Study On Magnetic Properties Of Low-Dimensional Cobalt Oxides

It's generally believed that the low-dimensional antiferromagnetic fluctuation is the key ingredient in the formation of Cooper pairs in the copper oxide high temperature superconductors, heavy Fermi superconductors, some organic superconductors and the recently discovered iron-based superconductors. The study of the physical properties of the low-dimensional magnetic systems provides an excellent research platform to understand the micron-mechanism of the superconductivity. So it's always an important direction of the condensed mater physics since the discovery of the high temperature superconductors in 1986. Besides, in the low-dimensional magnetic systems, novel phenomena, which are the consequence of electron correlations have often been found, such as spin gap behavior, Spin-Peierls transition, Bose-Einstein condensation and so on. On the other hand, for the low-dimensional (in particular, quasi-one-dimensional) magnetic systems, there is realistic possibility that experimental results can be successfully analyzed through theoretical models.Compared with the other magnetic materials, the cobalt oxides always have rich properties, because the cobalt ions can be probably distributed into various valence states and spin configurations. In this dissertation, we have synthesized the quasi-one-dimensional BaCoO3, Ba5Co5ClO13, and Sr6Co5O15-x single crystals successfully and studied the transport properties and magnetic properties systematically. We have also studied the substitution effect by partially substitution of Ru for Co in the two-dimensional NaxCoO2 system. The main conclusions are arranged as follows:(i) We have successfully synthesized the BaCoO3 single crystal for the first time and determined the structure. The resistivity displays a typical semiconductor behavior in the whole measuring temperature range. At high temperatures, the resistivity is found to follow the thermal excited model, which is contrast with the Anderson localization as previously reported. Through study of the magnetic properties, a large anisotropy is found for the external field parallel or perpendicular to the c axis. Around 15K, the compound undergoes an antiferromagnetic transition, which is consistent with the neutron results.(ⅱ) By improving the method, the single crystals of Ba5Co5ClO13 with high quality were synthesized successfully. In this compound, a large anisotropy of the magnetic susceptibility and resistivity is observed. Forχc, it exhibits an antiferromagnetic peak at TN～108K; forχab, it's more flat at high temperatures, and displays an upturn transition below TN. The large anisotropy of the magnetic susceptibility is consistent with the competition of the FM intra-blocks coupling and AFM inter-blocks coupling, where the "block" is composed by the three face-sharing CoO6 octahedra and the CoO4 tetrahedra in the two ends. The temperature dependence of the resistivity displays a hump in pab and a kink in pc around 108K, suggesting the strong coupling between the transport and magnetic properties. Above and below the transition, the transport properties in ab plane follow the 3D VRH mechanism.(ⅲ) The quasi-one-dimensional single crystal samples of (Sr1-xCax)-Co-O (x= 0.0,0.079,0.246) were synthesized successfully. For the Ca free sample, the transport property undergoes a transition around 100K. Below 100K, the resistivity follows the 1D VRH mechanism. Along the c axis, a large thermoelectric power is detected above 200K. For magnetic field parallel to the c axis, the susceptibility exhibits an broad peak around 27K, and a typical spin gap behavior was observed at low temperatures. For the Ca doped sample with x=0.079, the physical properties are very similar with the Ca free sample. At low temperatures, the spin gap behavior is almost unchanged. With the Ca doping, the modification of the Co-O chain has been detected in the x= 0.246 sample. For the sample with x=0.246, a stronger localized behavior is observed. Unfortunately, the partially substitution of Ca for Sr doesn't improve the thermoelectric properties. At low temperatures, a ferromagnetic correlation is detected along the c axis, instead of the spin gap behavior.(iv) In the Na0.75Co1-xRuxO2 system, we find that the y structure can be retained even up to x=0.5. The cell parameters increase with increasing Ru content. With the Ru doping, a metal-insulator transition is observed around x=0.02. For x> 0.02, an Anderson localization is induced in the CoO2 plane. The study of the magnetic properties indicates that there is a maximum in the value of magnetic susceptibility, and the effect paramagnetic moment with increasing Ru content.