Origin of ferromagnetism in ZnO/CoFe multilayers: Diluted magnetic semiconductor or clustering effect?

Epitaxial growth of (0001) oriented [ZnO(20 Å)/Co _0.7Fe_0.30(x Å)]₂₅ multilayers (MLs) with nominal thickness x=1, 2 and 5 has been prepared on α-Al₂O ₃ (0001) substrate by ion-beam sputtering. The magnetic properties over a temperature range of 6-350 K and structures probing by x-ray absorption spectroscopy (XAS) are reported. Above room-temperature ferromagnetism has been observed for x=1 and x=2 MLs, while superparamagnetic behavior dominates for x=5 ML. The field-cooled magnetization-temperature M(T) curves of x=1 and x=5 MLs can be fitted by a standard three-dimensional (3D) spin-wave and a Curie-Weiss model, respectively. For x=2 ML, however, neither a 3D spin-wave nor a Curie-Weiss model, but a combination of the two fits the M-T curve. The XAS studies together with the magnetic measurements further reveal that x= 1 sample behaves as a diluted magnetic semiconductor (DMS) ML, while x=2 ML shows a mixed structure consisting of a minor component of DMS and a major component of CoFe clusters. A predominant clustering phase appears for x=5 ML.