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The magnetic ordering of highly perfect single-crystal multilayer films of alternate layers of magnetic Dy and nonmagnetic Y prepared by molecular beam epitaxy has been studied by neutron diffraction. Results on a series of films with Dy thicknesses of approximately 16 atomic planes (≊45 Å) and Y thicknesses ranging from 10 to 22 planes have confirmed the existence of long-range helimagnetic ordering of the Dy 4f spins which is propagated through the intervening Y layers in phase coherence. The propagation vectors in both Dy and Y layers have been calculated from the wave vector of the magnetic satellites and the intensity of the bilayer harmonics. The propagation vector for Dy decreases continuously with temperature, while that in the Y is temperature independent and equal to 0.31 Å−1. The nature of the ordering and the noninteger multiple of π for the phase change of the propagation wave vector across the Y, suggests that the mechanism of long-range coupling is a conduction band spin-density wave in both Y and Dy stabilized by the 4f spins of the Dy. No intrinsic ferromagnetic transition is observed due to the clamping effect of the Y layers on the Dy magnetostriction. The application of a field along basal plane directions destroys the helical order and produces a ferromagnetic state with all spins aligned along the field direction. The moment in the induced ferromagnetic state is 10 μB, while that calculated for the zero-field helical state ranges from 7.5μB to 9.5μB for different samples, suggesting a possible static disorder of some Dy spins superimposed on the intrinsic long-range helical state.

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