Influence of Cu Underlayer on the High-Frequency Magnetic Properties of FeCoSiO Thin Films

A series of soft magnetic FeCoSiO films was fabricated on Si (100) substrates using Cu as the underlayer by radio frequency magnetron sputtering. The influence of Cu underlayers on the dynamic magnetic properties of FeCoSiO thin films was studied in detail. The results show that the Cu underlayers have a significant influence on the high-frequency properties and the damping factor of as-sputtered FeCoSiO films, while the static magnetic properties of sputtered films do not demonstrate obvious improvement. In the absence of Cu underlayers, the optimal films with the desired properties of low coercivity, H_c ~ 2.5 Oe; high saturation magnetization, 4πMs ~ 11.6 kGs; and high natural ferromagnetic resonant frequency, f_r ~ 4.05 GHz, were obtained. With the increase of the underlayer´s thickness, the complex permeability of FeCoSiO films has a significant change. The damping factor, which can be determined by curve fitting, based on the theoretical equation changed from 0.04 to 0.19 and reaches its maximum value when the Cu thickness is 2 nm. The increasing extent of damping factor is much larger than that in the previous reports. The reason is suggested to be related to change the film´s stress, surface, and interface roughness by the underlayer. It shows that using a Cu underlayer is an effective way for tuning the high-frequency properties and damping factor of soft magnetic thin films. It also implies that the effect of underlayer must be considered when the magnetic films are used in the gigahertz band.