Exchange coupling and spin-flip transition of CoFe₂O₄/α-Fe₂O₃ bilayered films

CoFe₂O₄/α-Fe₂O₃ (ferrimagnetic/antiferromagnetic) bilayered films were prepared on α-Al₂O₃(102) single-crystalline substrates by helicon plasma sputtering. A well-crystallized epitaxial α-Fe₂O₃(102) layer was formed on the substrate, while CoFe₂O₄ grown on α-Fe₂O₃ (102) was a polycrystalline layer with a (100)-preferred orientation. The α-Fe₂O₃(102) films without CoFe₂O₄ layers clearly showed a spin-flip transition at about 400 K. The spins aligned perpendicular to the film plane at room temperature changed their direction within the film plane above 400 K. However the α-Fe₂O₃ base layers of CoFe₂O₄/α-Fe₂O₃ bilayered films did not show any spin-flip transition. The CoFe₂O₄ layer on α-Fe₂O₃ had a large in-plane magnetic anisotropy, while the spin axis of the α-Fe₂O₃ (102) base layer was directed perpendicular to the film plane. The magnetization of ferrimagnetic CoFe₂O₄ layers was coupled perpendicularly to the spin axis of antiferromagnetic α-Fe₂O₃ layers due to the exchange coupling at the interface between CoFe₂O₄ and α-Fe₂O₃.