Crystal Growth And Low-Temperature Physical Properties Of Magnetically Frustrated RMnO₃and Dy₂Ti₂O₇

Geometrically frustrated magnetic systems, in which the magnetic interactions can not be satisfied simultaneously due to the frustrated lattice, have attracted many interests of scientists in condensed matter physics, because of their exotic ground-state properties and quantum-phase-transition behaviors. Moreover, such systems are closely related to some hot issues in condensed matter physics, such as high-Tc superconductivity and multiferroics. Therefore, the study of this field is of great significance. This thesis focuses on two kinds of geometrically frustrated magnetic materials. The crystal growth and low-temperature physical properties of hexagonal multiferroic materials RMnO3(R=Y, Lu, Ho, Er, Tm) and spin-ice material Dy2Ti2O7are carefully studied. This thesis is consists of three chapters and the main contents of each chapter are as follows:In chapter one, we introduce the main progresses on frustrated magnetic systems. First, the fantastic properties of R2Ti2O7systems are summarized. Second, the research progresses of spin-ice material Dy2Ti2O7are presented. Third, the research progresses of hexagonal multiferroic materials RMnO3(R=Y, Lu, Ho, Er, Tm) are introduced. Finally, the research progresses of these two systems by using the heat transport technique are also briefly introduced.In chapter two, we introduce study the crystal growth of the hexagonal rare-earth manganites RMnO3(R=Y, Lu, Ho, Tm and Er) by using an optical floating-zone method. High-quality single crystals and the optimized growth conditions are successfully obtained. It is found that the deficiency of the rare earth elements can make the melton zone unstable during the growth of TmMnO3and ErMnO3. This can be effectively resolved by appropriately increasing the ratio of the rare-earth oxide in the feed rod. Laue photograph and X-ray diffraction measurements demonstrate the good crystallinity of these crystals. The magnetic susceptibility, specific heat, resistance, dielectric constant and heat transport measurements are also carried out on these crystals to study the low temperature physical properties.In chapter three, we mainly study the low-temperature heat transport of Dy2Ti2O7single crystals. The thermal conductivities in zero field indicate a pure phonon heat transport behavior. The κ(H) isotherms in [111] magnetic field show clear anomalies at the field-induced magnetic transitions and the data above8T also show field dependence. These phenomena can not be understood with a single magnetic-monopole mechanism and the non-Ising terms magnetic excitations should also be considered in Dy2Ti2O7system. Moreover, the κ(H) isotherms show clear irreversibilities when the magnetic field along [111],[100] and [110] directions. The irreversibility in [111] magnetic fields surprisinglyhas no correspondence with the well-known magnetization hysteresis and a pinning effect of magnetic monopoles in spin-ice compound by the weak disorders is proposed to explain this phenomenon. The irreversibilities in [110] and [100] magnetic fields coincide with the magnetization hysteresis, which is due to the very slow spin dynamics at very low temperatures in Dy2Ti207system.