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The saturation magnetization and the hyperfine magnetic field of different f.c.c. Fe-Ni based alloys containing nearby 29 at .% Ni were studied as a function of temperature and for different Carbon and Manganese contents. We have observed abnormal behaviors that are explained in terms of mixed exchange interactions between atomic spins: J NiNi (r i ) < 0, J FeFe (r i ) > 0, J NiFe (r i ) < 0.

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F.C.C. Fe-30.3%Ni and Fe-30.5%Ni-1.5%C (wt.%) alloys were studied by means of Mössbauer spectroscopy in external magnetic field B ext =2.5, 5, 7 T parallel to the gamma-beam. It is shown that distribution of effective magnetic field in the alloys is broad and that carbon expands the range of B eff . The external magnetic field increases B eff in the Fe-Ni alloy and decreases it more evidently in the Fe-Ni-C alloy. Antiferromagnetic spin coupling along the ferromagnetic component is proposed to explain data.

78)%C alloys in the varying temperature range 150-293 K and in external magnetic field of 1,5 -5 T were conducted. Low value of the Debye temperature  D = 180 K was estimated using the temperature dependence of the integral intensity of Mössbauer spectra for specified temperature range. The inequality B eff = (0.7÷0.9)B ext was obtained in Mössbauer measurement applying external magnetic field 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 supposedly 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.