The role of the nature of magnetic coupling on the martensitic transformation in Fe-Ni

Fe100-xNix with x less than or equal to 30 at.%, known as anti-Invar alloys, undergo martensitic transformations. Total energy calculations have shown that these alloys incorporate a strongly fluctuating magnetic moment in the fee state. In order to understand the relationship between the moment fluctuations and the martensitic transformation, we have investigated the magnetic moment in the paramagnetic fee state of these alloys by polarized neutron diffraction. We find that the magnetic correlations are ferromagnetic in nature although, the ground state of these alloys are expected to be antiferromagnetic. We further observe that the magnetic moment of the fee state at M-8 increases with increasing Ni concentration, indicating that the fee state is stabilized by increasing ferromagnetic exchange. The results are found to be in agreement with thermodynamical predictions.