Thermodynamics and transport properties of the ferromagnetic metal SrRuO3 and the frustrated magnet Cs3Fe2Br9

In this thesis, the ferromagnetic metal SrRuO3 and the frustrated antiferromagnet Cs3Fe2Br9 are investigated. New high-quality single crystals of SrRuO3 grown with the floating-zone technique are used to study the critical phenomena at the ferromagnetic phase transition. A shape-memory effect is found below TC, which makes the material ferroelastic and thus multiferroic. Spin-orbit coupling yields spin-split bands and a Weyl point close to the Fermi energy acts as a magnetic monopole in momentum space, which causes an intrinsic anomalous Hall effect. Furthermore, the anomalous softening of the magnon gap and the stiffness can be described with the Hall conductivity, which makes the explanation via the Berry-curvature contribution conclusive. The new material Cs3Fe2Br9 consists of face-sharing Fe2Br9 bi-octahedra that are arranged in triangular layers stacked to form a hexagonal structure and orders antiferromagnetically. In contrast to isostructural classical spin systems that form a singlet ground state, the two S = 5/2 moments in each bi-octahedron are oriented parallel along c. The in-plane nearest-neighbor interaction overcompensates the antiferromagnetic dimer coupling. The antiferromagnetic ground state is substantially frustrated. For a magnetic field along the easy-axis, an extremely rich phase diagram with ten phases is found. Two phases host a fractional magnetization and one phase shows a ferroelectric polarization. The results are compared to analytical and numerical predictions for triangular and hexagonal lattices and related materials