Magnetic Properties of the Manganese Chromite-Aluminates

The mixed-crystal spinel series MnCr_2-tAlₜO₄ has been synthesized and found to form a single cubic phase with a cell edge that is a linear function of the aluminum content. An x-ray study indicates that the series is an almost-normal spinel series with the A sites occupied by divalent manganese ions and about 5% of the trivalent aluminum ions.The magnetization-temperature curves have approximately zero slope at absolute zero and exhibit no peaks or compensation points. The saturation moment is 1.16μB for $t=0.0$, increasing to 1.37μB at $t=0.8$, and dropping to 1.25μB at $t=1.0$. The reciprocal-susceptibility vs temperature curves have the hyperbolic shape characteristic of ferrimagnets. The observed magnetic properties cannot be explained by the N\'eel theory but can be accounted for, at least qualitatively, by the five-parameter Yafet and Kittel theory.