Paramagnetic Relaxation in Dilute Potassium Ferricyanide

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°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 $\frac{1}{T_1}=4.3\mathrm{}\times {10}^{-3\mathrm{}}{T}^9$ ${\sec }^{-1\mathrm{}}$ over the entire temperature range and over five decades of time. At 8.5 Gc/sec this rate is augmented by a direct rate $\frac{1}{T_1}=3.1\mathrm{}T$. 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.