A THERMAL SPIKE MODEL FOR NANOPHASE FORMATION IN YTTRIUM-IRON-GARNET UNDER SWIFT HEAVY-ION BEAMS

Amorphous latent tracks can be produced in the magnetic insulators, li ke yttrium iron garnet (Y3Fe5O12 or YIG), by irradiations with swift h eavy ions in the electronic slowing down regime. For this, the electro nic stopping power must be higher than a threshold, around 4.0 keV nm( -1) in the case of YIG, to obtain the amorphization of the material, w hen the irradiation proceeds with heavy ions in the 0.8-6 MeV amu(-1) range. In the same energy range, we find that a nanophase can be forme d in YIG by track overlapping. This occurs at fluences depending on th e electronic stopping power, and corresponding to the random impacts o f the heavy ions on the amorphous tracks. Yet, no subsequent amorphiza tion of the nanophase is observed even at high fluences. These results are interpreted with a model based on the thermal spike induced by th e high density of ionizations and electronic excitations in the materi al.