Study On The Phase Transformation And Magnetic Properties Of Two Compounds

Colossal magnetoresistive (CMR) materials such as rare-earth manganites is still a hot topic up to now. For one thing, scientists are looking forward to potential applications for their CMR effect; for another, the essence of the CMR is related to the basic issues in strongly correlated physics. The main purpose of this paper is to study in details the possible origin of insulator-metal (I-M) transition and CMR effect found in manganese perovskite. Up to now, there are many different theories and models trying to explain this phenomenon but no one can provide a feasible explanation.In this paper, the electrical and magnetic characters of manganese perovskite have been briefly summarized. Some theories and models introduced here also include phenomenon model based on double exchange theory, charge ordering model, spin-polarons model and 2-phase phenomenon model based on phase separation.The I-M transition temperature T_c,h in different magnetic fields has been obtained in the paper. The content about percolation includes the concept of percolation, critical percolation exponent and percolation probability about random resistor network model. The method of constructing model and calculating model resistance is presented here in details. The simulated result of the system through percolation theory is also discussed. Clearly, with the different parameters, just like the size of network and the generating method of random matrix, the model will give different result. Based on the analysis on experiment data, it is proposed that there is possibility of kinetic PS near T_c in manganese perovskite oxide. Thus, a random resistor network consisted of PMI and FMM is generated through Monte Carlo method. The model qualitatively analyses the main character of manganese perovskite which is PMI at high temperature, FMM at low temperature and insulator transiting to metal near Curie temperature. The model also simulates electrical transport behavior of sample when applied magnetic field and the result shows that I-M transition moves to high temperature and resistance near T_c falls sharply because of field. It is found there is linear dependence of the critical temperature on the magnetic field, provided the magnetic field is not too strong. The theoretical predications coincide well with the result of experiments.In addition, low-dimensional magnetism in organic polymers or molecules has attracted considerable interest from scientists since the discovery of ferromagnetism in organic polymer, such as poly-BIPO, m-PDPC, p-NPNN. The polymer chain, m-PDPC, is described by the Kondo-Hubbard Hamiltonian, which exhibits the ferromagnetic order by means of the mean-field theory and the quantum Monte Carlo simulation. In the present paper, we choose poly-BIPO and describe it as a simplified quasi-one- dimensional Ising-Heisenberg zig-zag spin chain model, and then we apply the quantum transfer-matrix method to study the specific heat of which. From the calculated results, we can find that, in the absence of the exchange interactions between side radicals and the main chain, the curves of specific heat show a round peak due to the antiferromagnetic excitations for the all antiferromagnetic interactions along the polymer chain, and considering the exchange interactions between the side radicals and the main chain, the curves of the specific heat show double-peak structure for ferromagnetic interactions between the radicals and main chain, indicating a competition between ferromagnetic and antiferromagnetic interactions and the possibility of the occurrence of the stable ferrimagnetic state along the polymer chain.