EXCHANGE COUPLING AND SPIN-FLIP TRANSITION OF CoFe_2O_4/α-Fe_2O_3 BILAYERED FILMS

摘要

CoFe_2O_4/α-Fe_2O_3 (ferrimagnetic / antiferromagnetic) bilayered films were prepared on α-Al_2O_3(102) single-crystalline substrates by helicon plasma sputtering. A well-crystallized epitaxial α-Fe_2O_3(102) layer was formed on the substrate, while CoFe_2O_4 grown on α-Fe_2O_3 (102) was a polycrystalline layer with a (100)-preferred orientation. The α-Fe_2O_3(102) films without CoFe_2O_4 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-Fe_2O_3 base layers of CoFe_2O_4/α-Fe_2O_3 bilayered films did not show any spin-flip transition. The CoFe_2O_4 layer on α-Fe_2O_3 had a large in-plane magnetic anisotropy, while the spin axis of the α-Fe_2O_3 (102) base layer was directed perpendicular to the film plane. The magnetization of ferrimagnetic CoFe_2O_4 layers was coupled perpendicularly to the spin axis of anitiferromagnetic α-Fe_2O_3 layers due to the exchange coupling at the interface between CoFe_2O_4 and α-Fe_2O_3.