Two types of magnetic multilayer structures with large magnetoresistance (MR) have been found. In the first type, such as Fe-Cr superlattices,[1,2] antiferromagnetic interlayer coupling forces the moments of the ferromagnetic layers to be antiparallel in zero applied field. The zero-moment configuration is thus the ground state for the coupled multilayers.
In the second type of ferromagnet-paramagnet- ferromagnet system which have large magnetoresistance, there is no significant interlayer coupling. The zero-field moment is therefore nearly equal to the saturated value. However, the moments of the ferromagnetic films may be rotated into antialignment by a finite applied field if the films have different coercivities. Different coercivities have been obtained in the Co-Au-Co system by growing the two Co films of a sandwich in a different fashion[3] or with different thicknesses,[4] and by exchange-biasing one of the ferromagnetic layers in the NiFe-noble metal-transition metal system.[5] In these uncoupled sandwiches, the role of the paramagnetic interlayer is basically that of a weak link, whose presence allows the moments of the two ferromagnetic layers to be decoupled, and thus separately manipulated. The paramagnetic layer in the uncoupled sandwiches is therefore analogous to the tunnel barrier in a Josephson junction, which permits the phase of the superconducting order parameter to be different on the two sides of the junction. The analogy breaks down in detail because in the Josephson effect the junction conductance is modulated by the cosine of the phase difference between the two superconductors, while in the spin- valve effect it is the resistance of the sandwich which has a cosine dependence on the relative orientation of the moments of the two ferromagnetic layers.[5,6]