Electron Spin-Lattice Relaxation in Dilute Potassium Chromicyanide at Helium Temperatures

Measurements have been made of the electron spin-lattice relaxation of the -{}, +{} line of Cr⁺⁺⁺ in K₃Co(CN)₆ at 9kMc/sec as a function of temperature, chromium concentration, and the proximity of the -3/2, -{} line. The experimental procedure, involving inversion of the line, is capable of distinguishing a "bottleneck" relaxation time from a true spin-phonon relaxation time, T₁. At Cr⁺⁺⁺ concentrations up to 0.5%, the relaxation data are fitted well by single exponential functions of time. Between 1.3^∘{}K and 4.8^∘{}K, T₁ varies approximately as T^-1.2 indicating that the single phonon process is dominant. No phonon-bath bottleneck is observed, in agreement with calculations based on the measured parameters. A "proximity effect" is observed in which the relaxation rate of the -{}, +{} line is enhanced when the -3/2, -{} line is within 20 linewidths. At one percent Cr⁺⁺⁺, the relaxation behavior is markedly different: the recovery is considerably faster and can no longer be described by a single time constant. This change and the proximity effect are interpreted qualitatively in terms of spin cross relaxation. The measured linewidth increases with concentration from 0.03% to 2% Cr⁺⁺⁺, even though the line is observed to be inhomogeneous at and below 0.5% Cr⁺⁺⁺.