Ferrimagnetic Resonance and Torque Measurements on Ytterbium-Substituted Yttrium Iron Garnet

It is pointed out that the comparison of microwave resonance and static torque measurements on single-crystal samples may enable relaxation effects in the former to be identified. Such a comparison is made for a crystal of nominal composition $({\mathrm{Yb}}_{\text{0.021}}{\mathrm{Y}}_{\text{0.979}}{)}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ in the temperature range 4.2–250°K. It is shown that for this crystal, as in ${\mathrm{Yb}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ , the anisotropy in $H_r$ , the field for resonance, is not due only to magnetocrystalline energy in a range of temperatures below 220°K. It is suggested that the extra reduction in $H_r$ observed, which occurs most strongly in the [111] directions, is associated with the heavily damped motion of the magnetization of the ytterbium ion. Both types of measurement show that between 20 and 4.2°K, the magnetocrystalline energy surface of the ytterbium ion changes very rapidly and develops sharp curvature.