Amorphization produced by ion implantation and subsequent crystallization produced by thermal processing have been investigated in films of (SmYGdTm)3Ga0.4Fe4.6O12garnet by transmission electron microscopy, incorporating a special cross‐sectioning technique. These films were produced by liquid‐phase epitaxy on {111} garnet substrates and subsequently implanted with ions of deuterium at 60 keV and doses ranging from 0.50 to 4.5×1016D+2/cm2and ions of oxygen at 110 keV and doses ranging from 0.95 to 8.6×1014O+/cm2. The amorphization process evolves in four separate stages: (1) an implanted (crystalline) band, delineated by the implantion strain profile, forms at doses of about 0.50×1016D+2/cm2/ and 0.95 O+/cm2, (2) isolated amorphous regions of about 10 nm in diameter form at doses of about 1.0×1016D+2/cm2and 1.9×1014O+/cm2, (3) the amorphous regions merge to form a continuous band below the implanted surface at doses of about 3.0×1016D+2/cm2and 5.7×1014O+/cm2, and (4) this band expands to the implanted surface at larger doses. Amorphization is caused by implantation with oxygen, but prior implantation with deuterium sensitizes the lattice by increasing the strain. The crystallization process evolves in three separate stages: (1) small crystallites, about 10 nm in size, form throughout the entire amorphous band after annealing for 10 min at 350 °C, (2) larger crystallites nucleate and grow from the implanted surface and amorphous/crystalline interface after annealing for 10 min at 450 °C, and (3) these crystallites grow in size until they merge to form a continuous polycrystalline layer. Some epitaxial regrowth of the monocrystalline into the amorphous region is also observed. Details of these processes are interpreted in terms of atomic displacement mechanisms and compared with corresponding changes of certain magnetic properties.