Dynamical behaviour of ultrathin [CoFeB (t(CoFeB))/Pd] films with perpendicular magnetic anisotropy

CoFeB-based ultrathin films with perpendicular magnetic anisotropy are promising for different emerging technological applications such as nonvolatile memories with low power consumption and high-speed performance. In this work, the dynamical properties of [CoFeB (t(CoFeB))/Pd (10 Å)](5) multilayered ultrathin films (1 Å ≤ t(CoFeB) ≤ 5 Å) are studied by using two complementary methods: time-resolved magneto-optical Kerr effect and broadband ferromagnetic resonance. The perpendicular magnetization is confirmed for multilayers with t(CoFeB) ≤ 4 Å. The effective perpendicular magnetic anisotropy reaches a clear maximum at t(CoFeB) = 3 Å. Further increase of CoFeB layer thickness reduces the perpendicular magnetic anisotropy and the magnetization became in-plane oriented for t(CoFeB) ≥ 5 Å. This behaviour is explained by considering competing contributions from surface and magnetoelastic anisotropies. It was also found that the effective damping parameter α(eff) decreases with CoFeB layer thickness and for t(CoFeB) = 4 Å reaches a value of ~ 0.019 that is suitable for microwave applications.