Nuclear magnetic resonance in [N(CH3)4]2CoCl4 single crystals: transferred hyperfine interaction and spin-lattice relaxation rate

The molecular susceptibility and paramagnetic shift of [N(CH3)4]2CoCl4 single crystals were measured, and from these experimental results we obtained the transferred hyperfine interaction, Hhf, due to the transfer of spin density from Co2+ ions to [N(CH3)4]+ ions. The transferred hyperfine interaction can be expressed as a linear equation, with Hhf increasing with increasing temperature. The remarkable change in Hhf near Tc5 (=192 K) corresponds to a phase transition. The proton spin-lattice relaxation times of [N(CH3)4]2CoCl4 single crystals were also investigated, and it was found that the relaxation process can be described by a single exponential function. The variation of the relaxation time with temperature undergoes a remarkable change near Tc5, confirming the presence of a phase transition at that temperature. From the above results, we conclude that the increase in Hhf with increasing temperature is large enough to allow the transfer of spin density between Co2+ ions and the nuclear spins of the nonmagnetic [N(CH3)4]+ ions in the lattice, and thus the increase in the relaxation time with temperature is attributed to an increase in the transferred hyperfine field.