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

Abstract

Measurements have been made of the electron spin-lattice relaxation of the -\textonehalf{}, +\textonehalf{} line of ${\mathrm{Cr}}^{+++}$ in ${\mathrm{K}}_{3}$Co${(\mathrm{CN})}_{6}$ at 9kMc/sec as a function of temperature, chromium concentration, and the proximity of the $\ensuremath{-}\frac{3}{2}$, -\textonehalf{} line. The experimental procedure, involving inversion of the line, is capable of distinguishing a "bottleneck" relaxation time from a true spin-phonon relaxation time, ${T}_{1}$. At ${\mathrm{Cr}}^{+++}$ concentrations up to 0.5%, the relaxation data are fitted well by single exponential functions of time. Between 1.3\ifmmode^\circ\else\textdegree\fi{}K and 4.8\ifmmode^\circ\else\textdegree\fi{}K, ${T}_{1}$ varies approximately as ${T}^{\ensuremath{-}1.2}$ 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 -\textonehalf{}, +\textonehalf{} line is enhanced when the $\ensuremath{-}\frac{3}{2}$, -\textonehalf{} line is within 20 linewidths. At one percent ${\mathrm{Cr}}^{+++}$, 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% ${\mathrm{Cr}}^{+++}$, even though the line is observed to be inhomogeneous at and below 0.5% ${\mathrm{Cr}}^{+++}$.