ISSN:
1090-6509
Source:
Springer Online Journal Archives 1860-2000
Topics:
Physics
Notes:
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 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)〉∝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 ∝ ((T−T c)/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 R c(T c)=R c max . The maximum correlation radius established (R c max =140 Å and 230 Å 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 T c, confirming the character of this magnetic transition as a first-order transition close to a second-order transition.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1134/1.558389
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