Both exchange-coupled[1] and uncoupled[2,3] ferromagnet- paramagnet-ferromagnet sandwiches and superlattices display a large negative magnetoresistance (MR) when the moments of adjacent ferromagnetic layers are turned from antiparallel to parallel. The isotropic positive magnetoresistance associated with the antiparallel alignment has been called the spin-valve effect.[3] In superlattices which have an antiferromagnetic interlayer exchange, the antialigned state occurs at zero applied field,[1,4] and is the ground state of the system. In the uncoupled sandwiches, the antialignment occurs for a finite range of applied fields, and is possible either because of an exchange bias pinning of one of the ferromagnetic moments,[3] or because of different coercivities in thick and thin Co layers.[2]
Another method of obtaining different coercive fields in alternate layers of a superlattice is to make those layers of different ferromagnetic metals. Shinjo and Yamamoto have reported results on Co-Cu-NiFe superlattices.[5] The Fe-Cu-Co system is another good choice because of the limited miscibility of the Fe and Co in Cu at low fabrication temperatures, and because the growth of these metals is already well understood.[6] For the present study, polycrystalline Fe, Cu, and Co layers were sequentially electron-beam evaporated from elemental sources onto untreated glass or silicon (100) substrates. Layer thicknesses were monitored during deposition with a quartz-crystal oscillator, and checked via x-ray fluorescence measurements. The background pressure of the vacuum system is about 10-7 torr.