Anti-Invar properties and magnetic order in fcc Fe–Ni–C alloy


Anti-Invar effect was revealed in the fcc Fe-25.3%Ni-0.73%C (wt%) alloy, which demonstrates high values of thermal expansion coefficient (TEC) (15–21)×10−6 K−1 accompanied by almost temperature-insensitive behavior in temperature range of 122–525 K. Alloying with carbon considerably expanded the low temperature range of anti-Invar behavior in fcc Fe–Ni-based alloy. The Curie temperature of the alloy TC=195 K was determined on measurements of temperature dependences of magnetic susceptibility and saturation magnetization. The Mössbauer and small-angle neutron scattering (SANS) experiments on the fcc Fe-25.3%Ni-(0.73–0.78)%C alloys with the varying temperatures below and above the Curie point and in external magnetic field of 1.5–5 T were conducted. Low value of the Debye temperature ΘD=180 K was estimated using the temperature dependence of the integral intensity of Mössbauer spectra for specified temperature range. The inequality Beff=(0.7–0.9)Bext was obtained in external field Mössbauer measurement that points to antiferromagnetically coupled Fe atoms, which have a tendency to align their spins perpendicular to Bext. Nano length scale magnetic inhomogeneities nearby and far above TC were revealed, which assumed that it is caused by mixed antiferromagnetically and ferromagnetically coupled Fe atom spins. The anti-Invar behavior of Fe–Ni–C alloy is explained in terms of evolution of magnetic order with changing temperature resulting from thermally varied interspin interaction and decreasing stiffness of interatomic bond.
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