Hentschel, Maria Katharina
(2018).
Structural Correlation and Magnetic Effects in Ferecrystalline Materials.
PhD thesis, Universität zu Köln.
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Abstract
Nanolaminates are an excellent model system to study the correlation of structural and magnetic disorder in magnetic nanomaterials in one dimension because they offer precise control over the structure on a sub-Ångström length scale. Ferecrystals are nanolaminate compounds of the misfit-layer family with the general formula [(MSe)1+δ]m(TX2)n (M = Pb, Sn, Bi, or rare earth metal; X = S, Se, Te; T = transition metal) where the individual constituents are stacked and rotationally disordered along the c-axis. Since m and n are tunable, they are excellent candidates to systematically investigate surface and finite size-induced structural distortions in nanomaterials. Detailed knowledge of the atomic structure and the structural correlation of nanomaterials is essential to predict and explain structure-property relations. Reciprocal space maps revealed interlayer correlations in ferecrystals with the structure [(MSe)1+δ]1(TSe2)1 opposed to the typical observed turbostratic disorder. Those results suggest that preferential nucleation and layer alignment during nucleation occur in ferecrystals and show that, at least for some 1:1 ferecrystals, ordered domains are present. Moreover, a new set of ferecrystalline materials with the general structure [(CuxCrySez)1+δ](NbSe2)n has been prepared. By varying the annealing temperature of these compounds, it was possible to form four different heterostructures. This is the first set of ferecrystals which gives access to multiple heterostructures by varying the annealing temperature. The different heterostructures have been analyzed by out-of-plane and in-plane diffraction as well as STEM/EDX imaging. Rietveld refinements have been used with different starting models to gain better understanding of the structure in the CuxCrySez layer. The CuxCrySez layer was best described by a symmetric variation of CuCrSe2 structure. Also the effect of the annealing temperature on the atomic layering and site occupancies has also been investigated via Rietveld refinement. By replacing traditional MX rock salt with a CuxCrySez layer, the first magnetic ferecrystals have been prepared. The CuxCrySez layer serves as the magnetic constituent and is separated by layers of a nonmagnetic ‘spacer’ constituent (NbSe2). Macroscopic magnetization measurements revealed a significant magnetic moment despite the only small magnetic volume. By tuning the number of spacer layers, we can systematically alter the magnetic properties of the ferecrystals and investigate the magnetic interlayer coupling with n.
| Item Type: | Thesis (PhD thesis) | ||||||||
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| URN: | urn:nbn:de:hbz:38-99331 | ||||||||
| Date: | 2018 | ||||||||
| Language: | English | ||||||||
| Faculty: | Faculty of Mathematics and Natural Sciences | ||||||||
| Divisions: | Faculty of Mathematics and Natural Sciences > Department of Chemistry > Institute of Physical Chemistry | ||||||||
| Subjects: | Chemistry and allied sciences | ||||||||
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| Date of oral exam: | 14 January 2019 | ||||||||
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| Refereed: | Yes | ||||||||
| URI: | http://kups.ub.uni-koeln.de/id/eprint/9933 |
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