Investigation Of Rare-Earth Oxides With Pyrochlore Structure And Their Low-Temperature Physical Properties

Spin-frustrated systems are one of the hot topics in condensed matter physics,due to their rich magnetism and quantum phase transitions.One of the frustration is the geometrical frustration,the magnetic interaction in the system cannot be satisfied at the same time in the state of minimum energy,resulting in the macroscopic degeneracy and spin fluctuation of the ground state.A typical class of three-dimensional spin-frustrated material is pyrochlore,which have many novel magnetic ground state at low temperature,such as spin liquid and spin ice.Chemical doping and element substitution in these materials can lead to the competition and transformation among various quantum states,resulting in some peculiar magnetism.In this thesis,we grow TR2AlSbO7(R=Er,Yb)and Tb2Ti2-xZrxO7 with pyrochlore structure and measure the low-temperature magnetism and heat transport properties.We can study the crystal field effect and the nature of ground state and provide more experimental basis to better understand the ground state characteristics of pyrochlore.We also study the low temperature heat transport properties of 1D Ising chain material Ca3Co2O6.In chapter one,firstly,we introduce the geometrical frustration in pyrochlore structure and the main progress of R2B2O7(B=Ge,Ti,Sn).Secondly,the theoretical research about the ground state of XY type pyrochlore and experimental progress about the disorder effect are presented.Finally,the ground state properties and research progress of Tb2Ti2O7 are introduced.In chapter two,we report a study of crystal growth and characterizations of a new rare-earth compound Er2AlSbO7,in which Al3+and Sb5+ions share the same positions with a random distribution.The magnetism is studied by magnetic susceptibility,specific heat,and thermal conductivity measurements at low temperatures down to several tens of millikelvin.Different from the other reported Er-based pyrochlores exhibiting distinct magnetically ordered states,a spin-freezing transition is detected in Er2AlSbO7 below 0.37 K,which is primarily ascribed to the inherent structural disorder.A cluster spin-glass state is proposed in view of the frequency dependence of the peak position in the ac susceptibility.In addition,the temperature and field dependence of thermal conductivity indicates rather strong spin fluctuations which is probably due to the phase competition.In chapter three,we study the magnetism and specific heat of Yb2AlSbO7 single crystal.Dc susceptibility indicates the antiferromagnetic interaction,which is different from Yb2B2O7.Specific heat shows that Yb2AlSbO7 does not have any magnetic order transition at low temperatures down to 0.4 K.More experimental investigations are needed to confirm the ground state.Chapter four reports the specific heat and heat transport of Tb2Ti2-xZrxO7 single crystals at low temperature.We quantitatively analyze the specific heat data and find that the energy level is not uniformly distributed,which might be caused by the disorder introduced by doping.The thermal conductivity shows no phonon peak and extremely low value at low temperature,which could be due to strong scattering of phonons by hybridization excitation.The dependence of thermal conductivity on magnetic fields exhibits complex behavior,which requires further theoretical and experimental studies.Chapter five introduces the low temperature heat transport of Ising material Ca3Co2O6.The zero-field heat transport shows a large phonon peak and can be well fitted by using the classical Deby model.Moreover,the low-T?(H)isotherms with H//c display a field-independent behavior.These results indicate that there is no contribution of magnetic excitations to the thermal conductivity in Ca3Co2O6 neither carrying heat nor scattering phonons,which can be attributed to the Ising-like spin anisotropy.