Crystallization process of amorphous Fe-Ta-C alloy films and thermal stability of the resultant soft-magnetic nanocrystalline state

Nanocrystallization behavior in sputter-deposited amorphous Fe-Ta-C films has been studied. By an analysis of kinetics combined with the microstructural characterization, it is found that the first stage reaction is primarily crystallization of alpha-Fe, which is similar to that occurring in amorphous Fe-Cu-Nb-Si-B and Fe-M-B (M = Zr, Hf, or Nb) alloys. The precipitation of TaC crystals follows this reaction, though some bonding between Ta and C seems to be already present in the amorphous matrix. In the early stage, the growth of primary α-Fe grain is considered to be suppressed by the solute-enriched amorphous matrix, being similar to the other aforementioned nanocrystalline alloys. However, such a partially crystallized state is not an optimum state for soft magnetic properties unlike the others. After an optimum annealing of the film with the optimum composition, the residual amorphous phase is almost absent, and the TaC particles dispersed at grain boundaries of α-Fe play an important role for retarding the grain growth instead. The temperature range where the residual amorphous phase exists is considerably narrow and hence the residual amorphous phase easily decomposes.