In this thesis, I study the electronic and structural phase diagrams of the superconducting 122 iron pnictides systems Ba1-xKx(Fe1-yCoy)2As2 and EuFe2(As1-xPx)2 by means of the local probe techniques 57Fe Mössbauer spectroscopy (MS) and muon spin relaxation (muSR). For both isovalent substitution strategies - Co/K for Fe/Ba and P for As, respectively - the antiferromagnetic Fe ordering and orthorhombic distortion of the parent compounds BaFe2As2 and EuFe2As2 are subsequently suppressed with increasing chemical substitution and superconductivity arises, once long-range and coherent Fe magnetic order is sufficiently but not entirely suppressed.
For Ba1-xKx(Fe1-yCoy)2As2 in the charge compensated state (x/2=y), a remarkably similar suppression of both, the orthorhombic distortion and Fe magnetic ordering, as a function of increasing substitution is observed and a linear relationship between the structural and the magnetic order parameter is found. Superconductivity is evidenced at intermediate substitution with a maximum Tsc of 15 K coexisting with static magnetic order on a microscopic length scale. The appearance of superconductivity within the antiferromagnetic state can by explained by the introduction of disorder due to nonmagnetic impurities to a system with a constant charge carrier density. Within this model, the experimental findings are compatible with the predicted s± pairing symmetry.
For EuFe2(As1-xPx)2, the results from 57Fe MS and ZF-muSR reveal an intriguing interplay of the local Eu 2+ magnetic moments and the itinerant magnetic Fe moments due to the competing structures of the iron and europium magnetic subsystems. For the investigated single crystals with x=0.19 and 0.28, 57Fe MS evidences the interplay of Fe and Eu magnetism by the observation of a transferred hyperfine field below Tafm at which the Eu subsystem orders into a canted A-type AFM magnetic structure. Furthermore, an additional temperature dependent out-of-plane tilting of the static Fe hyperfine field is observed below the onset of static Eu ordering. ZF-muSR shows a strong increase of the local field at the muon site below Tafm=20 K and a crossover from isotropic to anisotropic Eu spin-dynamics between 30 and 10 K. The temperature dependence of the spin dynamics, as derived from the muSR dynamic relaxation rates, are related to a critical slowing down of Eu-spin fluctuations which extends to even much higher temperatures (~100 K). They also effect the experimental linewidth observed in the 57Fe MS experiments. The strong influence of the Eu magnetic order onto the primary observables in both methods prevents conclusive interpretation of the experimental data with respect to a putative interplay of Fe magnetism and superconductivity.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:29145 |
| Date | 28 October 2015 |
| Creators | Goltz, Til |
| Contributors | Klauß, Hans-Henning, Fink, Jörg, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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