Composition and Annealing Temperature Dependence of Magnetic Properties in MgO/Co–Fe–B/Ta/Co–Fe–B/MgO Films

We study the dependence of anisotropy and damping in MgO/Co-Fe-B/Ta/Co-Fe-B/MgO films on annealing temperature and Co-Fe-B composition by vector network analyzer ferromagnetic resonance. The effective areal magnetic anisotropy K_eff · t_eff and Gilbert damping α in samples with Co₄₀Fe₄₀B₂₀ were found to be constant for 240 °C and 300 °C annealing. As the annealing temperature is increased to 350 °C, both K_eff · t_eff and α decrease. Increasing the Fe content in the Co-Fe-B layer to 60% increases its saturation magnetization, but the resulting increase in demagnetization energy is counteracted by the improvement in interfacial anisotropy. Fe-rich layers also exhibit lower α, although the difference in α is not observed for effective Co-Fe-B thickness less than 1.5 nm-the thickness regime where the largest increase in K_eff · t_eff occurs.