Crystalline-Field Effects in the Electron-Spin Resonance of Rare Earths in the Noble Metals

The effect of a low-lying crystalline-field excited state on the electron-spin-resonance (ESR) linewidth and $g$ shift of the ground state in a dilute magnetic alloy is calculated. Departures from linearity are found for the linewidth at temperature substantially below the excitation energy of the first excited state. Measurements on Ag:${\mathrm{Dy}}^{164}$ and (especially) Au:Er are reported which exhibit this behavior. Upon comparison with Ag:Gd and Ag:Er measurements, it is found possible to estimate the energy separation of the first excited level from the ground state from ESR linewidth and $g$-shift measurements alone.