Mechanisms of paramagnetic relaxation

An experimental study has been made of the paramagnetic spin-lattice relaxation properties of some transition metal ions. Three trivalent metal ions were studied, namely those of titanium, chromium and ruthenium, in the form of acetyl-acetonate compounds of the general formula M (C(_5)H(_7)Q(_2))(_3). By the use of the corresponding diamagnetic aluminium or cobalt compounds, mixed single crystals were grown with various paramagnetic concentrations. Measurements were made at low temperatures in the liquid helium range, at X band microwave frequencies (9.3 KMc/s). Some measurements were also made at higher microwave frequencies (35.5 KMc/s).In all cases the relaxation behaviour observed in magnetically dilute crystals is in good agreement with the well established theories relating to single ion relaxation. Particular attention has been given however to the study of the relaxation processes in more concentrated crystals, where concentration dependent relaxation is found. The results obtained have been compared with the two theoretical suggestions that have been proposed to account for these effects. Although neither proposal is entirely satisfactory, the one involving exchange interactions within paramagnetic ion clusters is shown to account in a qualitative way for the observed behaviour. A detailed study of the E.F.R. spectrum of chromium doped crystals has been made which shows that significant exchange interactions do exist in these materials.