Critical scattering in the helimagnets Fe1-xCoxSi

The critical scattering in the cubic noncentrosymmetrical helimagnets with Dzyaloshinskii-Moriya interaction Fe1-xCoxSi (x = 0.15, 0.2, and 0.5) has been studied. The samples with small Co concentration (x <= 0.2) are found to give the critical scattering with the q-dependence of the typical Lorentzian shape. The critical index of the inverse correlation length v = 0.48 +/- 0.05 is found for the compound with x = 0.15. The critical scattering above T-C has not been observed for x = 0.5. The critical index of the order parameter for the compounds with x = 0.15 and 0.20 are equal to beta = 0.220 +/- 0.005 and 0.230 +/- 0.007, respectively. The value of beta could not been established for x = 0.50 because of the first order character of the phase transition. In our opinion, the change of the character of the phase transition from the weakly first order to the real first order type with increase of the Co concentration is related to the decrease of the anisotropy and increase of the itinerancy of the compounds under study. (C) 2009 Elsevier B.V. All rights reserved.