Low-Temperature Relaxation Effects in Pulsed-Field Electron-Paramagnetic-Resonance Spectra of Rare-Earth Ions

Spin-lattice relaxation effects have been observed in the pulsed-field EPR spectra of rare earths in yttrium aluminum garnet, yttrium gallium garnet, and CaF₂. An analysis has been developed that permits evaluation of the spin-lattice relaxation rates. It is shown that for a number of Kramers-doublet rare-earth ions, at frequencies above 70 Gc/sec and at liquid-helium temperatures, the only important relaxation mechanism is the direct spin-phonon interaction due to modulation of the crystalline field by lattice vibrations. The effects of paramagnetic impurity concentration and low-lying excited states on relaxation rates have also been observed, and, where appropriate, their influence is included in the analysis and discussion.