Magnetic microstructures of nano-granular CoPt-Al-O thin films studied by Lorentz microscopy and electron holography

Microstructures and magnetic domain structures of nano-granular CoPt-Al-O thin films are investigated by Lorentz microscopy and electron holography. It is found that the CoPt-based nanocrystalline phases are basically surrounded by the Al₂O₃-based amorphous phases and the area of the Al₂O₃-based amorphous phases increases with the increase in oxygen content. Typical mazy domains and domain walls like fishbone are observed in the Co_45.8Pt_18.6Al_17.3O_18.3 and in localized area of the Co_38.7Pt_15.6Al_16.3O_29.4, respectively, while no magnetic domain walls are observed in the Co_34.4Pt_13.5Al_15.6O_36.5. With the increase in magnetic field, domain walls like fishbone start to disappear and the magnetic flux density increases in the Co_38.7Pt_15.6Al_16.3O_29.4, indicating the mixed-phase state of ferromagnetic and superparamagnetic phases. In the Co_34.4Pt_13.5Al_15.6O_36.5 of superparamagnetic state, the fluctuated lines of magnetic flux are considered to reflect the randomly magnetized distribution of CoPt-based single domain particles.