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\ifmmode\cdot\else\textperiodcentered\fi{}${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$ to give the systems $\mathrm{NiO}\ifmmode\cdot\else\textperiodcentered\fi{}{\mathrm{Fe}}_{2\ensuremath{-}t}{\mathrm{Al}}_{t}{\mathrm{O}}_{3}$ and $\mathrm{NiO}\ifmmode\cdot\else\textperiodcentered\fi{}{\mathrm{Fe}}_{2\ensuremath{-}t}{\mathrm{Ga}}_{t}{\mathrm{O}}_{3}$, 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 N\'eel as $Q$; ${M}_{1}$, ${M}_{2}$, or $P$; and type $R$. The saturation magnetization extrapolated to 0\ifmmode^\circ\else\textdegree\fi{}K (${\ensuremath{\mu}}_{0}$) for the nickel ferrite-aluminates decreases linearly with increase in $t$ approaching zero in the region of $t=0.63$; for further increase in $t$, ${\ensuremath{\mu}}_{0}$ increases to a maximum at about $t=1.0$ beyond which it decreases again to 0 at $t=2$. The increase in ${\ensuremath{\mu}}_{0}$ 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 ${\ensuremath{\mu}}_{0}$ 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 N\'eel theory, it is indicated that the $\mathrm{BB}$ interaction is negative, both for the aluminates and the gallates, at least for values of $t$ in the region 0.63 to 1.00.