Metal magnetic/nonmagnetic ͑Dy/Y͒ multilayer structures posses a coherent spin helix propagating through many Dy/Y bilayers. Samples of different Dy and Y layer thicknesses were investigated using polarized neutron scattering to determine the average chirality of the magnetic structure. The in-plane applied magnetic field induces the chirality since it couples to the uncompensated moments of the helix in a Dy layer. The value of the chirality has a complex behavior in dependence on the temperature and the magnetic field. It is shown that the chirality is affected by the competition between the Ruderman-Kittel-Kasuya-Yoshida interactions of the intralayer exchange within the Dy layers and the interlayer coupling across the nonmagnetic Y layers. The set of the experimental data allows one to make the reconstruction and schematic visualization of the spin structure within the multilayer system. In spite of a complex dependency of the average chirality on the temperature and the magnetic field, the twist angle between the spin planes of two neighboring Y/Dy interfaces has the same sign for all samples at any conditions. It is antisymmetric Dzyaloshinsky-Moriya interaction that reveals itself through the sign of this twist angle and as a result through the nonzero average chirality of the magnetic multilayer structure.

Measurements of the spin excitations propagating normal to the interfaces in Dy/Y superlattices using neutron inelastic scattering are presented. For a given magnon momentum, a neutron-scattering spectrum shows multiple peaks at different energies, which indicates discrete energy spectra. The results are compared with theoretical calculations developed here to describe magnetic excitations in rare-earth superlattices. The theory accounts for Ruderman-Kittel-Kasuya-Yosida ͑RKKY͒ and Dzyaloshinsky-Moriya interactions in incommensurate helicoidal structures and achieves a quantitative agreement with the experimental data. This work demonstrates that neutron inelastic scattering can be used for systematic studies of the exchange interactions and spin dynamics in nanomagnetic systems over wide areas of the Brillouin zone.

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