Static and dynamical behaviour of magnetically coupled Co/Cu/CoFeB trilayers

Abstract

Engineering the magnetic behaviour of stacks of ferromagnetic and non-magnetic thin films is mainly controlled by the anisotropy of individual layers plus the interlayer exchange coupling constant. Then, Co/Cu/CoFeB trilayers with different Cu layer thicknesses, tCu, were fabricated by sputtering. The effects of tCu on the magnetization reversal processes, the interlayer coupling, FMR frequencies and damping parameters were investigated. The analysis of the hysteresis loops have demonstrated how the two ferromagnetic films can be strongly coupled and behave like a single system for tCu ≤ 3 nm, or like two almost independent or uncoupled layers for tCu ≥ 3 nm. It is shown that the interlayer coupling is at the origin of the observed two FMR resonance modes: the acoustic and the optic modes. Particularly, the resonance frequency of the optic mode is very sensitive to the interlayer coupling constant and high resonance frequencies can be achieved (over 25 GHz) for ferromagnetically coupled Co/CoFeB bilayers with interlayer exchange coupling Jeff = (2.7 ± 0.8) erg/cm2. However, the technological application of such high resonance frequencies of the optic mode can be limited by the increase in the damping parameter mainly due to the spin-pumping effect.