Evolution of the structure and morphology of latent tracks as a function of elec- tronic energy deposition

The creation of latent tracks in insulating materials exposed to energetic heavy ions has been studied in a large number of targets. In particular detailed results were obtained in yttrium iron garnet (YIG) Y3Fe5O12 irradiated in a broad range of electronic energy deposition (dE/dx)e both with monoatomic and cluster ions [1-4]. By means of Transmission Electron Microscopy (TEM) and channeling Rutherford backscattering, it was demonstrated that with increasing projectile energy (i) discontinuous latent tracks appear at a threshold of dE/dx)t ≈ 4 keV/nm, (ii) the track diameter increases until (dE/dx)e reaches the Bragg peak (the maximum effective diameter is estimated to be 14 nm [1]), and (iii) above the Bragg maximum, the track diameter decreases with increasing projectile velocity. This so-called “velocity effect” has been related to the evolution of the density of deposited energy [1,4]: the higher the projectile velocity, the larger the lateral spread of deposited energy resulting in a reduction of the energy density. The aim of this work is to study the morphology and structure of ion tracks in the very high velocity regime unexplored up to now. For this purpose, small crystalline grains of YIG powder were deposited on TEM copper grids covered by a thin amorphous carbon film. The grid samples were exposed at 300 K and at normal incidence to U ions at the UNILAC and SIS applying fluences between 2 and 5×10 ions/cm, sufficiently low to avoid spatial track overlap. The beam energy was varied between 11.1 and 417 MeV/u corresponding to (dE/dx)e ranging from 49 to 9 keV/nm, respectively. High-resolution TEM investigations performed just after irradiation confirm that also in the high-velocity regime the track diameters decrease with increasing projectile velocity. Moreover, the track structure gradually evolves from a circular, well-defined amorphous zone into a more and more diffuse track of defective crystalline structure, and finally at very high beam velocities, the tracks show a very faint contrast in the TEM. They can only be imaged in high-resolution conditions in very thin sample regions and consist of almost perfect crystalline structures which are in perfect epitaxy with the surrounding material. Figure 1 compares the high-resolution TEM micrographs of two tracks created at different ion velocities: at 11.1 MeV/u ((dE/dx)e= 49 keV/nm), the track is continuous, amorphous and has a diameter of about 13 nm, whereas at 291 MeV/u ((dE/dx)e=11 keV/nm), the track is quasi-continuous showing a non constant diameter. The damaged zone exhibits a very weak contrast and is crystalline with some point defects. The track diameter is reduced to about 3 nm. The large differences in the morphology, size and structure of tracks produced at different beam velocities is a clear illustration that structural modifications induced by swift heavy ions become much weaker if the deposited energy is spread into a large volume thus decreasing the deposited energy density.