Magnetic structure of Dy-Y superlattices

Two samples of Dy-Y superlattices produced by molecular-beam-epitaxy techniques are shown by neutron diffraction to order magnetically in a helix which is incommensurate with the bilayer thickness. One sample consists of 64 bilayers, each bilayer made up of about 15 growth planes (42 Å) of Dy atoms followed by 14 planes (38 Å) of Y atoms. The second sample has 90 layers, each layer consisting of 9 Dy atomic planes and 8 ${\mathrm{Dy}}_{0.5}$ ${\mathrm{Y}}_{0.5}$ alloy planes. The phase coherence of this ordering extends over several bilayers, and is especially striking in the sample where the layers of localized Dy spins are separated by 14 atomic planes of nonmagnetic Y. The fact that the helix chirality propagates across several bilayers rules out a simple scalar Ruderman-Kittel-Kasuya-Yosida coupling between the Dy planes on either side of an Y layer, but suggests instead that a helical spin density wave is induced in the Y conduction electrons. A simple model for the superlattice structure factor demonstrates that observed asymmetries in the magnetic diffraction-peak intensities can be ascribed to the existence of different magnetic modulation wave vectors in each layer type (Dy and Y). In these superlattices the strain clamping by the intervening non-Dy layers and the substrate suppresses the first order ferromagnetic transition found in bulk Dy in both zero and finite fields. Although the planar magnetostriction is clamped, it is observed that the application of a magnetic field in the basal plane produces at low temperatures a second order irreversible transition to a metastable ferromagnetic state. At high temperature the magnetization process is initiated by a reduction of the helical coherence length due to a random-field coupling to the uncompensated Dy layer moment. This allows us to estimate the strength of the interaction through the Y layers.