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

Abstract Influence of thermal annealing on the ion beam sputtered FeCo(Nb)B quaternary alloy thin films 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.