Spin-lattice relaxation in two organic copper complexes

Spin-lattice relaxation measurements have been carried out by the pulse saturation technique on single crystals of copper (αα'Br) dipyrromethene and copper phthalocyanine, in the temperature range 1.4-4.2 °K and at a frequency of 9.375 Gc/s. With the former crystals, non-exponential recoveries were observed below 2.5 °K, and there was a temperature dependence of the relaxation time T₁ of the form (T₁)^-1 = AT² + BT⁹, T₁ being measured when thermal equilibrium was almost restored in the case of non-exponential recoveries. A phonon bottleneck at low temperatures was thus clearly indicated and the non-exponential recovery law, predicted by Faughnan and Strandberg for such a system, was found to fit the experimental results remarkably well. This law had not previously been the subject of rigorous experimental test.

With the phthalocyanine crystals, exponential recoveries were always found, with a law (T₁)^-1 = CTⁿ + DT⁹. For one specimen n = 1, but for two others 1 < n < 2. Phonon heating is shown to be an unlikely explanation of this phenomenon.