Damage induced in garnets by heavy ion irradiations: a study by optical spectroscopies

The damage induced by heavy-ion irradiation has been studied in yttrium iron garnet (Y₃Fe₅O₁₂ or YIG) films, doped with Ca, Tb and Tm, grown by liquid-phase epitaxy on gadolinium gallium garnet (Gd₃Ga₅O₁₂ or GGG) substrates. Irradiations of doped-YIG epitaxial films and GGG substrates with 36-MeV ¹⁸³W and 12-MeV ¹⁹⁷Au ions were applied for fluences between 1 x 10¹³ and 3 x 10¹⁵ cm^-2 near room temperature. The radiation damage was monitored by micro-Raman spectroscopy and UV-visible optical absorption spectroscopy. Raman spectra revealed that amorphization was achieved in YIG for both ions, whereas a high lattice disorder was induced in GGG without reaching amorphization for the Au ion irradiation. Raman spectra also showed that a major damage of the tetrahedral sites was induced in GGG, as previously found for YIG. It is concluded that with such ions reaching the stopping power threshold of track formation in YIG and GGG the observed rate of amorphization may result from a combination of electronic and nuclear energy losses as calculated using the unified thermal spike model. (authors)