Magnetic properties of amorphous films of Bi2O3 -Fe2O3-CaO

The magnetic properties of amorphous films with the composition (Bi₂O₃)_0.5-x(Fe₂O₃ )_0.5(CaO)_x are investigated. Samples with x=0-0.05 were prepared in a pure oxygen or mixed oxygen-argon atmosphere by the RF sputtering technique. As-sputtered films are X-ray amorphous and paramagnetic at room temperature. The films show a spontaneous magnetic moment when annealed in air at temperatures T _a above 400°C. Data on the room temperature saturation induction 4πM with T_a, followed by a maximum in 4πM for T_a=600-700°C, depending on the x-value. At higher T_a, a decrease in 4πM is observed. The room temperature 4πM is found to decrease with increase in x, from 970 G for x=0.1 to 300 G for x=0.5. The Curie temperature is lower than the crystallization temperature. Ferromagnetic resonance studies show a uniaxial or planar anisotropy in the films. X-ray diffraction data do not indicate the precipitation of crystalline ferrimagnetic oxides in the annealed films and the observed magnetization is therefore attributed to ordered amorphous clusters