Interlayer Coupling and High-Frequency Performance in Magnetic Anisotropic FeCoB/Hf/FeCoB Trilayers with Various Hf Thicknesses

Abstract

FeCoB (25 nm)/Hf(tHf)/FeCoB (25 nm) sandwich films with different hafnium thicknesses tHf were fabricated using a modified compositional gradient sputtering method to obtain self-biased high-frequency performances. The effects of tHf on the interlayer coupling and FMR frequency were investigated. It is revealed that interlayer coupling enhanced the resonance frequency by 48%, and a ferromagnetic coupling between the FeCoB films occurred for the trilayers with tHf < 3.0 nm, likely due to the interface roughness and pinhole effect. In this case, only acoustic mode resonance was observed with the same high-frequency performance as the corresponding FeCoB single layer. In contrast, a tHf-dependent antiferromagnetic interlayer coupling appeared at tHf > 3.0 nm. The coupling coefficient J1 was antiferromagnetic, and a biquadratic coupling J2 appeared at tHf > 3.5 nm. The coupling mechanism was simulated and verified by Layadi’s rigid model, and the simulation was consistent with the experimental results.