Magnetic Properties Of Low-Dimensional Transition Metal Oxides

Low-dimensional transition metal oxides have shown rich characteristics,including ferroelectric coupling,superconductivity,colossal magnetoresistance and quantum disorder et al.These remarkable properties are mostly due to the complex interactions between electron spin,lattice,charge and orbital degrees of freedom in materials.The competition mechanisms of them usually lead to complex physical properties.Particularly,the quantum spin liquid,which is from highly entangled between spins,is a new type of quantum disordered state.Discovery of solid quantum spin liquid material experimentally has become a frontier research in condensed matter physics.Co²⁺-based honeycomb lattice materials（such as A₃Co₂SbO₆and A₂Co₂Te O₆（where A is an alkali metal））are an important platform to investigate the Kitaev quantum spin liquids behaviors.Compared with 4d（Ru）or 5d（Ir）systems,3d electrons are more localized and can suppress the random interactions outside the nearest neighbor.In addition,the extra electrons can introduce lots of interaction channels,which can effectively suppress the non-Kitaev terms in the Hamiltonian.In this dissertation,we study the Li₃Co₂SbO₆polycrystalline samples,with polymorphic;one is a monoclinic structure,and the other is an oblique structure.We successfully sythetized the orthorhombic phase and honeycomb phase Li₃Co₂SbO₆polycrystallines,and obtained some experimental results as listed below:At first,we revealed the magnetic properties of orthorhombic Li₃Co₂SbO₆by using DC and AC susceptibility,specific heat,muon spin relaxation（μSR）and neutron diffraction measurements.Continuous magnetic transitions were observed at115 K,89 K and 71 K in low-field DC susceptibility measurements.The transition below T_N（115 K）is suppressed in the higher magnetic field.However,the zero-field ac susceptibility shows that there are several obvious frequency-independent transitions at 114 K,107 K,97 K,79 K and 71 K.The measurements of specific heat,μSR and neutron diffraction confirm the long-term magnetic order state,indicating a single transition around 115 K.The difference between different measurements is due to the fact that ferromagnetic zigzag chains may have stacked faults or local disorders,resulting in ferromagnetic boundaries in the overall antiferromagnetic background.Secondly,we reveal the magnetic properties of the honeycomb phase Li₃Co₂SbO₆by means of X-ray diffraction,DC magnetic susceptibility and specific heat measurement.The study of direct current susceptibility shows that there is a significant ferromagnetic contribution above the antiferromagnetic transition temperature.In the measurement of isothermal magnetization,there is a significant metamagnetic transition at about 0.7 T at 2 K（below T_N）,indicating the ferromagnetic order driven by spin flipping in honeycomb antiferromagnets.The behavior of specific heat is consistent with the expectation under the anisotropic Kitaev model.Fianllly,we also need to prepare the orthogonal phase Li₃Co₂SbO₆single crystal,and further study the local crystal structure and magnetic structure of the system to explain the ultimate reason for the abnormal ferromagnetic behavior observed in the polycrystalline samples.A series of Zn-doped samples with low concentration were prepared.The antiferromagnetic order of Li₃Co₂SbO₆was suppressed to the quantum spin liquid state by the random substitution of non-magnetic ions for Co²⁺to dilute the magnetic lattice.