Preparation and the magnetic properties of Co-γ-Fe2O3 perpendicular magnetic films on NiO underlayer

Abstract

The NiO underlayer, which had 〈100〉 orientation, with NaCl-like structure could be prepared onto the glass substrate by reactive rf sputtering at ambient temperature. The CoO/Fe3O4 multilayers were grown epitaxially on the underlayer at 200 °C; the Fe3O4 and CoO on the NiO grew along the 〈100〉 axis independent of the bilayer thickness Λ, though those on the glass substrate did not show the 〈100〉 orientation for Λ≳100 Å. Followed by annealing of the multilayers in air at 280–350 °C, the 〈100〉-oriented Co-γ-Fe2O3 perpendicular magnetic films were obtained. The (400) of the Co-γ-Fe2O3 on the (200) of the NiO has tensile stress at the interface because of the smaller (400) spacing than the (200) spacing normal to the film plane. Therefore, the (400) spacing measured parallel to the film plane (2.072 Å) was smaller than that of bulk, 2.086 Å. The magnetic properties of the Co-γ-Fe2O3 films on the NiO, Co-γ-Fe2O3/NiO, obtained above were evaluated. The squareness compensated by a demagnetization coefficient of Co-γ-Fe2O3/NiO were more than 0.94, though those of the Co-γ-Fe2O3 on the glass substrate, Co-γ-Fe2O3/glass, were about 0.85. The anisotropy energy of Co-γ-Fe2O3/NiO was estimated 3.3×106 ergs/cm3, which value was larger than that of Co-γ-Fe2O3/glass (2.5×106 ergs/cm3). Coercivity of the perpendicular hysteresis loop of Co-γ-Fe2O3/NiO increased with Co contents and could be controlled in the region of 0.8–5.0 kOe, although Hc of Co-γ-Fe2O3/glass were 5.0 kOe constantly. The columnar grains of the Co-γ-Fe2O3, whose diameter is 300–500 Å, grew continuously along the columnar grains of NiO.