The magnetic phase transition in the iron-nickel fcc alloy (Formula presented) 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 (Formula presented) 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 (Formula presented) the larger-scale correlations with characteristic length (Formula presented) cannot be described by the Ornstein-Zernike expression and its shape could be roughly modeled by the “squared Ornstein-Zernike” formula: (Formula presented).