Controllable Interfacial Coupling Effects on the Magnetic Dynamic Properties of Perpendicular [Co/Ni]₅/Cu/TbCo Composite Thin Films

Dynamic magnetic properties in perpendicularly exchange-coupled [Co/Ni]₅/Cu (t_Cu = 0-2 nm)/TbCo structures show strong dependences on the interfacial antiferromagnetic strength J_ex, which is controlled by the Cu interlayer thickness. The precession frequency f and effective damping constant α_eff of a [Co/Ni]₅ multilayer differ distinctly for parallel (P) and antiparallel (AP) magnetization orientation states. For samples with a thin t_Cu, f of the AP state is apparently higher, whereas α_eff is lower than that in the P state, owing to the unidirectional exchange bias effect (H_EB) from the TbCo layer. The differences in f and α_eff between the two states gradually decrease with increasing t_Cu. By using a uniform precession model including an additional H_EB term, the field-dependent frequency curves can be well-fitted, and the fitted H_EB value is in good agreement with the experimental data. Moreover, the saturation damping constant α₀ displays a nearly linear correlation with J_ex. It decreases significantly with J_ex and eventually approaches a constant value of 0.027 at t_Cu = 2 nm where J_ex vanishes. These results provide a better understanding and effective control of magnetization dynamics in exchange-coupled composite structures for spintronic applications.