Weyl semimetals feature linear band crossings with non-trivial topological properties. These (Weyl) points can have a strong impact on the transport response, when they are located close to the Fermi level. The Weyl points can lead to, among other things, a large anomalous Hall effect. Co3Sn2S2 is a prototypical candidate of this material class and displays intriguing physics, originating from its topological as well as magnetic properties. Many interesting transport responses, including the anomalous Hall effect and the anomalous Nernst effect have been found to be realised in this compound.
Previous experiments have been carried out exclusively in (bulk) single crystals of Co3Sn2S2. It is demonstrated in this thesis, that microsturctures with well defined contacts can fabricated from bulk single crystals using focused ion beam cutting. Laser lithography is employed to define the electrical contacts to the micro ribbons. The microstructured devices are used to study the evolution of the magnetoresistance and magnetothermopower with magnetic field and temperature.
The magnetoelectric transport response in Co3Sn2S2 is addressed in the first part of this thesis. Co3Sn2S2 exhibits a particularly large anomalous Hall effect in the transverse magnetoresistance. The anomalous Hall conductivity is found to be independent of the longitudinal conductivity, suggesting an intrinsic origin of the anomalous Hall effect. This notion is further corroborated by comparing the experimental results with band structure calculations, using the Berry curvature. Furthermore, the agreement between measurements on the microstructures with measurements on single crystals demonstrates the high quality of the microstructured devices.
Additionally to the transverse magnetoresistance also the longitudinal magnetoresistance of Co3Sn2S2 is studied. The magnetoresistance decreases with increasing magnetic field for temperatures above 100 K. Futhermore, the magnetoresistance is found to be highly anisotropic with respect to the direction of the magnetic field. The negative magnetoresistance can be consistently explained invoking the magnon magnetoresistance. This effect is present in the ferromagnetic as well as the paramagnetic phase and is the dominant magnetoresistance effect above 100 K. The remarkably strong magneto-crystalline anisotropy of Co3Sn2S2 along its crystalline c-axis leads to the interesting observation, that the magnon magnetoresistance appears to depend on the projection of the external magnetic field on the magnetization. At low temperatures, magnon modes are frozen out and the magnon magnetoresistance vanishes. However, the anisotropy of the carrier mobilities gives rise to an anisotropic orbital magnetoresistance. Interestingly, no clear signs of crystalline/non-crystalline anisotropic magnetoresistance, depending on the magnetization direction, can be observed. The salient features in the magnetoelectric response can be understood through the comprehensive investigations reported in this thesis.
In the second part, the thermal counterparts of the magnetoelectric transport responses are investigated. Again, a surprisingly large transverse (Nernst) effect is found. The Nernst conductivity reaches up to 4 A/(m K) in Co3Sn2S2. Unlike their magnetoelectric counterparts, the Nernst coefficient and the Nernst conductivity exhibit a sign change as a function of temperature. These experimental results strengthen the theoretical framework, that the anomalous Hall and Nernst effect probe different states. Furthermore, an additional contribution at the magnetic phase transition can be identified using the Mott relation. This contribution indicates that the anomalous Nernst conductivity can be enhanced by magnetic fluctuations near the magnetic ordering temperature. It appears to be a generic contribution in ferromagnetic materials and it is so far not accounted for by this particular formulation of the Mott relation. This subtle feature is uncovered when comparing the large magnetoelectric and magnetothermal transport response in the magnetic Weyl semimetal Co3Sn2S2.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:72645 |
| Date | 30 October 2020 |
| Creators | Geishendorf, Kevin |
| Contributors | Nielsch, Kornelius, Woltersdorf, Georg, Technische Universität Dresden, Leibniz Insitute for Solid State and Materials Research |
| 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 |
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