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

Abstract 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 T C =195 K was determined on measurements of temperature dependences of magnetic susceptibility and saturation magnetization. The Mossbauer 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 Θ D =180 K was estimated using the temperature dependence of the integral intensity of Mossbauer spectra for specified temperature range. The inequality B eff =(0.7–0.9) B ext was obtained in external field Mossbauer measurement that points to antiferromagnetically coupled Fe atoms, which have a tendency to align their spins perpendicular to B ext . Nano length scale magnetic inhomogeneities nearby and far above T C 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.