Spectral Diffusion in Electron Resonance Lines

Spectral diffusion in the inhomogeneous paramagnetic resonance lines of ${\mathrm{Ce}}^{3+}$ and ${\mathrm{Er}}^{3+}$ in CaW${\mathrm{O}}_4$ at concentration ∼${10}^{18}$ spins/cc has been studied by electron spin echo techniques. Measurements show that the spreading of excitation through the line can be approximately described by a diffusion kernel in the form of a Lorentzian function whose width is linear in time. Local field fluctuations due to spin-spin flips and spin-lattice flips appear to be the primary cause of diffusion. In (Ca,Ce)W${\mathrm{O}}_4$ below 4.2°K the lattice time is long, and spin-spin flips predominate, leading to a diffusion rate which is independent of temperature. In (Ca,Er)W${\mathrm{O}}_4$ lattice relaxation is more rapid, and the diffusion parameter varies as the lattice time. Nuclei of ${\mathrm{W}}^{183}$ in the host crystal have been observed to give a modulation in the envelope of spin echoes, but the corresponding local fields are smaller than those arising from electron spins at the concentrations studied.