Paramagnetic Relaxation in Dilute Potassium Ferricyanide

Abstract

Paramagnetic relaxation times have been measured for the ground-state doublet of iron present as a dilute substitutional impurity in potassium cobalt cyanide. Measurements were carried out over the temperature range 1.25 to 4.5\ifmmode^\circ\else\textdegree\fi{}K and over a range of Fe/Co concentrations 0.24 to 3.5 at.%. Two frequencies were used, 1.8 and 8.5 Gc/sec, to provide a direct test of the frequency variation of the relaxation time. The "fast-passage-recovery" technique was employed, and a description of the apparatus is included. At 0.24% the 1.8-Gc/sec times are found to be well fitted by a Raman rate $\frac{1}{{T}_{1}}=4.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}{T}^{9}$ ${\mathrm{sec}}^{\ensuremath{-}1}$ over the entire temperature range and over five decades of time. At 8.5 Gc/sec this rate is augmented by a direct rate $\frac{1}{{T}_{1}}=3.1T$. At 0.5% the $X$-band direct rate is the same, but the Raman rate appears slightly higher; and the $L$-band rates are substantially faster. All relaxation rates increase with further increases in concentration, but the effect is stronger at 1.8 Gc/sec, so that the rates at both frequencies become equal at 1.7%. Still higher concentrations give rise to behavior not describable by a simple relaxation rate.For low concentration, the theoretical fourth-power frequency dependence of the direct relaxation process is not verified directly because the low frequency rates are always dominated by the Raman process; however, the frequency dependence must be as at least the third power to be consistent with the data. Some attempts are made to interpret the concentration dependence in terms of cross relaxation between single ions and coupled pairs.