Strain Engineering of Magnetic Anisotropy in Epitaxial Films of Cobalt Ferrite

Abstract: Perpendicular magnetic anisotropy (PMA) energy up to $K_{\mathrm{u}}=6.1\pm0.8$ MJ m$^{-3}$ is demonstrated in this study by inducing large lattice-distortion exceeding 3% at room temperature in epitaxially distorted cobalt ferrite Co$ {x} $Fe$ _{3-x} $O$ _{4} $ (x = 0.72) (001) thin films. Although the thin film materials include no rare-earth elements or noble metals, the observed $ K{u} $ is larger than that of the neodymium-iron-boron compounds for high-performance permanent magnets. The large PMA is attributed to the significantly enhanced magneto-elastic effects, which are pronounced in distorted films with epitaxial lattice structures upon introducing a distortion control layer of composition Mg$ {2-x} $Sn${1+x}$O$ {4} $. Surprisingly, the induced $ K{u} $ can be quantitatively explained in terms of the agreement between the local crystal field of Co$ ^{2+} $ and the phenomenological magneto-elastic model, indicating that the linear response of induced $K_u$ is sufficiently valid even under lattice distortions as large as 3.2%. Controlling tetragonal lattice deformation using a non-magnetic spinel layer for ferrites could be a promising protocol for developing materials with large magnetic anisotropies.