Temperature Dependence of the Linewidth of Garnets Containing Fe4+ Ions

The linewidth of garnets containing Fe⁴⁺ ions was measured from 4.2° to 300°K at 10 and 25 kMc/sec for small, well‐polished single‐crystal spheres in the (110) plane. Within the measuring range the linewidth is proportional to the concentration of Fe⁴⁺ ions. For both $\mathrm{YIG}$ and $\mathrm{LuIG}$ at 25 kMc/sec, the temperature dependence in all three major crystallographic directions exhibits two well‐defined peaks. At 10 kMc/sec, all peaks have shifted toward lower temperatures by different amounts, while the linewidth at the peaks has increased by about 20%. In the [100] direction, however, the two peaks seem to overlap and only one peak, with no detectable structure, is observed at 60°K and at 10 kMc/sec. Complementary measurements of the polarity of the Seebeck voltage indicate the presence of holes as majority carriers in our samples. The existence of two peaks in the temperature dependence of the linewidth can be explained by the slow‐relaxation theory if one assumes anisotropy of the relaxation time of the Fe⁴⁺ ion.