Extraordinarily large perpendicular magnetic anisotropy in epitaxially strained cobalt-ferrite Co_xFe_3-xO₄(001) (x = 0.75, 1.0) thin films

Perpendicular magnetic anisotropy (PMA) of cobalt-ferrite Co _xFe_3-xO₄ (x = 0.75 and 1.0) epitaxial thin films grown on MgO (001) by a reactive magnetron sputtering technique was investigated. The saturation magnetization was found to be 430 emu/cm³ for x = 0.75, which is comparable to that of bulk CoFe ₂O₄ (425 emu/cm³). Torque measurements afforded PMA constants of K_eff^u = 9.0 Merg/cm 3 (Ku = 10.0 Merg / cm³) and K_eff^u = 9.7 Merg / cm³ for x = 0.75 and 1.0, respectively. The value of K_eff^u extrapolated using Miyajima's plot was as high as 14.7 Merg/cm³ for x = 1.0. The in-plane four-fold magnetic anisotropy was evaluated to be 1.6 Merg/cm³ for x = 0.75. X-ray diffraction measurement revealed our films to be pseudomorphically strained on MgO (001) with a Poisson ratio of 0.4, leading to a considerable in-plane tensile strain by which the extraordinarily large PMA could be accounted for.