Crystallisation of amorphous rare-earth-iron and transition-metal-boron thin films

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Abstract
The amorphous to crystalline transition in a series of ultra-high-vacuum (UHV) vapour-deposited thin films has been investigated by transmission electron microscopy (TEM). Annealing of the greater part of the films is performed in a conventional TEM hot stage attachment. All films possess a columnar microstructure in which the columns are surrounded by a network of material with lower density. Heating of the RE-Fe films results in the formation of RE-oxide preferentially within the anisotropic network. At the same time Fe-rich clusters are formed by a nucleation and growth mechanism. Recrystallisation sets in at temperatures in excess of 700 degrees C, when the columnar structure disappears. The TM-B films are more stable against oxidation. In the early stage of the amorphous to crystalline transition, precipitates are formed consisting of the TM element and TM borides. This is followed by a rapid transition of the remaining amorphous matrix to the crystalline state. Some compositions, however, crystallise via the formation of radially expanding crystals that consist of monocrystalline regions with either lath-like or plate-like morphology. The growth rate of these crystals is easily controlled by adjusting the heat input into the film.