| Summary: | 碩士 === 國立中正大學 === 物理所 === 98 === Fe3O4 and Mn3O4 are the most typical spinel ferrimagnetic oxides. The magnetic coupling between Fe3O4 and Mn3O4 has been studied through superlattices but with relatively smaller length scale of each constituent layer. In this thesis, a Fe3O4/Mn3O4/Fe3O4 trilayered structure on MgO(001), MgO(011) and STO(001) substrates, fabricated by molecular beam epitaxy(MBE), provides a simple system to test the magnetic response to a much larger length scale. These trilayered structure have been also characterized by X-ray diffraction(XRD) and scanning electron microscopy(SEM) for both crystalline quality and surface morphology.
X-ray diffraction along the perpendicular direction reveals that the lattice parameters along the z-direction are off the bulk value -1.3% for Mn3O4 and over 4 % for Fe3O4, respectively indicating that both constituent layers are heavily under strain. This modification of the lattice parameters along the z-axis results from the in-plane matching which is due to probably similar oxygen sublattices. The in-plane matching is further confirmed by a model calculation and suggests a strong coupling between two constituent layers.
The remanence magnetization vs. temperature (Mr-T) shows a magnetic compensation point at ~22K. Although this observation indicates antiparallel coupling at the interface and consistent with the previous superlattice results, this is unexpected for the present configuration. The compensation state is thus attributed to a domain wall structure formed in Fe3O4 along the perpendicular direction due to the strong anisotropic energy of the tetragonal Mn3O4 relative to cubic Fe3O4. Magnetic hysteresis measurements at various temperatures support this domain wall configuration and also map out an H-T diagram which includes a low-field compensation and a high-field spin flop phase.
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