Investigation of extrinsic damping caused by magnetic dead layer in Ta-CoFeB-MgO multilayers with perpendicular anisotropy

We systematically investigated the influence of the dead layer, the oxidation degree of naturally oxidized MgO, the structure of adjacent nonmagnetic metal layers on the damping parameter, and the perpendicular anisotropy of Ta(Ru)/Co₂₀Fe₆₀B₂₀/MgO and MgO/Co₂₀Fe₆₀B₂₀/Ta films using the vector network analyzer ferromagnetic resonance measurement technique. MgO/Co₂₀Fe₆₀B₂₀/Ta film shows almost twice larger extrinsic damping than that for Ta/Co₂₀Fe₆₀B₂₀/MgO film, whereas the perpendicular anisotropy is much smaller. This two-fold enhancement of damping parameter is successfully explained by the extrinsic damping arises from the dead layer at the CoFeB-Ta interface through the Elliott-Yafet mechanism in addition to the conventional spin-pumping model. Furthermore, we found that the oxidation degree of naturally oxidized MgO has no significant impact on the damping parameter, while the perpendicular anisotropy for MgO/Co₂₀Fe₆₀B₂₀/Ta film is enhanced by longer oxidation time.