Hyperfine Splitting of a Localized Moment in a Metal

The hyperfine splitting of the erbium localized moment resonance in silver metal has been observed for nominal concentrations of 28-, 65-, 86-, and 1500-ppm solid solution Ag: Er alloys. A hyperfine coupling constant $A=75\mathrm{}\pm 1.0$ Oe was found to fit the individual line positions to within experimental accuracy (±5 Oe). The magnetic-resonance bottleneck is discussed; it is argued that this bottleneck is absent for these measurements, and is the reason for the failure of previous attempts to observe such hyperfine splittings in transition-metal alloys.