Crystallization Process of Amorphous Fe–Ta–C Alloy Films and Thermal Stability of the Resultant Soft-Magnetic Nanocrystalline State

Abstract

Nanocrystallization behavior in sputter-deposited amorphous Fe-Ta-C films has been studied. By an analysis of kinetics combined with the microstructural characterization, it was found that the first stage reaction is primary crystallization of α-Fe, which is similar to that occurs 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 nanocrystalline alloys mentioned above. However, such a partially crystallized state is not an optimum state for soft magnetic properties unlike the others, because the residual amorphous phase has only a low magnetization resulting in an insufficient intergranular magnetic coupling. 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 compared with the other nanocrystalline alloys, probably because the reactivity between Ta and C toward TaC is stronger than that between M and B, and hence the residual amorphous phase easily decomposes. The relation between structural evolution after nanocrystallization and magnetic softness was also investigated for the films outside the optimum composition.