Magnetic Damping Constant of CoFeB/Pt Thin Films With Varying the Thicknesses of Pt and Insertion Layer of Al

The effect of the thicknesses of Pt and the insertion of Al layer on the effective magnetic damping constant of CoFeB/Pt thin films caused by the spin-pumping effect has been studied by an angular-dependent ferromagnetic resonance method. In CoFeB/Pt bilayers, a roughly linear dependence of the effective damping constant on Pt thickness (tPt) was observed before saturated at t_Pt = 2 nm. While in CoFeB/Al/Pt trilayers, a sharp drop of damping constant was observed while Al thickness is around 0.5 nm, then the damping constant nearly remains when Al thickness further increases to 5 nm. It indicates a decreased spin pumping efficiency due to Al insertion between CoFeB and Pt layers. The decreased spin pumping efficiency was ascribed to the spin memory loss at CoFeB/Al and/or Al/Pt interfaces rather than the spin backflow effect. The results also suggest that the linearly increased damping constant in CoFeB/Pt layers mainly comes from the magnetic proximity effect (MPE) of Pt, which could be eliminated by insertion a thin layer of Al. However, in order to obtain a large-spin current injection efficiency and less MPE to improve the spin-pumping effect in CoFeB/Pt, a proper insertion layer rather than Al with small-spin memory loss need to be investigated.