Microstructure and Magnetic Properties of FePt/MgO Multilayered Thin Films
FePt alloys with the CuAuI L10-ordered structure are important magnetic materials since their large uniaxial magnetocrystalline anisotropy can overcome the thermal stability problem of high-density magnetic recording media. In this study, transmission electron microscopy (TEM) and energy-filtered TEM (EFTEM) are employed to characterize the microstructure of a series of FePt/MgO multilayered thin films with different FePt layer thicknesses. The as-deposited FePt layers transform into the L10-ordered phase after annealing at 700oC for 30 minutes. The laminated FePt/MgO structure effectively preserves a strong L10  texture normal to the thin film plane during annealing. The annealed films exhibit large coercivities from 6.5 kOe to 11.5 kOe due to the formation of the L10-ordered phase. A reduction in coercivity with increasing FePt layer thickness is attributed to the differences in the microstructure after annealing. For the films with thinner FePt layers, annealing produces discontinuous FePt layers that reduce exchange coupling and increase coercivity. As the FePt layer thickness increases, the magnetic layers become more continuous after annealing and thereby the exchange coupled layers have lower coercivity.