Spin-Spin and Crystal-Field Interactions in the Rare-Earth Ethyl Sulfates. II. The Spectrum of Dysprosium

Paramagnetic-resonance measurements under the condition $h\nu >\mathrm{kT}$ have been performed on dysprosium in dysprosium ethyl sulfate (DyES) and in yttrium ethyl sulfate. Although crystal-field considerations predict that no resonance is to be expected from Dy ions in the $C_{3h}$ symmetry of the ethyl sulfate, a weak absorption has been observed in both salts. The nearest-neighbor interaction in DyES is found to be dipolar. As the temperature is lowered below 1°K, the resonance spectrum exhibits an unusual behavior, splitting up into a complex pattern of overlapping lines. A similar but simpler behavior observed in the dilute salt has been quantitatively explained in terms of a defect model. It is shown that the large $g$-tensor anisotropy of Dy precludes the possibility that the splitting in the concentrated paramagnet is due to magnetostatic modes.