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
$_100- xhboxFe_ x$
)
$_88 hboxB_12$
/Ru/ (Co
$_100- x$
Fe
$_ x$
)
$_88$
B
$_12$
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
$(J_1)$
and biquadratic
$(J_2)$
coupling energy. As the
$d_ Ru$
decreases, the
$J_1$
oscillates and
$J_2$
monotonously increases. Although the
$J_2$
effect appeared at thinner
$d_ Ru$
, i.e.,
$d_ Ru ≪ 0.7$
nm, the
$J_1$
effect was rather stronger than the
$J_2$
one. The magnitude of the first peak of
$J_1$
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
$J_1$
was realized in FeCoB/Ru/FeCoB system even in ultrathin
$d_ Ru$
of 0.3 nm.