Investigation of a magnetic phase transition in fcc iron-nickel alloys

Abstract

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 Tc. Small-angle scattering by critical correlations with radius Rc 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)〉∝exp(−r/Rd), where Rd is the correlation length of the second scale. The temperature dependence of the correlation radius Rc was obtained: Rc ∝ ((T−Tc)/Tc)−ν, where ν≈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 Rc(Tc)=R c max . The maximum correlation radius established (R c max =140 A and 230 A 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=T c exp , 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 Tc, confirming the character of this magnetic transition as a first-order transition close to a second-order transition.