Inducing in-plane structural and magnetic anisotropy in a thick polycrystalline nickel film coupled to LiNbO₃ substrate

Abstract This study unveils mechanisms of thermally induced in-plane (IP) magnetic anisotropy in a 2 µ m thick polycrystalline film of nickel sputtered on lithium niobate Y-cut substrate at room temperature. We show that the appearance of IP magnetic anisotropy is correlated to IP anisotropic strain in nickel films, resulting from the coupling between the film and the substrate. Thermal annealing at moderate temperatures until 250 ∘ C under vacuum has permitted to modify significantly the strain state in the nickel film. The native IP contractive strain in the as-grown film has been gradually released and becomes anisotropic with annealing. It is demonstrated that the resulting strain is mainly due to the mismatch in thermal expansion between the film and the substrate, but also to the anisotropic strain relaxation counting for 25% of the total IP anisotropic strain. IP magnetic anisotropy of the nickel film is then highly enhanced and completely released once the film is peeled off from the substrate. This study sheds light on coupling between magnetostrictive and piezoelectric materials, providing a better understanding of the magnetoelectric effect in composite heterostructures.