Magnetic structure of Fe1-xCoxSi in a magnetic field studied via small-angle polarized neutron diffraction

The magnetic structure of Fe1-xCoxSi 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 [100] axes due to the anisotropic exchange, that, however, decreases with increasing Co concentration x. The magnetic structure of Fe1-xCoxSi 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 (2006)] for cubic magnets with Dzyaloshinskii-Moriya (DM) 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.