Evidence for multimagnon-mediated nuclear spin relaxation in the   intertwining double-cain ferrimagnet $Ca_3 Cu_3 (PO_4)_4$
  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 $1/T_1$. 
Shoji Yamamoto, Hiromitsu Hori, Yuji Furukawa, Yusuke Nishisaka,   Yuzuru Sumida, Kiyotaka Yamada, Ken-ichi Kumagai, Takayuk...