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

The interactions between magnetic rare-earth ions in a set of isomorphic salts have been investigated under the experimental condition gβH>kT such that the spins are highly aligned by the external magnetic field. Paramagnetic resonance under these conditions is found to show behavior qualitatively similar to ferromagnetic resonance. The high degree of spin alignment results in a nearly uniform internal field for samples having a regular shape, so that it is possible to observe narrow line spectra in concentrated paramagnetic salts. The spin-spin interactions may be directly determined from the line shifts at low temperatures, and from the observation of resolved structure that results from the resonance absorption of ions having a near neighbor ion whose spin is antiparallel to the magnetic field. In dysprosium ethyl sulfate containing gadolinium as an impurity, the interaction of a Gd ion with its first-, second-, and third-nearest-neighbor Dy ions, as well as the total interaction with all Dy neighbors, has been measured. The interaction may be explained in terms of a dipolar coupling between the spins. The influence of the spin-spin interactions on the angular variation of the Gd spectrum is also examined. The spin-lattice relaxation time of a Gd ion is found to depend on its nuclear spin.