Effect of Oxygen Stoichiometry on Microstructural and Magnetic Properties of FePt/TaO $_{\rm x}$ Bilayer Fabricated by Ion-Beam-Bombardment Deposition

The effect of TaO_x capping layer on microstructures and magnetic properties of FePt thin films via annealing was studied. The structural ordering of FePt from face-centered cubic to face-centered tetragonal phases depends strongly on the oxygen contents in the capping TaO_x layer. The role of the TaO_x layer is used to separate the FePt grains, as revealed by TEM. The annealed FePt/TaO_x (15%O₂/Ar) exhibited the largest out-of-plane coercivity (~2.4 kOe) amongst all samples, compared to that (H_c ~ 4.2 kOe) in the reference FePt layer. At low oxygen content in FePt/TaO_x bilayers, the Ta atoms may act as defects to obstruct the FePt ordering whereas at high oxygen contents, the excess oxygen atoms diffuse into the FePt layer and react with Fe to form iron oxides which give rise to the low coercivities, as characterized by the XPS depth profile and binding energy analysis.