Perpendicular magnetic anisotropy (PMA) of cobalt-ferrite Co xFe3-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 Keffu = 9.0 Merg/cm 3 (Ku = 10.0 Merg / cm3) and Keffu = 9.7 Merg / cm3 for x = 0.75 and 1.0, respectively. The value of Keffu 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.
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