Nuclear magnetic relaxation of H-1 in the organic radical salts m-MPYNN+center dot ClO4- and m-MPYNN+center dot BF4-

We have measured nuclear spin-lattice relaxation limes T-1 of H-1 in the or ganic salts mMPYNN(+). ClO4- and m-MPYNN+. BF4- in the temperature range be tween 2 K and 100 Ii under external fields up to 70 kOe. In these salts, tw o cationic (spin-1/2) radicals constitute a dimer due to the ferromagnetic interaction J(1). We interpreted the experimental results of T-1(-1) below about 10 K (similar to J(1)/k(B)) in terms of two relaxation processes base d on the energy levels of the ferromagnetic dimer. The flip-flop and the di rect processes which are due to longitudinal and transverse spin fluctuatio ns become dominant at higher and lower fields, respectively, the crossover field being around 10 kOe. For the field dependence of T-1(-1) at higlrer t emperatures, the spin diffusive model of spin-1/2 in a two-dimensional syst em is satisfactorily applied.