INFLUENCE OF NEUTRON IRRADIATION ON THE TEMPERATURE DEPENDENCE OF MAGNETIC SUSCEPTIBILITY IN Fe - Ni INVAR ALLOYS WITH VARYING NICKEL CONTENT

This study systematically examines the effects of neutron irradiation and alloy composition on the magnetic behavior of Fe - Ni Invar alloys. Three compositions Fe - 22 % Ni, Fe - 30 % Ni (classical Invar composition), and Fe - 50 % Ni were selected to elucidate the interplay between chemical disorder, irradiation-induced modifications, and magnetic properties. The samples were irradiated using a 100keV pulsed neutron source at a dose of 1015 neutrons cm-2. Magnetic characterization was carried out using a vibrating sample magnetometer (VSM) in conjunction with temperature-dependent magnetic susceptibility measurements performed at different orientations between the neutron beam direction and the applied alternating magnetic field. The results demonstrate a pronounced dependence of magnetic behavior on both alloy composition and irradiation conditions. For the Fe - 30 % Ni alloy, the Curie temperature was determined to be approximately 315K. Following neutron irradiation, a significant extension of magnetic ordering was observed, with locally ferromagnetic regions persisting up to approximately 460K. This behavior is attributed to irradiation-induced defects that locally modify the electronic structure, enhance 3d electron localization, and increase effective exchange interactions. The magnetic susceptibility also exhibits a clear angular dependence, indicating anisotropic defect distribution. Complementary structural characterization using X-ray diffraction (XRD) and scanning electron microscopy (SEM) reveals the coexistence of ordered and disordered phases and a heterogeneous microstructure, which correlate strongly with the observed magnetic properties. These findings provide evidence of defect-mediated ferromagnetism and highlight the potential of controlled irradiation for tailoring magnetic properties in Fe - Ni alloys.