Anomalous Magnetizations in the Yttrium Gallium Iron Garnet System and the Exchange Constant of Yttrium Iron Garnet

Magnetization curves for the yttrium gallium iron garnet system ${\mathrm{Y}}_3{\mathrm{Ga}}_x{\mathrm{Fe}}_{5-x}{\mathrm{O}}_{12}$ in the region $x=1.3\mathrm{}-1.5$ are presented. These show anomalous temperature dependences. They are very similar in character to those anticipated by Néel and found for some spinel ferrites by Gorter. The molecular field theory is applied to these curves using an appropriate set of exchange constants. The best fit is achieved using rather small values for $J_{\mathrm{aa}}$ and $J_{\mathrm{dd}}$. This is in good agreement with the most reliable determination of these constants for yttrium iron garnet and with the current belief that these constants should indeed be small.