Anomalous shift of Curie temperature in iron-nickel Invar alloys by high-energy heavy ion irradiation

Fe _0.68Ni _0.32 Invar alloys are irradiated with 3.54 GeV Xe ions, and with 2.71 GeV U ions at room temperature. Measurements of the magnetic moment of specimens under AC magnetic field before and after irradiation show that the Curie temperature, T_c, for the irradiated region increases with increasing the ion fluence. This phenomenon appears even at low ion fluence, implying that it is attributed to ion-induced high-density electronic excitation. It is well known that T_c of such Fe-Ni Invar alloys strongly depends on the Ni concentration and the external pressure. With increasing the Ni concentration from ∼30%, T_c increases, and with decreasing the lattice parameter by the external pressure, T_c decreases. Therefore, the increase in T_c by high-energy ion irradiation can be explained as originating from the lattice expansion and/or the composition change, which are induced by the high-density electronic excitation.