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@PHDTHESIS{Thoma:1048346,
      author       = {Thoma, Henrik},
      title        = {{G}ermanium-based multiferroic compounds with melilite
                      structure},
      school       = {RWTH Aachen University},
      type         = {Dissertation},
      reportid     = {FZJ-2025-04566},
      pages        = {243 p.},
      year         = {2023},
      note         = {Dissertation, RWTH Aachen University, 2023},
      abstract     = {This work is dedicated to the detailed investigation of the
                      atomic and magnetic structurein a selection of
                      germanium-based melilite-type compounds, namely Ba2TGe2O7(T
                      = Cu, Co and Mn) and the Ba2Cu1-xMnxGe2O7 solid solution
                      series. These materialsgained high interest in the current
                      condensed matter research as they revealed astrong
                      magnetoelectric coupling that could not be explained by the
                      conventional modelsof spin-driven multiferroicity. Inspired
                      by their peculiar magnetoelectric effects, newmechanisms
                      have recently been proposed, which might be of high
                      potential for spintronicapplications.Detailed structural
                      parameters of the Ba2Cu1-xMnxGe2O7 solid solution series are
                      providedas a function of the concentration x by X-ray and
                      neutron powder diffraction on aseries of recently
                      synthesized polycrystalline compounds. Moreover, spurious
                      phases areidentified and characterized. Additional heat
                      capacity measurements reveal the emergenceof two distinct
                      low-temperature magnetic phases for the Cu-rich and
                      Mn-richcompounds, separated by a critical concentration of x
                      ≈ 0.57. The macroscopic propertiesof these phases and
                      their evolution with applied magnetic field are mapped outby
                      magnetization measurements. The magnetic ground state in
                      Ba2Cu1-xMnxGe2O7 isidentified and specified by
                      high-resolution neutron powder diffraction, disclosing an
                      antiferromagneticcone-like structure for the Cu-rich and a
                      commensurate antiferromagneticstructure for the Mn-rich
                      phase.A comprehensive study of the magnetic structure in
                      Ba2TGe2O7 (T = Cu, Co and Mn)single crystals is provided by
                      polarized neutron diffraction. For this study, the
                      flippingratio setup of the POLI instrument at the Heinz
                      Maier-Leibnitz Zentrum in Garching,Germany, is extended for
                      a new high-field magnet with a dedicated supermirror
                      bender.This setup is numerically optimized and the neutron
                      polarization confirmed to be $above99\%.$ Corresponding
                      flipping ratios are measured on the Ba2TGe2O7 single
                      crystalsin the para- and antiferromagnetic phases, revealing
                      precise values for the ordered andfield-induced magnetic
                      moments, the susceptibility tensors and local anisotropies.
                      Basedon these results and a careful symmetry analysis, the
                      sign of the Dzyaloshinskii-Moriyainteraction vector in T =
                      Cu and Co is reported for the first time. Additional
                      sphericalneutron polarimetry on Ba2CoGe2O7 confirms
                      P2′1212′ as the magnetic space group of theground state
                      and demonstrates the high sensitivity of the remanent domain
                      configurationon applied magnetic fields.In the context of
                      this thesis, an advanced maximum entropy approach for the
                      reconstructionof magnetization densities from polarized
                      neutron diffraction data was developedand implemented in a
                      new software tool. Applied to Ba2TGe2O7, it
                      providesaspherical density shapes for the transition metal
                      ions that directly relate to their 3dorbital configuration
                      resulting from crystal field effects. This is confirmed by
                      multipoleexpansion, revealing orbital populations close to
                      the expected values.},
      cin          = {JCNS-2 / JCNS-FRM-II / JARA-FIT},
      cid          = {I:(DE-Juel1)JCNS-2-20110106 /
                      I:(DE-Juel1)JCNS-FRM-II-20110218 /
                      $I:(DE-82)080009_20140620$},
      pnm          = {632 - Materials – Quantum, Complex and Functional
                      Materials (POF4-632) / 6G4 - Jülich Centre for Neutron
                      Research (JCNS) (FZJ) (POF4-6G4)},
      pid          = {G:(DE-HGF)POF4-632 / G:(DE-HGF)POF4-6G4},
      typ          = {PUB:(DE-HGF)11},
      url          = {https://juser.fz-juelich.de/record/1048346},
}