Evolution of microstructure and magnetic properties of Fe-Co-N films sandwiched by AlN layers

Abstract

A series of Fe-Co-N films sandwiched by AlN layers have been sputter-deposited at different N2 flow ratios in an Ar and N2 gas mixture. It is revealed that the main phase of films transforms from polycrystalline α-FeCo(N) to amorphous-like γ′-(Fe,Co)4N, polycrystalline ε-(Fe,Co)3N and ξ-(Fe,Co)2N with increasing N2 flow ratio. The coercivity of the films initially decreases with increasing N2 flow ratio, reaching the lowest value of 8–10 Oe at N2 flow ratio around 20% to 30%. This is due to the formation of amorphous-like phase with extremely fine microstructure. Furthermore, vacuum annealing has been carried out over a temperature range of 100 to 600 °C for the films with N2 ratios ranging between 20% and 40%. The saturation magnetization of the films largely increases after the films are annealed at 350 °C, which is caused by the decomposition of the nitride phases into α-FeCo(N) during the annealing process. The coercivity of the 20% and 30 % N2 films shows strong annealing temperature dependence, and it increases at first and then decreases.