SUPERPARAMAGNETIC BEHAVIOR OF FE-NI ALLOYS AT LOW NI CONCENTRATION

Mossbauer spectroscopic method has been used to determine the magnetic behavior of Fe-Ni fine particles (approximate to 200 Angstrom) in the concentration range 10 to 35 at% Ni. The Mossbauer spectra of the all oys having 20, 22.5 and 25 at% Ni show the existence of a broad centra l line (which can be fitted with a Lorentzian doublet) along with a se xtet whereas the spectrum of Fe72.5Ni27.5 alloy shows only a broad cen tral line (which can also be fitted with a Lorentzian doublet). The ce ntral line in each of the above cases disappears for large particles, suggesting that this line originates from the superparamagnetic behavi or of the ferromagnetic fine particles. Mossbauer spectroscopic studie s reveal that ferromagnetic order appears in fine particles (approxima te to 200 Angstrom) of Fe70Ni30 and Fe65Ni35 alloys. The experimental results suggest that the anisotropy energy changes with the change of Ni concentration in these alloys.