Spin Fluctuations and Concentration Magnetic Transitions in Chiral Helical Ferromagnets Fe1 – xCoxSi

Abstract

The fluctuation theory is applied to the study of concentration transformations in Fe1 – xCoxSi disordered quasi-binary chiral helical ferromagnetic alloys with the Dzyaloshinsky–Moriya interaction. The ground state is described on the basis of the LDA + U + SO approximations used in ab initio calculations, with additional allowance for concentration fluctuations associated with the difference in the potentials of the intraatomic Hubbard interaction at sites occupied by iron and cobalt atoms. The solutions of the obtained magnetic equations of state for the long- and short-range ordered phases with right and left magnetic chirality are considered. The concentration dependences of the parameters of the intermodal interaction and the area of compositions, in which the first-order magnetic phase transitions induced by thermal fluctuations are accompanied by the occurrence of spin helix fluctuations, are studied. It is shown that the transition with a change in the sign of magnetic chirality is accompanied by the appearance of a minimum on the concentration dependence of the mode–mode parameter and the emergence of quantum helical ferromagnetism with noticeable intensification of zero spin fluctuations.