Crystal Growth And Physical Properties Of Low-Dimensional Organic Magnetics

Low-dimensional organic magnetic materials have aroused great attention owing to their rich magnetic phase transitions and novel ground-state properties. Their different spin quantum numbers and magnetic structures, such as spin ladder with S=1/2, spin chains with S=1,1/2, produce various quantum ground states and magnetic phase diagrams. Therefore, researches on such low-dimensional magnetic materials at low temperatures and strong magnetic field have great contribution to deep understanding of the quantum phase transitions and fundamental properties.In this paper, a detailed study on the synthesis and the heat transport properties of single crystals (CH3)2NH2CuCl3(MCCL) and Br-doped MCCL has been performed. For comparison, the crystal synthesis, crystal structures and physical properties of two novel organic magnetic materials6[SC(NH2)2]NiBr2and4[SC(NH2)2]NiBr2have also been studied in detail. The details are summarized as follows:The first part of Chapter1is the review on the research progress of low-dimensional magnetic materials. We first briefly introduced the physical properties of low-dimensional magnetic materials and development of relevant theories, and then focused on the discussion of several one-dimensional magnetic structures. We discussed their magnetic phase diagrams and theoretical models especially for the systems with spin gap in their ground states. Finally, the magnetic thermal conductivities of various magnetic structures were carefully compared and discussed. In the second part, the importance and the development history of the crystal growth are summarized, especially for the low-temperature solution growth method, including their growth methods and the factors that affect crystal morphology.In Chapter2, we mainly discussed the synthesis of the MCCL by slow evaporation method. Its crystal structure was characterized by X-ray diffraction and physical properties measurement. The optimal growth conditions have been explored by optimizing various parameters, including the solvents, temperature, and super-saturation, so as to obtain MCCL with high quality and good morphology. Furthermore, the Br-doped (CH3)2NH2CuCl3(1-x)Br3x single crystal with multiple compositions have also been synthesized successfully. It was discovered that the Br-doping did not change the MCCL crystal structure. For T>2K, Br-doped MCCL displays a paramagnetic behavior with the temperature and the specific heat data does not show any long-rang order state down to0.4K. However, at very low temperatures, both the susceptibility and specific heat of MCCL show long-range order.In chapter3, the thermal transport properties of MCCL have been investigated, which show a complex dependence on magnetic field and temperature. Because of the strong phonon scattering caused by strong spin fluctuations in low-dimensional systems, the thermal conductivity in zero field is significantly suppressed. However, the magnetic scattering becomes very weak in the state of spontaneous antiferromagnetic ordering, and is increased with the gradual suppression of the ordering state with increasing magnetic field. Moreover, from the spontaneous antiferromagnetic ordering state to the field-induced state and then to the phase boundaries of spin polarized state, the thermal conductivity shows the diplike features, which is caused by the strong phonon scattering resulting from the critical spin fluctuation. In high field, the thermal conductivity increased significantly for the completely polarization of spin and the weakness of the spin fluctuation.In chapter4, we have studied the crystal growth, crystal structure and properties of a novel organic magnetic material6[SC(NH2)2]NiBr2. This new material possesses a layered monoclinic crystal structure with β=133.674°. At high temperature, the susceptibility is weak anisotropy with temperature at inner-layer; at low temperature, the susceptibility is sharply declined or increased along different directions. Furthermore, according to the measurements of the susceptibility and specific heat, the phase diagram reveals complicated magnetic structure transitions from antiferromagnetic ordering phase to spin flop phase and then to spin polarized state. The thermal conductivity at zero field with decreasing temperature present the phonon peak, diplike behavior, and an approximation to the limits of phonon boundary scattering with the T-dependence of T2.6. The thermal conductivities show a weak dependence on the magnetic field both at high and low temperatures, while the dip is suppressed as the field increases, because the magnetic excitation scattering is significantly weakened. Thermal conductivity presents a minimum at the transition field, which shows the strong scattering of phonons caused by spin fluctuations at the phase transition.In chapter5, we introduced a novel organic magnetic material4[SC(NH2)2]NiBr2prepared by the water solution method. It has the same crystal structure with4[SC(NH2)2]NiCl2(DTN), whose ground state is a disordered spin gap state. Magnetic field induce a phase transition in DTN from disordered to a long-range antiferromagnetic state, which is known a magnon BEC. However, due to the difference of lattice parameters between these two materials, their magnetic exchange interactions are different. As a result,4[SC(NH2)2]NiBr2is found to have a different ground state with long-range AF order.