Amorphization and recrystallization of yttrium iron garnet under swift heavy ion beams

Abstract

The room-temperature dc conductivity is used to monitor the damage and structural modifications induced by swift heavy ion irradiations in yttrium iron garnet (Y3Fe5O12 or YIG) epitaxial layers doped with calcium (CaYIG) or silicon (SiYIG), with a variable conductivity due to a variable degree of compensation, and amorphous YIG layers. Irradiations are performed with heavy ions in the 0.8–6 MeV amu−1 energy range, in the electronic slowing down regime, with an electronic stopping power ranging between 7 and 41 MeV μm−1 above the amorphous track formation threshold (4.5 MeV μm−1) in this low-ion velocity range. A conductivity decrease versus ion fluence is found in the case of the high-conductivity uncompensated epilayers whereas an increase occurs for the low-conductivity compensated ones, either p-type (CaYIG) or n-type (SiYIG). These results are discussed by considering the competing effects of disorder on the carrier density and mobility in the case of compensated and uncompensated semiconductors. In both cases, the low-fluence data display a plateau at around the same conductivity value corresponding to the amorphous YIG above an amorphous fraction around 50% regardless of the ions. All the high-fluence data exhibit a power-law behavior without saturation, above a threshold fluence decreasing with increasing amorphization cross section (A). These results are interpreted by the formation of amorphous tracks and of a more conducting nanophase after recrystallization of the tracks under ion impacts. All the data are rescaled versus the product of A times fluence (φ) where amorphization dominates for Aφ⩽1, whereas recrystallization dominates for Aφ>10. However, significantly larger A values than the ones previously determined from the RBS-channeling data are derived from a mean-field analysis of the low-fluence conductivity data with a 2D Bruggeman model. These deviations are ascribed to a contribution of the crystalline track halos where internal stresses are accumulated due to the atomic density difference between the crystal and amorphous phase. A simple phenomenological approach of the amorphization and recrystallization processes is proposed on the basis of two kinetic rate equations with a recrystallization cross section (S) at least one order of magnitude smaller than A. These S values are in agreement with a thermal spike model assuming vaporization of the amorphous YIG phase along the ion path. At such high temperatures in the ion tracks, the garnet phase may decompose into a more conducting nanocrystalline phase. Finally, an exp(−T)−1/4 law for the thermal dependence of conductivity at low temperature is found in the nanophase like in the amorphous one, most probably because of the strong contribution of the disordered grain boundary cores in the conduction process.