  The nuclear spin-lattice relaxation time $T_1$ of $^{31}P$ nuclei in the
title compound is measured for the first time and interpreted in terms of a
modified spin-wave theory. We establish a novel scenario for one-dimensional
ferrimagnetic spin dynamics -- it is three-magnon processes enhanced by
exchange scattering, rather than Raman processes, that make the major
contribution to $1/T_1$.
