Spin-lattice relaxation in the polymer resin poly-4-vinylpyridine doped with transition ions Cu2+, Cr3+, Mn2+, and Gd3+ possessing weak spin-orbit coupling

The temperature dependency of the spin-lattice relaxation rate (T-1(-1)) in poly-4-vinylpyridine (P4VP) resin samples doped with 0.5-5.0 % weight conc entrations of ions characterized by weak spin-orbit coupling, Cu2+, Cr3+, M n2+, and Gd3+ was investigated over the range 4.2-250 K. It was found to be quadratic and linear at very low and intermediate temperatures, respective ly, well accounted for by the exchange interaction between ions modulated b y lattice vibrations, the ions being distributed uniformly over the sample. At higher temperatures, and with a concentration of up to 3%, the relaxati on rate became almost independent of temperature, described well by cross r elaxation between exchange-coupled pairs of spins and individual spins. At concentrations higher than 3%, the Bloembergen-Wang three-reservoir model w as found to account for the data satisfactorily at all temperatures. [S0163 -1829(99)01614-8].