Austenitic stainless steels are widely used in fast-reactor cores; they represent metastable solid solutions. They work at high fast-neutron fluxes and elevated temperatures, which accelerate the decomposition substantially [I]. The decomposition produces segregations of carbide, nitride, intermetallide, and other phases [2], which largely determine the properties [3]. Electron microscopy is widely applied to irradiated materials but does not always enable one to identify these segregations. When the decomposition products are ferromagnetic, there are certain advantages in magnetic methods. Interest attaches to the magnetic parameters for stainless steels irradiated to high fast-neutron fluences because they may be used in fusion reactors.