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The magnetic structure of Fe 1−x Co x Si single crystals with x = 0.10, 0.15, 0.20, 0.50 has been studied by small angle polarized neutron diffraction and superconducting quantum interference device measurements. Experiments have shown that in zero field the compounds with x = 0.1, 0.15 have a well-defined tendency to order in the one-handed spiral along ͗100͘ axes due to the anisotropic exchange, that, however, decreases with increasing Co concentration x. The magnetic structure of Fe 1−x Co x Si with x = 0.2, 0.5 consists of spiral domains with randomly oriented spiral wave vector k. The applied magnetic field produces a single domain helix oriented along the field. The process of the reorientation starts at the field H C1 . Further increase of the field leads to a magnetic phase transition from a conical to a ferromagnetic state near H C2 . In the critical range near T C the integral intensity of the Bragg reflection shows a well-pronounced minimum at H fl attributed to a k flop of the helix wave vector. On the basis of our experiments we built the H-T phase diagram for each compound. It is shown that the same set of the parameters governs the magnetic properties of these compounds k, H C1 , H fl , and H C2 . Our experimental findings are well interpreted in the framework of a recently developed theory ͓Phys. Rev. B 73, 174402 ͑2006͔͒ for cubic magnets with Dzyaloshinskii-Moriya ͑DM͒ interaction. In particular, the theory suggests an additional quantum term in the magnetic susceptibility caused by the DM interaction which is in good agreement with the experiment.

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Polarized neutron scattering experiments have demonstrated that Dy/Y multilayer structures possess a coherent spin helix with a preferable chirality induced by the magnetic field. The average chirality, being proportional to the difference in the left- and right-handed helix population numbers, is measured as a polarization-dependent asymmetric part of the magnetic neutron scattering. The magnetic field applied in the plane of the sample upon cooling below T(N) is able to repopulate the otherwise equal population numbers for the left- and right-handed helixes. The experimental results strongly indicate that the chirality is caused by Dzyaloshinskii-Moriya interaction due to the lack of the symmetry inversion on the interfaces.