Magnetic phase transition in disordered Fe-Ni alloys studied by means of small-angle neutron scattering and three-dimensional analysis of the neutrondepolarization - art. no. 094426

The magnetic phase transition in iron-nickel fcc alloys Fe70Ni30 doped by c arbon (0.7% at.) is investigated using the three-dimensional analysis of th e neutron depolarization (ND) and small-angle neutron scattering (SANS). Th e coexistence and the growth of two different scale magnetic correlations a re 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> a re 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 correlation s 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 variatio ns (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) lo cally ordered ferromagnetic regions appear in paramagnetic phase and form a large percolative cluster. In this case the connectivity length of the ord ered regions dominates at the correlation length of thermal fluctuations. T herefore 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.