Two-phonon spin-lattice relaxation of rigid atomic clusters
  Spin-phonon relaxation due to two-phonon processes of a spin cluster embedded in an elastic medium has been studied. For the case of uniaxial anisotropy, relaxation rates due to Raman processes and processes involving the emission of two phonons have been obtained. For a biaxial spin Hamiltonian, the rates of transitions between tunnel-split levels have been computed. By comparison with the rates of corresponding direct processes, we have established temperature ranges where the Raman mechanism dominates over the one-phonon decay mechanism. 
C. Calero, E. M. Chudnovsky, and D. A. Garanin