Crystal Growth And Low-temperature Properties Of Spin-ice Materials R₂Ti₂O₇(R = Dy, Ho)

Physics of the frustrated magnetic materials is considered as an important branch in condensed mater physics. The geometric spin frustration, which is caused by the frustration of the geometric symmetry of the crystal structure, is particularly notable. The presence of frustration effect and the incompatible interactions among spins result in the macroscopic ground state degeneracy, showing rich quantum magnetic properties. Pyrochlore structure R₂Ti₂O₇ (R = Dy, Ho) is a typical geometrically frustrated of magnetic materials. Its crystal field effect is so strong that the magnetic ions with Ising anisotropy lie on the tetrahedron vertices only along the crystal axis of [111] direction. The competition among frustration effect, spin antiferromagnetic interaction between the nearest neighbor, and the long-range ferromagnetic interaction, which is originated from the magnetic dipole interaction, result in the spin-ice ground state with a six-fold degenerate and the "two-in-two-out" spin configuration at low temperatures. In addition, the magnetic field in different directions can lift the degeneracy of different levels, showing rich magnetic phase transition behaviors. However, there are still many controversies on the spin-ice ground state and magnetic phase transition behaviors. One of the most attractive progress in this field is the discovery of magnetic monopoles in spin-ice R₂Ti₂O₇, which can explain a lot of exotic phenomena based on the magnetic monopole model. It is still not very clear which method is more perfect between magnetic dipole model and the magnetic monopole model. Recently, thermal conductivity measurement is recognized as an effective method in studying the magnetic materials. It can be not only used to detect the excitations, such as phonon, magnon, but also to study the spin-phonon coupling, magnetic structure and the quantum phase transitions. In this thesis, we explore the methods of the crystal growth of R₂Ti₂O₇ in order to get the higher quality single crystals, and then we carefully study the magnetic susceptibility, specific heat, thermal conductivity and other properties of the spin-ice materials.The first chapter reviews the previous works on the research of spin-ice materials R₂Ti₂O₇ (R = Dy, Ho), including magnetic susceptibility, specific heat, neutron diffraction etc.,which describe the magnetic properties and the magnetic structure, and give a brief description of the magnetic dipole moment model and the model of magnetic monopoles. It can be seen that there are still many problems and disputes on the spin ice materials. The second chapter focuses on the grow conditions of the R₂Ti₂O₇ (R = Dy, Ho) single crystals using the floating-zone method. After many attempts, we succeed in getting some ideal growing conditions, and obtain high-quality single crystals. The low-temperature data of magnetic susceptibility and specific heat verified the spin-ice ground state, which is consistent with the previous works. In addition, we study the low-temperature heat transport behaviors of Dy₂Ti₂O₇ single crystals, which are annealed under different conditions. It is found that the oxygen defects have little effect on the phonon heat transport. The small phonon peak observed in R₂Ti₂O₇ single crystals is mainly due to strong magnetic scattering induced by the magnetic fluctuations of the spin-ice ground state.