Annealing effect and interlayer modulation on magnetic damping of CoFeB/interlayer/Pt thin films

The annealing effect and interlayer (IL) modulation on the magnetic damping properties of CoFeB/IL/Pt (IL = Ta or Ru) multilayers have been systematically studied by the time-resolved magneto-optical Kerr effect. It is found that after inserting a thin IL, the saturated magnetic damping factor α_s drops considerably due to the reduced spin pumping effect. By fitting the curves of α_s versus IL thickness, spin diffusion lengths of Ta and Ru are determined to be 3.07 and 3.59 nm, respectively. Interestingly, for the CoFeB samples with different capping layers of Pt, Ta/Pt, or Ru/Pt, the α_s values exhibit an identical non-monotonic variation tendency as annealing temperature (T_a) increases. It first rises to a maximum at T_a ∼ 100 °C and then decreases, reaching a minimum value (α_s = 0.021-0.011) at T_a ∼ 300 °C. After that, α_s starts to increase again. Such a complicated variation behavior is attributed to two-magnon scattering contribution, which originates from the change in the CoFeB surface roughness and interfacial atomic intermixing induced by thermal annealing. This study provides a deep understanding and effective control of magnetic damping for practical design of high performance spintronic devices.