| Summary: | 博士 === 國立臺灣大學 === 物理研究所 === 103 === Being one of the most studied materials, magnetite, Fe3O4, has inspired scientists and researchers for a very long time -- a long running mystery. It is also a potential material for the applications in magnetic storage, magnetic anisotropy and spin injection. A special phase transition (so called Verwey transition) in crystal structure, electric transport, magnetization and heat capacitance was observed at ~123K and the origin of this phenomenon is still under debates. Toward to the success of fabricating spintronic devices, the growth and characterization of Fe3O4 thin film is essential and extremely important.
In our present work, a Molecule-Beam-Epitaxy is adopted to grow high quality Fe3O4 films with various thicknesses and the growth condition is monitored by a real-time Reflex High Energy Electron Diffraction. After the confirmation of the sample quality, the best samples are chosen for the ferromagnetic resonance (FMR) studies. Temperature dependent FMR spectra and magnetocrystalline anisotropy are studied at various temperatures. Experimental results show that the transitions of lattice vibration, electrical conductivity and magnetocrystalline anisotropy all occur at different temperatures. Therefore, the origin of Verwey transition is suggested to be a Jahn-Teller type distortion. Furthermore, a lower transition temperature of magnetocrystalline in thinner film is attributed to a competition between Jahn-Teller effect and strain. This study provides a better understanding on the magnetocrystalline transition as well as the influence of the strain effect for Fe3O4 film.
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