In this thesis the thermodynamic properties of the spin-1/2 chain compounds Cs2CoCl4 and Cu(C4H4N2)(NO3)2 (CuPzN) are investigated. Measurements of the specific heat, the thermal expanison, the magnetization and the magnetocaloric effect in a temperature range from 0.25 to 30 K and in magnetic fields up to 17 T are compared to model calculations.
In Cs2CoCl4 a crystal field anisotropy of about 7 K leads to a splitting of the orbital spin-3/2 states of the magnetic Co^{2+} into Kramers doublets. It is shown that the inclusion of excited crystal field states
in the application of a spin-1/2 XXZ model to the system is essential for a consistent description of the thermodynamics of Cs2CoCl4. The anisotropy Delta~0.12 of the spin chain is smaller than previously assumed in literature.
Below 2 K and for magnetic fields smaller than 4 T the magnetism of Cs2CoCl4 is well described by the XXZ model in a transverse magnetic field. Signatures of a quantum phase transition at 2.0 T are found in the thermodynamic data.
Below T_N=0.22 K magnetic order arises. The magnetic phase diagrams for different field directions are derived from the experimental data and the microscopic origins of the appearing phases are discussed. Bordering antiferromagnetism a low-temperature phase is identified that possibly is not a spin-liquid phase as previously suggested in literature. For fields along the crystallographic b axis an additional two-stage spin-flop transition arises.
In the mixed compounds Cs_2CoCl_{4-x}Br_x and Cs_3CoCl_{5-y}Br_y a site-selective doping is indicated by structural and thermodynamic investigations. While in the first system an easy-plane type magnetism, similar to the parent compound is found, the latter shows an easy-axis anisotropy and a non-continuous evolution of the low-temperature magnetism.
The magnetism of Cu(C4H4N2)(NO3)2 is highly isotropic and well described by the spin-1/2 Heisenberg chain, which shows a quantum phase transition as a function of an external field. In the thesis, the thermodynamics of Cu(C4H4N2)(NO3)2 are compared to exact results of the Heisenberg model including magnetic fields up to and above the critical field of the spin chain. All investigated thermodynamic quantities are described by the model with a high precision and show clear signatures of a quantum phase transition.
A coupling constant of 10.60 K is found and all data consistently indicate a critical field of the spin chain of 13.90 T.
Approaching the critical field, the specific heat acquires the expected power law, the magnetocaloric effect diverges and a universal scaling law is obeyed
