Magnetic diffuse scattering of neutrons from Fe-Ni invar alloy

Neutron diffuse scattering around the (111) Bragg peak from a single crystal of Fe-Ni alloy with 35 atomic percent nickel concentration has been measured. Magnetic contributions are extracted from the diffuse scattering by varying temperature from room temperature up to 450°C. The intensities of the scattering are best fitted with gaussian distribution in a form exp [-0.693( q /κ) 2 ], where q is a distance from (111) point in the reciprocal space. The inverse correlation range κ as a function of temperature is determined; κ is nearly constant with the value κ=0.11A -1 below T c and gradually decreases with temperature above T c down to the value κ=0.07A -1 at 100 degrees above T c . A model which takes into account a fluctuation of the magnetic correlation range is proposed and discussed for the invar alloy.