Effects of Foreign Co2+ and Fe2+ Ions on the Magneto-Optical Properties of Gadolinium Iron Garnet

Ferrimagnetic single crystals derived from gadolinium iron garnet by cosubstitution with cobalt and germanium, ${\mathrm{Gd}}_3{\mathrm{Fe}}_{\text{5−2x}}^{\text{3+}}{\mathrm{Co}}_x^{\text{2+}}{\mathrm{Ge}}_x^{\text{4+}}{\mathrm{O}}_{12}$ , are shown to exhibit a drastic change of the visible Faraday rotation. This effect is attributed to a spin‐allowed crystal field transition in tetrahedrally coordinated Co²⁺ ions. Another system, ${\mathrm{Gd}}_3{\mathrm{Fe}}_{\text{5−2x}}^{\text{3+}}{\mathrm{Fe}}_x^{\text{2+}}{\mathrm{Ge}}_x^{\text{4+}}{\mathrm{O}}_{12}$ , was studied also, and it was found that the Faraday rotation is suppressed by formation of Fe²⁺ ions. It is suggested that certain transition‐metal ions like Co²⁺ are useful in achieving materials of improved magneto‐optical performances.