Quasi-one-dimensional Spin System Ca₃ (Co, T)₂O₆ (T = Fe, Mn) : Preparation, Structure And Magnetic Properties

In recent years, quasi-one-dimensional spin system Ca₃(Co,T)₂O₆ (T=Co, Rh etc) had attracted much interest. On the one hand, these materials show thermoelectric properties, magnetic capacitance, ferroelectricity, and have potential applications. On the other hand, rich, anomalous magnetic behavior had been observed in these materials, which are considered as one of model materials for studying one-dimensional strongly correlated systems. In this paper, we performed a study on crystal structure and magnetic properties of Fe- and Mn-doped compounds. The main contents and conclusions can be summarized as follows:In Chapter 1, we introduce the background and potential applications of quasi-one-dimensional compounds as well as our purpose of present studies.In Chapter 2, we generally introduce some materials preparation methods and their features. Specially, we chose the sol-gel to prepare samples which have higher solubility limit than that of solid state method. We introduce polycrystalline powder X-ray diffraction theory and data acquisition. Finally, we introduce methods for magnetization measuremnts.In Chapter 3, we studied the crystal structure and magnetic properties of Fe-doped Ca₃Co_2-xFe_xO₆ samples with different compositions. Room temperature x-ray diffraction data show that the solubility limit of Fe in Ca₃Co₂O₆ is about x=0.7, above which the crystal structure changes gradually from the rhombohedral (R-3c) to triclinic (P-1) phase. We performed a systematic study on magnetic properties of Fe-doped compounds within x=0-1.5. We found that for the rhombohedral samples, the inverse magnetic susceptibility follows the Curie-Weiss law. The M(H) curves exhibit a well-defined ferrimagnetic state, which becomes smeared out with increasing temperature or Fe composition. The magnetic properties of the triclinic compounds are quite different. The magnetization curves of x=1.1 characterize the presence of low-temperature antiferromagnetic state. The inverse magnetic susceptibility measured up to 300 K does not follow the Curie-Weiss law, suggesting the presence of an ordering temperature well above 300K or short-range ferromagnetic (FM) correlations.In Chapter 4, we studied the crystal structure and magnetic properties of Ca₃CoMnO₆. X-ray diffraction shows that the structure of the sample is orthorhombic structure. Unusual short-range FM correlations, which can be understood in terms of Griffiths-like singularity, have been illustrated in spin chain compound Ca₃CoMnO₆ by systematic magnetization measurements. Its main characteristics are: First, these FM correlations can be dramatically suppressed by a small stoichiometric mismatch of Co/Mn atoms. Second, these FM correlations develop at T_G=¹25 K, a temperature much higher than the ordering temperature of T_N=¹3 K. Third, these FM correlations survive in magnetic fields of over 2 T. This feature is quite different from the general case. Griffiths-like anomaly was usually observed in very low magnetic fields and in many cases was suppressed in a field of several kilo-oersted.