Formation and transformation of metastable phases during electrodeposition and annealing of cobalt–iron alloy films

During the electrodeposition of Co–Fe alloy films from a CoSO₄·7H₂O-FeSO₄·7H₂O bath, the formation of metastable phases, such as a complex cubic Co–Fe phase isostructural to α-Mn and the HCP ε-Co/Fe and Ω-Co/Fe phases, appears to be related to the incorporation of metal hydroxide/oxide precipitates into the plated alloy films. In the absence of the incorporated precipitates, the plated films are the equilibrium α-Fe solid solution BCC phase. Thus, the addition of stabilizing reagents (such as ammonium citrate), and/or a lowering of solution pH, prevents the formation of the precipitates and promotes the formation of the BCC phase. On the other hand, increasing temperature causes the formation of metastable phases, possibly through the weakening of the stabilizing effect of the ammonium citrate, or the promotion of the formation of metal hydroxides/oxides precipitates. The BCC phase has higher saturation magnetic flux densities and lower coercivities than the metastable phases. Annealing of the films transforms the metastable phases, if present, into the BCC phase, leading to a decrease in the coercivity. An increase in the magnetic flux density after annealing is, however, not observed, possibly due to the cracking or delamination of the films as a result of annealing. Cracking and delamination make the determination of the film volume, which is required for magnetic flux density calculation, questionable.