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@INPROCEEDINGS{Kovcs:827182,
      author       = {Kovács, András and Li, Zi-An and Caron, Jan and
                      Dunin-Borkowski, Rafal},
      title        = {{M}agnetic imaging of skyrmions in {F}e{G}e using off-axis
                      electron holography},
      address      = {Weinheim, Germany},
      publisher    = {Wiley-VCH Verlag GmbH $\&$ Co. KGaA},
      reportid     = {FZJ-2017-01380},
      pages        = {739 - 740},
      year         = {2016},
      comment      = {European Microscopy Congress 2016: Proceedings / Kovács,
                      András ;ISBN: 9783527808465},
      booktitle     = {European Microscopy Congress 2016:
                       Proceedings / Kovács, András ;ISBN:
                       9783527808465},
      abstract     = {Magnetic skyrmions are topologically protected spin
                      structures that have recently attracted considerable
                      interest as a result of their physical properties and
                      potential applications in energy-efficient spintronic
                      devices for information technology [1]. Magnetic skyrmions
                      were first observed in B20 compounds, whose
                      non-centrosymmetric crystal structure gives rise to strong
                      spin-orbit coupling. In these materials, the
                      Dzaloshinskii-Moriya interaction results in the formation of
                      a particle-like chiral spin structure in a regular hexagonal
                      lattice. Transmission electron microscopy (TEM) offers a
                      variety of methods for imaging the magnetic structure of
                      skyrmions, including the Fresnel mode of Lorentz TEM
                      combined with phase retrieval based on the transport of
                      intensity equation, scanning TEM combined with differential
                      phase contrast imaging and off-axis electron holography
                      (EH). Here, we discuss recent advances in EH-based methods
                      and related techniques for imaging skyrmion and helical spin
                      structures in B20 FeGe single crystals as a function of
                      temperature and applied magnetic field.Focused ion beam
                      (FIB) milling was used to prepare TEM specimens of FeGe with
                      a homogenous thickness of ˜100 nm and a large surface area
                      of ˜50 µm2. FIB-induced damage was reduced by using low
                      energy (<1 kV) ion milling. In order to form helical and
                      skyrmion spin structures [2], the FeGe specimens were cooled
                      below 280 K using a Gatan 636 liquid nitrogen cooling
                      holder. Fresnel images and off-axis electron holograms were
                      recorded using an FEI Titan 60-300 TEM operated at 300 kV in
                      magnetic field free conditions (<0.5 mT) in
                      aberration-corrected mode. The microscope was equipped with
                      a conventional Gatan Ultrascan 2k × 2k charge-coupled
                      device (CCD) camera and two biprisms, which were located in
                      the first and second selected area aperture planes.Figure 1
                      shows experimental magnetic phase images and corresponding
                      magnetic induction maps of skyrmion and helical spin
                      structures in FeGe recorded using EH. The mean inner
                      potential and magnetic contributions to the total phase
                      shift were separated by taking differences between
                      measurements recorded at low temperature and at room
                      temperature (when the FeGe is non-magnetic). The phase
                      resolution of the EH experiments was optimised by acquiring
                      multiple series of electron holograms and combining them
                      after cross-correlation, as well as by recording electron
                      holograms with a direct electron detection (Gatan K2-IS)
                      camera, which offers an improved detective quantum
                      efficiency and modulation transfer function when compared
                      with standard CCD cameras [3]. Skyrmions were studied as a
                      function of both temperature and magnetic field, which was
                      applied parallel to the electron beam direction using the
                      objective lens of the microscope (in free lens control
                      mode). The twin construction of the objective lens used
                      allowed the strength and polarity of the magnetic field to
                      be changed continuously, in order to study the magnetization
                      reversal dynamics of the skyrmions in situ in the TEM. The
                      recorded magnetic phase images were also used to calculate
                      the projected in-plane magnetization distribution in the
                      sample using a model-based iterative reconstruction
                      technique. As the inverse problem of reconstructing the
                      magnetization distribution is ill-posed, regularisation
                      parameters were used to constrain the solution. Examples of
                      the resulting magnetisation maps are shown in Fig. 2 for the
                      helical and skyrmion structures.},
      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.EMC2016.6276},
      url          = {https://juser.fz-juelich.de/record/827182},
}