FMR study of interlayer exchange coupling in FeCoB|Ta|FeCoB trilayers with in-plane anisotropy

Abstract

In this work, we present a detailed ferromagnetic resonance (FMR) study of two FeCoB layers coupled across a Ta spacer. The structures of studied samples are FM1/Ta(d)/FM2 where FM1 is a magnetic layer composed of Fe/FeCoB, FM2 is a magnetic layer composed of FeCoB/NiFe, d is the thickness of the Ta layer in nm, and d is varied from 0.375 nm to 4 nm. The FeCoB within FM1 is strongly coupled to high saturation magnetization Fe, and the FeCoB in FM2 is strongly coupled to lower saturation magnetization NiFe in order to separate the FMR resonance positions of these two layers. This is required to determine the strength of interlayer exchange coupling (J) between FM1 and FM2. We solved a system of coupled Landau Lifshitz Gilbert equations, representing the coupled magnetic layers FM1 and FM2, and used it to fit the FMR data and determine J, magnetic anisotropy, Gilbert damping, and g-factor of each magnetic layer as a function of Ta spacer layer thickness and annealing temperature. This study reveals that the dependence of the coupling strength on the Ta thickness is the same for non-annealed samples and for those annealed at 200 °C: the coupling drops to 0 above approximately 0.475 nm and increases rapidly below 0.45 nm. For samples annealed at 300 °C coupling begins to increase below approximately 0.7 nm. It is found that the coupling between FM1 and FM2 for non-annealed samples goes to zero for a Ta thickness two times less than the spin diffusion length.