Interlayer coupling in Co/Si multilayers (abstract)

Abstract

Although the picture of antiferromagnetic (AF) coupling in metal/metal multilayers is well-established, it is not clear how the picture can be extensively modified for coupling across insulating or semiconducting spacer layers. Unusual temperature sensitive interlayer coupling has recently been observed in Fe/Si/Fe sandwiches and Fe/Si multilayers.123 The spacer has been claimed to be a metastable metallic FeSi with CsCl structure in the Fe/Si multilayers,4 and a transition from AF coupling to biquadratic coupling has been observed with decreasing temperature in Fe/FeSi multilayers.56 In this study, interlayer coupling has been investigated as a function of nominal Si layer thickness tSi in Co/Si multilayers. The multilayers were grown at ambient temperature on water cooled thermally oxidized Si substrates by ion beam sputtering at a base pressure of 3×10−7 Torr. The sputtering was conducted using 1.8×10−4 Torr Ar ions and an acceleration voltage of 400 V. Co was shown to have hcp structure with (002) texture by x-ray and transmission electron diffractions. From the magnetization curve, the coupling was ferromagnetic below tSi=0.8 nm, changed to antiferromagnetic above 0.8 nm Si and disappeared at tSi>1.7 nm, exhibiting nonoscillation. We have estimated interlayer coupling J between Co layers across the spacer from the saturation field Hs using the relation of Hs=4J/dMs, where d is the Co layer thickness and Ms is the saturation magnetization. The interlayer coupling was also investigated using FMR for Co/Si/Co sandwiches, which demonstrates an exponentially decay of the coupling with increasing tSi, and gives AF coupling of J=−3.6×10−2 erg/cm2 for a 10 nm Co/1.5 nm Si/10 nm Co sandwich. The cross-sectional transmission electron microscopy exhibited the spacer consisting of Co silicide with amorphous nature, formed in the interface. Temperature dependence of the coupling was investigated using FMR and compared with that of Fe/Si multilayers. Substrate dependence of the interlayer coupling was also investigated.