Study On The Stability Of Organic Ferromagnet

Functional organic polymers have become one of recent research focuses because of their ample physical properties, being flexible and easy on manufacture as a kind of special polymers. During last thirty years, many kinds of functional organic polymers have been demonstrated in the laboratory, such as conductive polymer, whose electrical conductivity being larger than copper and strength exceeding steel, electro-optical devices, and organic ferromagnets.The traditional ferromagnets are mainly made by transitional metals or lanthanon elements, since the atoms of these materials have unpaired electrons in d-or f- orbitals and unfilled stable electronic shells to supply stable magnetic moments. Recently these traditional ferromagnets are limited in application since they cannot satisfy many special requirements because of their high density and hardness of manufacture.The research on the theoretical and experimental studies of the organic ferromagnets is one of the new directions in the macromolecular polymers, in that, as the organic functional material, they have very special electrical, magnetic and optical properties comparing with traditional metals and semiconductors. Moreover, they can be used to further construct polymers with special structures with different kinds of synthesize methods.It is very interesting topic to find effective methods to synthesize the organic polymers into stable magnetic materials, and study their properties. The research on the origin and mechanisms of the organic solid magnetisms is one of the challenges because, in one hand, the C, H, O and N elements have only s- or p- electrons but not d- or f- electrons (the latter is the reason for the magnetism), and on the other hand, the electronic shells of these elements are closed in general and exhibit diamagnetism. Ovchinnikov and Spector have found that the π-conjugated polymer with side radicals will exhibit fenprnagnetism and they have synthesized the ferromagnetic polymer poly-BIPO.In 1970's, Su, Schrieffer and Heeger constructed the SSH model, using the semi-classical tight-binding method to study the electronic structures and optical properties of one conjugated polymer, polyacetylene, and their success opened a new era on the research of organic polymer especially the quasi-one-dimensional polymers. Later, Xie, Yao, Fang and Liu studied the properties of poly-BIPO based on the SSH model considering the electron-electron correlation and electron-phonon interaction in one-dimensional polymers. They found stable magnetic state in one-dimensional polymers. These kinds of studies not only enrich the knowledge on the microscopic structure of the organic polymers, but also have much application value. They also impelled the investigation and development of the new type organic polymer devices.In this thesis, we reformed the Hamiltonian in the SSH model, and investigated the ground state properties of poly-BIPO polymer considering the electron-lattice, electron-electron interaction and Heisenberg spin antiferromagnetic coupling. The main contents and the basic results are showed as following:The stability of the ground state of organic ferromagnet, especially the magnetic stability, is very important in the synthesis and application of such materials. Using the numerical methods, we investigated the effects of the antiferromagnetic coupling, electron-phonon interaction, electron-electron interaction, size of polymers on the stability of the quasi-one-dimensional organic ferromagnets. Under the periodic boundary conditions, we find:1. The generation of the spin density wave in the main chain can be ascribed to the anti-ferromagnetic coupling of π electron and unpaired electrons of side radical. And the coupling strength will affect the amplitude of the spin density wave, but not the distribution of spin density wave. The spin density wave(SDW) will increase along the improvements of ferromagnetic coupling J, and order parameter of SDW m_s →1 when J →∞.2. The electron-phonon coupling will improve the localization of the spin density wave, so as to suppress the excitation of SDW.3. The effect on the spin density wave coming from the two terms of electron-electron interaction in extended Hubbard models will be different: on-site electron-electron Coulomb interaction will improve the form of spin density wave, and the site-site electrons interaction will suppress the form of the spin density wave.4. The small size of the one-dimensional organic ferromagnetic polymers will affect the properties of the systems. When the sites is larger than 30, the effect can be neglected.We also discussed the effects of disturbance with square and Gaussian random distribution on the stability of the quasi-one-dimensional organic ferromagnets. When the disturbance is present, the order parameter of the spin density wave will change correspondingly and the rules of variation is that, the order parameters will first improve along with the improvement of the perturbation to a maximum value, and then descent. This phenomena can be explained by the results of dimerization.