Fe${\mathrm{Al}}_2$${\mathrm{O}}_4$-Mg${\mathrm{Al}}_2$${\mathrm{O}}_4$: Growth and Some Thermal, Optical, and Magnetic Properties of Mixed Single Crystals

Synthetic single crystals of Fe${\mathrm{Al}}_2$ ${\mathrm{O}}_4$ have been grown from the melt. Measurements of the thermal conductivity $K$ and optical absorption coefficient $\alpha$ have been made on a combination of synthetic and natural single crystals of ${\mathrm{Mg}}_{1-x}{\mathrm{Fe}}_x{\mathrm{Al}}_2{\mathrm{O}}_4$ for $0\le x\le 1$. The $\alpha$ measurements at 300°K in the absorption band of tetrahedral ${\mathrm{Fe}}^{2+}$ between 0.4 and 0.6 eV have been calibrated and then used to measure $x$. The thermal conductivity results between 3 and 300°K show that the heat is conducted by phonons and that tetrahedral ${\mathrm{Fe}}^{2+}$ is a very effective phonon scatterer at all temperatures because it possesses a series of low-lying energy levels in the $3d$ shell. This type of scattering is absent in Co${\mathrm{Al}}_2$ ${\mathrm{O}}_4$. The phonon scattering by ${\mathrm{Fe}}^{2+}$ is dominant for $x>\mathrm{}0.1\mathrm{}$, and for these crystals $\mathrm{K}\sim \frac{T}{\surd x}$. The magnetic susceptibility $\chi$ of a single crystal of Fe${\mathrm{Al}}_2$ ${\mathrm{O}}_4$, measured between 2 and 290°K, shows a peak at 9.5±0.5°K. The analysis of the $\chi$ and $K$ measurements suggests that the ∼10°K transition is caused by the individual ${\mathrm{Fe}}^{2+}$ ions, and is not associated with a longrange, antiferromagnetic ordering of the Fe${\mathrm{Al}}_2$ ${\mathrm{O}}_4$.