Exchange-relaxation narrowing of the fine structure of Gd in single crystals of Pd

The magnetic-resonance spectrum of single crystal Pd: Gd (300 and 500 ppm) is reported at temperatures from 1.32 to 5.16 K. Fine structure appropriate to a cubic local environment is observed. The theory of Barnes and Zimmerman for fine-structure narrowing in dilute magnetic alloys is applied and a cubic crystalline field splitting parameter $b_4=29.6\mathrm{}$ G extracted, in agreement with a previous report for a more concentrated sample (1300 ppm) by Devine et al. The theoretical spectrum does not exhibit structure in the vicinity of the field for resonance of the $M_s=\frac{1}{2}\leftrightarrow M_s=-\frac{1}{2}$ transition, whereas the experimental spectrum exhibits a peak at that position. The 1300-ppm sample reported by Divine et al. exhibits a substantially larger peak at this position (compared to the other fine-structure lines) than the 300-ppm sample, while the 500-ppm line is only slightly more intense than the 300-ppm sample. It is argued that long-range spin-spin interactions are responsible for the collapse of the fine structure of a fraction of the Gd spins to the ½ ↔ -½ position, while the remainder of the Gd are isolated spins in the Pd matrix. The observed superposition of collapsed and isolated Gd spectra allows one to draw inferences about the spatial distribution of the spin-spin field at very low solute concentrations. In particular, the spin-spin interactions in Pd: Gd are consistent with a model wherein they are large within a sphere containing ∼200 Pd lattice sites, and negligible outside.