Magnetoelastic coupling in multiferroic GdMnO3 and metamagnetic Ca2-xSrxRuO4

Subject of the present thesis is the magnetoelastic coupling in multiferroic GdMnO3 and the metamagnetic Ca2-xSrxRuO4 with x between 0.2 and 0.5. GdMnO3 belongs to a class of new multiferroic materials where ferroelectricity shows up inside a magnetically ordered phase and a strong coupling between the magnetic and the electric properties is present. It possesses two magnetic transitions, one at TN into the ICAFM phase and one at Tc into the cAFM phase. Furthermore, for H parallel b, a ferroelectric transition occurs at TFE. Based on thermal-expansion and magnetostriction data, a modified H-T-phase diagram is derived. Due to large hysteresis effects in the low-field and low-temperature region, the pure cAFM phase cannot be reached upon cooling in zero magnetic field. The transition into the cAFM phase is accompanied by a jumplike drop of the orthorhombic splitting, which recovers upon entering the ferroelectric phase. Moreover, the uniaxial pressure dependencies of all three transitions are analysed. For the compound Ca2-xSrxRuO4 a change of the relevant magnetic correlation from ferromagnetic to antiferromagnetic is observed as soon as the RuO6 octahedra start tilting upon decreasing the Sr content below x=0.5. In Ca1.8Sr0.2RuO4, a metamagnetic transition occurs in a magnetic field, which comes along with strong structural changes. However, a complete suppression of the tilt upon the magnetic-field induced crossover from antiferromagnetic to ferromagnetic correlations can be excluded. At low temperatures, strong and anisotropic thermal expansion anomalies are observed. Both, these anomalies and the structural changes at the metamagnetic transition point towards a rearrangement of the orbital occupation induced by temperature as well as by magnetic field. For Ca1.8Sr0.2RuO4, a sign change of the low-temperature anomalies of the thermal expansion and the strong increase of the Grueneisen parameter in fields close to the metamagnetic transition can be identified as characteristic features of the metamagnetic transition. The observation of similar features in Ca1.5Sr0.5RuO4 provides evidence for an equivalent metamagnetic transition, but with a transition field close to zero. Within a phenomenological description, the anomalous behavior of both, the thermal expansion and the specific heat at the metamagnetic transition can be attributed to an enhanced entropy at the metamagnetic transition at low temperatures. Furthermore, similarities to other metamagnetic systems as well as the uniaxial pressure dependencies of the magnetic properties are discussed. (orig.)