Study On The Exotic Magnetic Property Of Correlated-electron System

The free electron theory of metal and band theory of solids are based on the single-electron approximation. In solid state physics, atom is composed of the valence electrons and ion. When we study the electron motions, the Born-Oppenheimer approximation is an important assumption that the electronic motion and ion motion in molecules are independent of each other. In the theory of free electron, ion can be regarded as background with positive charge to maintain charge neutral. While in the single-electron approximation of band theory, the effect of ion is summed up as the periodic potential and the other electrons move in the mean field. The effects of electron-electron interaction are included in the exchange-correlated term by various methods, such as Hartree-Fork theory and Density-functional theory. As the correlated interaction becomes stronger, the single-electron approximation will be invalidated. Especially for the d-electron transition metal oxides, the correlated interaction among the electrons is so strong that the novel properties can not be well explained by the traditional condensed matter theories.The correlated interaction among electrons induces a series of important physical phenomena, such as novel magnetic properties, giant magnetoresistance, superconductivity, and so on. In the correlated system, there also exist the correlated interaction among spins, between spins and orbits, and between spins and lattices. Such correlated interaction is the physical origin of novel magnetic properties in solids. The magnetic effect comes from the correlated interaction among electrons, rather than the effect of quantum relativism.In this thesis, we explored some typical kinds of materials with correlated interaction and studied their synthesis processes, lattice structure, magnetic properties and electronic properties. We also offered our perspective on their novel and abundant magnetic phenomena.(1)In most manganites, Mn3+ ion is in High-spin state with three 3d electrons in t2g orbits and the last in the eg orbits. Some interesting and important properties are concerned with the two eg orbits under the strong electron correlation. Our experimental and theoretical study find that rhombohedral Li(Mn,Cr)O2 displays abnormal spin state with four 3d electrons occupying all three t2g orbits, S=1. Furthermore, a magnetic transition from paramagnetism to paramagnetism is found in high-temperature region, which shows changes of magnetic moment. Spin-orbit coupling plays an important role in this transition. The materials shows spin-glass-like behavior in low-temperature region, which should comes from the effect of geometrical frustration and abnormal t2g properties.(2)Rhombohedral Li0.8V0.8O2 was synthesized by hydrothermal method, and the influence of alkalinity on the hydrothermal treatment is also studied. The measurement of dc/ac susceptibility shows paramagentism in the whole temperature region between 2 K and 300 K. Typical semiconducting behaviors of resistivity is found, which can be explained by the variable-range-hopping mechanism. In Li0.8V0.8O2, Li+ and V4+ ion are disorderly distributed in the 3a and 3b sites. Disorder can induce the localization of electron state. Model of Anderson weak localization is employed to explain the low-temperature paramagnetism and semiconducting behavior.(3)Low-temperature quantum magnets have attracted considerable attentions because of their rich and interesting quantum magnetic phenomena, which have no analogy in high dimensions due to the enhanced quantum fluctuations in the one-dimensional (1D) structure. We systemically study the effect of hole-doping on the structure, valence state and magnetic properties of one-dimensional spin chain material LiCuVO4. XPS results showed that the valence state of V and O ion maintain unchanged before and after hole- doping. But the valence of Cu ion changes and partly +3 Cu ion appear, which is in correspondence with results of magnetic moment calculation. Part nonmagnetic [Cu3+O4] units suppress the magnetic susceptibility and induce new magnetic ordering at low temperature.(4)Despite the same chemical formula with that of LiCuVO4, LiNiVO4 has a totally different lattice structure, because of the John-Teller effect of Cu2+ ion. LiNiVO4 is of the inverse spinel structure with space group Fd 3 m, whose Li+ and Ni2+ ion disorderly occupy the space of octahedron, and V5+ ion is in the space of tetrahedron. The chemical formula can also be represented as V(LiNi)O4. There exists spin-orbit coupling in the high-temperature region, and LiNiVO4 displays short-range antiferromagnetic order.