Fe Content Dependence of Synthetic-Antiferromagnetic Coupling in Subnano-Crystalline FeCoB/Ru/FeCoB Films

Ru thickness (d_Ru) dependence of flopping field (H_f) was investigated in subnano-crystalline (Co_100-x Fe_x)₈₈B₁₂/Ru/ (Co_100-xFe_x)₈₈B₁₂ films with various Fe content, x. The H_f with d_Ru showed oscillatory behavior with the period of 1.5 nm of d_Ru, which was independent of the Fe content. The first peak of H_f takes the maximum value at x=65 at.%, and the second peak was degenerated with increasing the Fe content. These results are analyzed in terms of interlayer coupling effect including the bilinear (J₁) and biquadratic (J₂) coupling energy. As the d_Ru decreases, the J₁ oscillates and J₂ monotonously increases. Although the J₂ effect appeared at thinner d_Ru, i.e., d_Ru<0.7 nm, the J₁ effect was rather stronger than the J₂ one. The magnitude of the first peak of J₁ was greatly enhanced with increasing Fe content, and took the maximum at x=65 at.%. Therefore, a smooth interface was established in subnano-crystalline FeCoB, and the intrinsic strong J₁ was realized in FeCoB/Ru/FeCoB system even in ultrathin d_Ru of 0.3 nm