Giant magnetoresistance and enhanced antiferromagnetic coupling in highly oriented Co/Cu (111) superlattices

We demonstrate the presence of large oscillatory antiferromagnetic (AF) interlayer coupling and ``giant'' magnetoresistance in crystalline (111)-oriented Co/Cu superlattices grown on Pt buffer layers on (0001) sapphire substrates. The AF coupling strength is {}4--5 times larger than previously reported for sputtered polycrystalline Co/Cu multilayers. However, a significant fraction of the superlattice ({}70%) remains ferromagnetically coupled. This may account for the comparatively low giant magnetoresistance values observed of {}40% at 3.5 K and {}26% at 295 K compared to values of >120% at 4.2 K and >65% at 295 K, respectively in sputtered structures. Similar crystalline (111) Co/Cu structures grown on Cu buffer layers on GaAs(110) show no evidence for antiferromagnetic coupling, although they are of nearly comparable structural perfection. The presence or absence of antiferromagnetic coupling in (111) Co/Cu superlattices appears to be a result of subtle structural imperfections giving rise to direct ferromagnetic coupling of neighboring Co layers.