Exchange stiffness and damping constants of spin waves in CoFeB films

Exchange stiffness and damping of spin waves in a soft ferromagnet of CoFeB are investigated using a time-resolved pump-probe technique. A pump pulse excites coherent spin waves in CoFeB via optical spin-orbit torque from Pt, and a probe pulse detects the phase of the spin waves via magneto-optic Kerr effect. From the frequency of spin waves, exchange stiffness is determined to be 13 pJ m⁻¹, which is nearly independent of the CoFeB thickness from 6 to 16 nm and the annealing temperature of 400 °C. Damping constant of spin waves is determined from the relaxation of spin waves. Importantly, the spin wave damping increases with decreasing the CoFeB thickness, suggesting the spin pumping effect at the interface between CoFeB and Pt. The thickness dependence of damping becomes stronger after annealing at 400 °C, and the spin wave damping of >0.08 is obtained at a CoFeB thickness of 6.3 nm. Such large damping of spin waves would have an important effect on the operation of the CoFeB-based memory devices.