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

We have measured nuclear spin-lattice relaxation times T1 of 1H in the organic salts m-MPYNN+·ClO4- and m-MPYNN+·BF4- in the temperature range between 2 K and 100 K under external fields up to 70 kOe. In these salts, two cationic (spin-1/2) radicals constitute a dimer due to the ferromagnetic interaction J1. We interpreted the experimental results of T1-1 below about 10 K (∼J1/kB) in terms of two relaxation processes based on the energy levels of the ferromagnetic dimer. The flip-flop and the direct processes which are due to longitudinal and transverse spin fluctuations become dominant at higher and lower fields respectively, the crossover field being around 10 kOe. For the field dependence of T1-1 at higher temperatures, the spin diffusive model of spin-1/2 in a two-dimensional system is satisfactorily applied. (author)