Magnetization in Nickel Ferrite-Aluminates and Nickel Ferrite-Gallates

Abstract
Trivalent aluminum and trivalent gallium have been substituted for trivalent iron in nickel ferrite NiO·Fe2 O3 to give the systems NiO·Fe2tAltO3 and NiO·Fe2tGatO3, respectively, where t goes from 0 to 2. The materials, annealed and prepared in a powdered form, were homogeneous with the length of the edge of the unit cell decreasing linearly from 8.332A to 8.043A and 8.247A, for the aluminum and gallium substitution, respectively, as t varies from 0 to 2. In the case of the aluminum substitution, thermomagnetic curves were obtained of the types classified by Néel as Q; M1, M2, or P; and type R. The saturation magnetization extrapolated to 0°K (μ0) for the nickel ferrite-aluminates decreases linearly with increase in t approaching zero in the region of t=0.63; for further increase in t, μ0 increases to a maximum at about t=1.0 beyond which it decreases again to 0 at t=2. The increase in μ0 is interpreted as an indication of a reversal in the direction of the resultant magnetization due to predominance of the magnetization of the ions on the A sites over those on the B sites. For the gallium substitution, the thermomagnetic curves follow the more conventional Q type with the appearance of the R-type curves for values of t from 0.63 to 1.5. As contrasted to the aluminates, the gallates show an increase in μ0 with t, with a maximum developing in approximately the same region where the aluminates showed the crossover from B dominance to A dominance. The Curie temperatures decrease near-linearly with increase in t; the points for the aluminates and the gallates are so close together that they can be represented by one smooth curve. From an application of the Néel theory, it is indicated that the BB interaction is negative, both for the aluminates and the gallates, at least for values of t in the region 0.63 to 1.00.