Cross Relaxation between Some Paramagnetic Ions in Crystals Observed by an Optical Method

Abstract

An optical-absorption method of monitoring the population changes within the ground Zeeman doublet of paramagnetic ions has been utilized in an optical-microwave double-resonance experiment to measure cross-relaxation effects in mixed crystals. The spin dynamics of the following systems are reported and discussed: Pr-Er(ethylsulfate), Ce-Nd(ethylsulfate), Pr-Ni(double nitrate), and Pr-Sm(ethylsulfate). Experimental results are compared to relevant theories of the spin dynamics and interaction mechanisms which are expected for these ions. The theories of the phonon bottleneck are discussed in terms of their ability to explain some nonexponential decays which are observed in the experiment. A method of estimating the cross-relaxation rate due to virtual-phonon exchange is developed which uses measured spin-lattice relaxation rates to eliminate unknown parameters. The relaxation measurements generally indicate very rapid energy transfer between dissimilar ions which have overlapping resonances, even in a case where the magnetic dipole-dipole interaction is absent.