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

The room-temperature dc conductivity is used to monitor the damage and stru ctural modifications induced by swift heavy ion irradiations in yttrium iro n garnet (Y3Fe5O12 or YIG) epitaxial layers doped with calcium (CaYIG) or s ilicon (SiYIG), with a variable conductivity due to a variable degree of co mpensation, 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 mu m(-1) above the amorphous track formation threshold (4.5 MeV mu m(-1)) in t his low-ion velocity range. A conductivity decrease versus ion fluence is f ound in the case of the high-conductivity uncompensated epilayers whereas a n increase occurs for the low-conductivity compensated ones, either p-type (CaYIG) or n-type (SiYIG). These results are discussed by considering the c ompeting effects of disorder on the carrier density and mobility in the cas e of compensated and uncompensated semiconductors. In both cases, the low-f luence data display a plateau at around the same conductivity value corresp onding to the amorphous YIG above an amorphous fraction around 50% regardle ss of the ions. All the high-fluence data exhibit a power-law behavior with out saturation, above a threshold fluence decreasing with increasing amorph ization cross section (A). These results are interpreted by the formation o f amorphous tracks and of a more conducting nanophase after recrystallizati on of the tracks under ion impacts. All the data are rescaled versus the pr oduct of A times fluence (phi) where amorphization dominates for A phi less than or equal to 1, whereas recrystallization dominates for A phi > 10. Ho wever, significantly larger A values than the ones previously determined fr om the RBS-channeling data are derived from a mean-field analysis of the lo w-fluence conductivity data with a 2D Bruggeman model. These deviations are ascribed to a contribution of the crystalline track halos where internal s tresses are accumulated due to the atomic density difference between the cr ystal and amorphous phase. A simple phenomenological approach of the amorph ization and recrystallization processes is proposed on the basis of two kin etic 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 ex p(-T)(-1/4) law for the thermal dependence of conductivity at low temperatu re is found in the nanophase like in the amorphous one, most probably becau se of the strong contribution of the disordered grain boundary cores in the conduction process. (C) 2000 American Institute of Physics. [S0021-8979(00 )04607-7].