Fig. 8-5 Temperature dependence of the ac susceptibility of an Fe-Ni Invar alloy irradiated with 2.7 GeV U ions As indicated by an arrow, the susceptibility signal shifts to the high-temperature side with increasing ion fluence, indicating an increase in Curie temperature.

Fig. 8-6 Fluence dependence of the Curie temperature for irradiation with 2.7 GeV U ions and 3.5 GeV Xe ions It is found that 2.7 GeV U-ion irradiation increases the Curie temperature 30 times more efficiently than 3.5 GeV Xe-ion irradiation.

When solid materials are irradiated with ions accelerated to GeV energies, lattice defects created densely along the ion paths and change material properties significantly. This phenomenon is caused by the transfer of a very high density of energy (several tens of keV/nm) from the ion to the target material. Up to now, ion irradiation has been strongly believed to "damage" the target material and as a result to "degrade" it. Therefore, most of our research themes have been to study the detailed behavior of the degradation or to develop a technique to control the irradiation-induced damage. On the other hand, the ion irradiation has also been recognized as a potential candidate for a material-processing technique, because high-energy ion irradiation enables us to spatially control the modified region and to control the physical properties in a well-defined way. In this study, an iron-nickel (Fe-Ni) Invar alloy was chosen as an irradiation target. This is because this alloy has magnetic properties sensitive to crystal structure and also to pressure and compositional change. This fact made us think that ion irradiation can cause certain magnetic property changes for this alloy. As a result of irradiation experiments, we have found that irradiation with GeV uranium (U) or xenon (Xe) ions results in a shift of the Curie temperature (paramagnetic-ferromagnetic transition temperature) towards higher temperatures. In other words, we found that ion irradiation can surely be a new means of changing the magnetic properties of the material (Fig. 8-5). In this case, ion-irradiation does not induce "degradation," but instead introduces materials qualitatively different from the matrix materials. Although irradiation probably creates lattice defects in the alloy, it is a rare example of a case where irradiation does not cause degradation, but causes a magnetic property change such as a change in the Curie temperature. The irradiation-induced magnetic property change is probably due to local changes in lattice spacing or local compositional change in the ratio of Fe to Ni concentration. Furthermore, as demonstrated in Fig. 8-6, it has been established through irradiation experiments that magnetic properties such as the Curie temperature can be controlled by varying species, energy, and fluence of the irradiating ions.

Reference A. Iwase et al., Anomalous Shift of Curie Temperature in Iron-Nickel Invar Alloys by High-Energy Heavy Ion Irradiation, Nucl. Instrum. Methods Phys. Res., Sect. B, 209, 323 (2003).

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