a significant increase in
                      computation time (Figs 2 and 3). Our forward simulation
                      approach is used within an iterative model-based algorithm
                      to solve the inverse problem of reconstructing the
                      three-dimensional magnetization distribution in the specimen
                      from tilt series of two-dimensional phase images recorded
                      about two independent tilt axes. Results will be presented
                      from studies of magnetite nanocrystals, lithographically
                      patterned magnetic elements and magnetic skyrmions examined
                      as a function of temperature and applied magnetic field. At
                      the same time, we are developing a similar algorithm for the
                      reconstruction of three-dimensional charge density
                      distributions in materials. Preliminary results will be
                      presented from studies of charge distributions in
                      electrically biased needle-shaped specimens, which require
                      the analysis of differences between phase images recorded
                      using two applied voltages, in order to subtract the mean
                      inner potential to the phase shift.The above studies are
                      part of a wider program of research aimed at recording
                      off-axis electron holograms of nanoscale materials and
                      devices in the presence of multiple external stimuli.
                      Further examples will be presented from studies of
                      electrically biased resistive switching devices and
                      two-dimensional flakes of transition metal dichalcogenides,
                      whose electrical properties can be influenced strongly by
                      the presence of contamination and defects, as well as by
                      their interfaces to metal contacts. We are grateful to J.
                      Ungermann, M. Riese, G. Pozzi, W. Williams, A. R. Muxworthy,
                      M. Farle, M. Beleggia, T. F. Kelly and N. Kiselev for
                      valuable contributions to this work and to the European
                      Research Council for an Advanced Grant.},
      month         = {Aug},
      date          = {2016-08-28},
      organization  = {16th European Microscopy Congress (EMC
                       2016), Lyon (France), 28 Aug 2016 - 2
                       Sep 2016},
      cin          = {PGI-5 / ER-C-1},
      cid          = {I:(DE-Juel1)PGI-5-20110106 / I:(DE-Juel1)ER-C-1-20170209},
      pnm          = {143 - Controlling Configuration-Based Phenomena (POF3-143)},
      pid          = {G:(DE-HGF)POF3-143},
      typ          = {PUB:(DE-HGF)8 / PUB:(DE-HGF)7},
      doi          = {10.1002/9783527808465.EMC2016.8663},
      url          = {https://juser.fz-juelich.de/record/827187},
}