Swift heavy ion-beam induced amorphization and recrystallization of yttrium iron garnet

Abstract

Pure and (Ca and Si)-substituted yttrium iron garnet (Y3Fe5O12 or YIG) epitaxial layers and amorphous films on gadolinium gallium garnet (Gd3Ga5O12, or GGG) single crystal substrates were irradiated by 50 MeV 32Si and 50 MeV (or 60 MeV) 63Cu ions for electronic stopping powers larger than the threshold value (~4 MeV μm−1) for amorphous track formation in YIG crystals. Conductivity data of crystalline samples in a broad ion fluence range (1011–1016 cm−2) are modeled with a set of rate equations corresponding to the amorphization and recrystallization induced in ion tracks by electronic excitations. The data for amorphous layers confirm that a recrystallization process takes place above ~1014 cm−2. Cross sections for both processes deduced from this analysis are discussed in comparison to previous determinations with reference to the inelastic thermal-spike model of track formation. Micro-Raman spectroscopy was also used to follow the related structural modifications. Raman spectra show the progressive vanishing and randomization of crystal phonon modes in relation to the ion-induced damage. For crystalline samples irradiated at high fluences (⩾1014 cm−2), only two prominent broad bands remain like for amorphous films, thereby reflecting the phonon density of states of the disordered solid, regardless of samples and irradiation conditions. The main band peaked at ~660 cm−1 is assigned to vibration modes of randomized bonds in tetrahedral (FeO4) units.