Inducing or suppressing the anisotropy in multilayers based on CoFeB

Abstract

Controlling the uniaxial magnetic anisotropy is of practical interest for a wide variety of applications. We study ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$ single films grown on various crystalline orientations of ${\mathrm{LiNbO}}_{3}$ substrates and on oxidized silicon. We identify the annealing conditions that are appropriate to induce or suppress in-plane uniaxial anisotropy. Anisotropy fields can be increased by annealing up to 11 mT when using substrates with anisotropic surfaces. They can be decreased to below 1 mT when using isotropic surfaces. In the first case, the observed increase of the anisotropy originates from the biaxial strain in the film caused by the anisotropic thermal contraction of the substrate when back at room temperature after strain relaxation during annealing. In the second case, anisotropy is progressively removed by applying successive orthogonal fields that are assumed to progressively suppress any chemical ordering within the magnetic film. The method can be applied to CoFeB/Ru/CoFeB synthetic antiferromagnets, but the tuning of the anisotropy comes with a decrease of the interlayer exchange coupling and a drastic change in the exchange stiffness.