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| Hauptseite > Publikationsdatenbank > Germanium-based multiferroic compounds with melilite structure |
| Hauptseite > Publikationsdatenbank > Germanium-based multiferroic compounds with melilite structure |
| Dissertation / PhD Thesis | FZJ-2025-04566 |
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.
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