  We studied the interlayer coupling in the quadrilayer consisting of IrMn/CoFe
(bottom layer)/NiFeOx/ CoFe (top layer). The in-plane perpendicular interlayer
coupling is observed between CoFe layers at room temperature. An anisotropy
transition from uniaxial to unidirectional in the perpendicular direction is
observed around Tt = 55 K. The nano-oxide layer NiFeOx shows no distinguishable
ferromagnetic signal in the high-temperature (uniaxial) phase, while a strong
signal appeared in the low-temperature (unidirectional) phase. A possible
two-component scenario, that the nano-oxide layer may contain both amorphous
short-range antiferromagnetic domains and superparamagneitc clusters, is
proposed to explain the phase transition.
