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

Abstract

Measurements have been made of the electron spin-lattice relaxation of the -½, +½ line of ${\mathrm{Cr}}^{+++}$ in ${\mathrm{K}}_3$Co${\mathrm{}\left(\mathrm{CN}\right)\mathrm{}}_6$ at 9kMc/sec as a function of temperature, chromium concentration, and the proximity of the $-\frac{3}{2}$, -½ 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°K and 4.8°K, $T_1$ varies approximately as $T^{-1.2\mathrm{}}$ 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 -½, +½ line is enhanced when the $-\frac{3}{2}$, -½ 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% ${\mathrm{Cr}}^{+++}$.