Abstract:
In this thesis different quasi one-dimensional copper oxide systems were investigated by means of neutron scattering and thermodynamic experiments. The compound copper nitrate (Cu(NO3)2*2.5D2O) can be described as an alternating Heisenberg chain system. The magnetic phase diagram of copper nitrate for magnetic fields applied parallel to the crystallographic b axis was mapped out in much detail using neutron diffraction and thermodynamic techniques. The phase diagram shows a dome-shaped phase boundary of a field induced long-range ordered state for temperatures below 166 mK and fields between 2.8 T and 4.3 T, which is discussed in terms of a Bose-Einstein condensation of triplons. Furthermore, features in the magnetocaloric effect data at temperatures up to 220 mK were interpreted as crossovers into a Luttinger liquid regime. Simulations could be used to describe and complement the experimental data. In the compound linarite (PbCuSO4(OH)2) the nearest neighbor interactions compete with the next nearest neighbor interactions leading to frustration. At 2.8 K the system shows a first order phase transition into a long-range ordered state in zero field. Thermodynamic measurements were used to map out the phase diagram for magnetic fields applied along the b direction and temperatures down to 250 mK. Four different magnetic phases were detected. By means of neutron diffraction the spin structures in these phases were determined. In the ground state a helical spin arrangement was found. For applied magnetic fields a complex spin-flop process was observed, which is realized in two steps and results in a collinear spin arrangement with spins lying in the ac plane. Furthermore, an extraordinary sine wave modulated spin structure was found. Here, the wave vector is shifting with the magnetic field. This phase was interpreted in terms of a spin density wave phase, which can be described with density waves of bound three-magnons.