A magnetic phase transition in carbon-doped (0.1 and 0.7 at. %) Fe70Ni30 Invar alloys was investigated by the method of depolarization of a transmitted neutron beam and by small-angle scattering of polarized neutrons. It is shown that for both alloys, two characteristic length scales of magnetic correlations coexist above T-c. Small-angle scattering by critical correlations with radius R-c is described well by the Ornstein-Zernike (OZ) expression. The longer-scale (second) correlations, whose size can be estimated from depolarization data, are not described by the OZ expression, and hypothetically can be modeled by a squared OZ expression, which in coordinate space corresponds to the relation [M(r)M(0)]proportional to exp(-r/R-d), where R-d is the correlation length of the second scale. The temperature dependence of the correlation radius R-c was obtained: R-c proportional to((T-T-c)/T-c)(-nu), where nu approximate to 2/3 is the critical exponent for ferromagnets, over a wide temperature range up to T-c(exp), at which the correlation radius becomes constant and equals its maximum value R-c(T-c)=R-c(max). The maximum correlation radius established (R-c(max)=140 Angstrom and 230 Angstrom for the first and second alloys, respectively) characterizes the length-scale of the fluctuation for which the appearance of critical correlations first results in the formation of a ferromagnetic phase, and the phenomenon itself exhibits a "disruption" of the second-order phase transition at T=(exp)(c), as a result of which a first-order transition arises. Temperature hysteresis was also detected in the measured polarization of the transmitted beam and intensity of small-angle neutron scattering in the alloy above T-c, confirming the character of this magnetic transition as a first-order transition close to a second-order transition. (C) 1997 American Institute of Physics. [S1063-7761 (97)015 12-6].
