Significance of the direct relaxation process in the low-energy spin dynamics of a one-dimensional ferrimagnet NiCu(C7H6N2O6)(H2O)3·2H2O

In response to recent nuclear magnetic resonance measurements on a ferrimagnetic chain compound NiCu(C7H6N2O6)(H2O)3·2H2O [Solid State Commun. 113 (2000) 433], we calculate the nuclear spin–lattice relaxation rate 1/T1 in terms of a modified spin-wave theory. Emphasizing that the dominant relaxation mechanism arises from the direct (single-magnon) process rather than the Raman (two-magnon) one, we explain the observed temperature and applied-field dependences of 1/T1. Ferrimagnetic relaxation phenomena are generally discussed and novel ferrimagnets with extremely slow dynamics are predicted.