Tetragonal-like distortion and enhanced magnetic anisotropy of the cubic spinel Fe₃O₄ film with a thin ZnFe₂O₄ capping layer

25-nm-thick Fe₃O₄ films without and with 1.5-nm-thick ZnFe₂O₄ capping layer was deposited on (110) SrTiO₃ substrate, and their structural and magnetic properties were studied. X-ray diffraction data showed that the position of the (110) peak shifted marginally to a higher angle after deposition of the capping layer in oxygen environment. Transmission electron microscopy images showed that the Fe₃O₄ film on the (110) SrTiO₃ substrate appeared epitaxial, and the capping layer on the Fe₃O₄ film appeared polycrystalline. Reciprocal space mapping and pole-figure measurements revealed that a structural distortion of the Fe₃O₄ layer from cubic to tetragonal-like was presumably induced by a slight change in oxygen content during the deposition of the capping layer in oxygen environment. Compared with the cubic Fe₃O₄ film, magnetic hysteresis loops of the tetragonal-like Fe₃O₄ showed increased coercivity and enhanced uniaxial magnetic anisotropy, which were presumably attributed to an increase of crystalline anisotropy level in the tetragonal-like structure.