Magnetic phase transition in disordered Fe-Ni alloys studied by means of small-angle neutron scattering and three-dimensional analysis of the neutron depolarization

The magnetic phase transition in iron-nickel fcc alloys Fe70Ni30 doped by carbon (0.7% at.) is investigated using the three-dimensional analysis of the neutron depolarization (ND) and small-angle neutron scattering (SANS). The coexistence and the growth of two different scale magnetic correlations are observed in a paramagnetic phase as temperature closes to T-C. The usual critical fluctuations of a Lorentzian shape with size R-0<200 <Angstrom> are found by the analysis of SANS intensity I(q). The ND analysis shows also presence of the large scale correlations (about 10(3)-10(4) Angstrom) With the "squared" Lorentzian shape. We attribute these large scale correlations to local variations of the Curie temperature T-C. The local TC variations are described by the disorder parameters of the system: <T-C>, a spread of T-C variations DeltaT(C), and characteristic size of the local areas R-0, where T-C variations occur. The ratio between the depth of the T-C variations (DeltaT(C)/<T-C>) and its characteristic correlation length Ro la (a is a lattice constant) determines a scenario of the transition: "percolative" or "homogeneous" ones. At R-0/a much greater than(DeltaT(C)/<T-C>)(-2/3) locally ordered ferromagnetic regions appear in paramagnetic phase and form a large percolative cluster. In this case the connectivity length of the ordered regions dominates at the correlation length of thermal fluctuations. Therefore this transition goes in accordance with the "percolative" scenario. All three parameters of the transition are obtained from the data of the three dimension analysis of the ND. From the temperature dependence of the magnetization, we found <T-C> = 397 +/-0.5 K and DeltaT(C) = 4.55 +/-0.05 K. From ND data we estimated the temperature independent characteristic size R-0 of the T-C variations. It is equal to 10(4) Angstrom.