From a comparison of the thermal expansion and magnetization behavior of uhv evaporated fcc FeNi films with the respective bulk we will show that neither the deviation of the moment from the Slater-Pauling curve nor the weak magnetic behavior (small spin wave stiffness) are relevant for the occurence of the Invar effect. Reason is that in the freshly evaporated films the premartensitic transition is prevented, which in the bulk (and 900K annealed films) leads to non propagating longitudinal spin fluctuations, not sensed in inelastic neutron scattering (“hidden excitation”) but contributing in a Bloch like form (T3/2 dependence) to the magnetization.
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