Fe content dependence of synthetic-antiferromagnetic coupling in subnano-crystalline FeCoB/Ru/FeCoB films

Ru thickness ([d.sub.Ru]) dependence of flopping field ([H.sub.f]) was investigated in subnano-crystalline [([Co.sub.100-x[Fe.sub.x]).sub.88][B.sub.12]/Ru/ [([Co.sub.100-x[Fe.sub.x]).sub.88][B.sub.12] films with various Fe content, x. The [H.sub.f] with [d.sub.Ru] showed oscillatory behavior with the period of 1.5 nm of [d.sub.Ru], which was independent of the Fe content. The first peak of [H.sub.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 ([J.sub.1]) and biquadratic ([J.sub.2]) coupling energy. As the [d.sub.Ru] decreases, the [J.sub.1] oscillates and [J.sub.2] monotonously increases. Although the [J.sub.2] effect appeared at thinner [d.sub.Ru], i.e., [d.sub.Ru] < 0.7 nm, the [J.sub.1] effect was rather stronger than the [J.sub.2] one. The magnitude of the first peak of [J.sub.1] 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 [J.sub.1] was realized in FeCoB/Ru/FeCoB system even in ultrathin [d.sub.Ru] of 0.3 nm. Index Terms--Interlayer coupling, perpendicular recording medium, soft magnetic underlayer, subnano-crystalline FeCoB.