Ferrimagnetic Resonance in Yttrium Iron Garnet at Liquid Helium Temperatures

Ferrimagnetic resonance experiments have been performed on single crystal spheres of yttrium iron garnet (${\mathrm{Y}}_3$ ${\mathrm{Fe}}_2$ ${\left(\mathrm{F}\mathrm{e}{\mathrm{O}}_4\right)}_3$) at 24 kMc/sec. At 4.2° the line width measured with the steady field along [100] is 15 oe, as compared with 30 oe at 295°, 400 oe at 65°K. At liquid helium temperatures the crystal is apparently cubic. There is a strong anisotropy in line width. Except for the case of the field along [100], the line width varies strongly with temperature in this temperature region. A large magnetocrystalline anisotropy is indicated by the variation in field for resonance with direction. This ranges over 3000 oe. The maxima and minima in this field indicate that terms of higher order than the $K_1$ and $K_2$ terms used hitherto will be necessary for the description of the anisotropy energy surface. Thus, at 4.2°K, apparently [100], [110], and [111] are all hard directions, and the easy directions are exactly or close to [112], [221], and [123].