Electron Spin Relaxation in Copper Tutton Salts at Low Temperatures

By means of a pulse saturation technique applied at $X$-band frequencies, electron-spin relaxation times have been observed as a function of crystal size in the liquid-helium temperature range for the CuN${\mathrm{H}}_4$ Tutton salt throughout the concentration range 0.02-100%. The concentration range 12-100% exhibits what is thought to be a phonon bottleneck. This is manifest in a marked size dependence of the observed decay constants. Alternative explanations are considered. For the concentration range 0.05 to 5%, size-dependent decay times have also been recorded. There appears to be a correlation between this kind of size effect and contamination by the paramagnetic impurity of iron. Rapid crystal growth is found to alleviate the size effects throughout the range of concentrations studied. The effects of deuteration have been examined in crystals of 1% concentration and it was found that there was an order-of-magnitude difference in the relaxation times of the hydrated and deuterated salts, size for size, below the helium $\lambda$ point. The hydrated and deuterated CuK Tutton salts at the same concentration showed no such difference. The supposed spinlattice coupling is concentration-dependent, and its nature has not yet been established theoretically.