The concept of magnetic frustration is a fundamental topic in modern solid-state physics having direct consequences in systems with rich magnetic phases hosting emergent excitations, such as the magnetic monopoles in the spin-ice compounds. One important ingredient of frustration is the lattice that constrains the magnetic spins on it to a site anisotropy and inter-site coupling. Therefore, strong magnetoelastic interactions between the magnetic system and the lattice are expected and investigated in this thesis in detail. At first, I investigate the dependence of the relative length change of single crystals of the classical spin ices \dto{} and \hto{} on the magnetic field and temperature by capacitive dilatometry. In terms of the magnetostriction and thermal expansion \dto{} and \hto{} show qualitatively similar behavior, that seems to be independent of the Kramer or non-Kramers character of the rare-earth ion. The magnitude of the magnetostrictive effect deep in the spin-ice phase at \SI{0.3}{\kelvin} is $\deltaL{} = \SI{2e-5}{}$ and $\SI{2e-4}{}$ for \dto{} and \hto{}, respectively.
In numerical simulations using a manifold model, the experimental results could be qualitatively reproduced by a combination of exchange and crystal-field striction. A second highlight of the dilatometric measurements of the spin-ice compounds is the observation of the lattice dynamics. The relaxation processes are rather slow, the longest relaxation times were observed at lowest temperatures and in the field range with magnetostrictive hysteresis, \ie{}, below \SI{0.9}{\tesla} for \dto{} and below \SI{1.5}{\tesla} for \hto{}. I find that the region of longest relaxation coincides well with the kagome-ice phase of the magnetic phase diagrams; the laxation time is of the order of \SI{5000}{\second} ($> \SI{80}{\minute}$). With increasing temperatures the time scale of the relaxation reduces to minutes at around \SI{0.7}{\kelvin} corresponding to the spin-freezing temperature obtained from ac-susceptibility measurements.
In the second study I investigate the variation of the magnetic properties in dependence of the lattice constant. A systematic reduction of the lattice constant of \dgsoxx{} can be achieved by substituting the non-magnetic germanium ion in the cubic pyrochlore oxide with silicon. Characteristic properties of a spin-ice phase could be observed in measurements of magnetization, ac susceptibility, and heat capacity. From the temperature shift of the peaks, observed in the temperature-dependent heat capacity, an increase of the strength of the magnetic exchange interaction by a changed ratio of the competing exchange and dipolar interaction is deduced. The new spin-ice compounds are, thus, closer to the phase boundary between spin-ice phase and antiferromagnetically ordered all-in-all-out phase consistent with a reduction of the energy of monopole excitations.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:36509 |
| Date | 10 December 2019 |
| Creators | Stöter, Thomas |
| Contributors | Wosnitza, Joachim, Geck, Jochen, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
| Relation | info:eu-repo/grantAgreement/Deutsche Forschungsgemeinschaft/SFB 1143/247310070//Korrelierter Magnetismus: Von Frustration zu Topologie |
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