Observation of two length scales of magnetic correlations in the Invar Fe75Ni25 alloy above TC by means of small-angle neutron scattering and neutron depolarization

.The magnetic phase transition in the iron-nickel fee alloy Fe,,Ni, doped by carbon is investigated. Combining neutron depolarization and small-angle neutron scattering analysis, we demonstrate the coexistence of two length scales of magnetic correlations above Te in this intrinsically disordered system. A smaller scale length is attributed to the critical fluctuations. The shape of these fluctuations is well described by the Ornstein-Zernike expression. According to the strong depolarization above Te the larger-scale correlations with characteristic length Rd cannot be described by the Omstein-Zernike expression and its shape could be roughly modeled by the "squared Ornstein-Zernike" formula: [M(r)M(0)]similar to exp( -r/R-d).