Nuclear Magnetic Relaxation of ¹H in the Organic Radical Salts m-MPYNN⁺·ClO₄^- and m-MPYNN⁺·BF₄^-

Abstract

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/k_B) 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, respec -tive -ly, 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.