Single crystal artificial superlattices prepared by molecular beam epitaxy and consisting of Dy, Er, and Gd layers alternated with Y all exhibit long-range magnetic order that is propagated through intervening non-magnetic Y layers as shown by neutron diffraction studies. In the case of Dy systems, the phase and chirality of the helical order are preserved from one Dy layer to another. The phase shift across the Y layer is proportional to the Y layer thickness, and so is in general a non-integral multiple of π, suggesting that the coupling is propagated by a spin-density-wave mechanism. The magnetic coherence range, obtained from a series of samples of constant Dy thickness and varying number of Y layers, shows a linear variation with the inverse of the Y layer thickness.