On the origin of magnetic anisotropy of FeCo(Nb)B alloy thin films: A thermal annealing study

•Effect of thermal annealing on FeCoNbB thin film having different thicknesses has been studied.•UMA gradually decreases with annealing and disappears upon annealing at 773 K.•Growth induced anisotropic tensile stresses as the possible origin of observed UMA.•Nb addition improves thermal stability, inhibits grain growth and stabilises bcc-FeCo phase.•FeCoNbB films are proposed as potential material for magnetic electrode in MTJ applications. Influence of thermal annealing on the ion beam sputtered FeCo(Nb)B quaternary alloythinfilms having thickness ∼20 nm and 80 nm was investigated. Regardless of amorphous microstructure, the as-deposited films possess uniaxial magnetic anisotropy (UMA) in the film plane. Annealing at temperature ∼503 K results in amorphous to nanocrystalline phase transformation with precipitation of bcc-FeCo phase. With increasing annealing temperature, magnetic anisotropy gradually decreases and thermal annealing at 773 K results in complete disappearance of UMA. In-plane strain distribution in such nanocrystalline FeCo(Nb)B thin films was obtained by using laboratory and synchrotron X-ray diffraction (XRD) measurements. In-plane and out-of-plane X-ray diffraction measurements suggest, growth induced long range non-uniform tensile stress, as the possible origin of in-plane uniaxial magnetic anisotropy in such films. It is demonstrated that the stress distribution becomes uniform after annealing at 773 K and results in complete disappearance of magnetic anisotropy. The study further suggests that the presence of Nb in FeCoB film improves the thermal stability, inhibits grain growth and stabilizes bcc-FeCo phase up to elevated temperatures. This information is useful for magnetic tunnel junction application, where annealing of FeCoB magnetic electrode is essential.