Reversal of interlayer exchange coupling through post-annealing process accompanying asymmetric crystallization in Ta/Co2Fe6B2/MgO/Co2Fe6B2/ MgO heterostructures

Abstract

The effect of the post-annealing process accompanying asymmetric crystallization on interlayer exchange coupling in the Ta/Co2Fe6B2/MgO/Co2Fe6B2/MgO heterostructure is systematically investigated using ferromagnetic resonance. To induce structural asymmetry, the bottom Co2Fe6B2 layer is in contact with a Ta buffer, which acts as a boron absorber, while the top Co2Fe6B2 layer is encapsulated between two MgO layers. Structural analysis and magnetization measurements confirm that the selective crystallization of the bottom Co2Fe6B2 layer is well established upon a thermal annealing process, while the top Co2Fe6B2 layer is remained as a disordered state. The ferromagnetic resonance spectra reveal distinct coupling regimes depending on the annealing state. In as-grown samples, an antiferromagnetic (AFM)-like coupling is observed for MgO thicknesses, tMgO above 2.0 nm. In particular, the post-annealing process can trigger a sign reversal of the interlayer exchange coupling to a ferromagnetic (FM)-like state in the thick spacer regime (tMgO ≥2.6 nm).