Coupling between Co and Gd-Co layers separated by nonmagnetic spacers

We have studied the interlayer coupling between Co and Gd-Co layers separated by nonmagnetic spacers in a series of [Co/X/(Gd-Co)/X]4/Co multilayers (X=Cu, Ti and Si). The exchange interaction at the interfaces causes 'macroscopic ferrimagnetic behavior' in the whole structure. The critical field for a spin-orientation transition H* leading to a non-collinear magnetic structure in multilayers gives an estimate of the coupling strength. The critical field decreases with the spacer thickness for all spacer materials. The temperature dependence of the critical field, however, depends strongly on the spacer material. The differences are attributed to the fact that, in contrast to Cu and Ti, using Si can reduce the Co magnetic moment.