Rotationally invariant theory of spin-phonon interactions in paramagnets

We propose a theory of spin-phonon interactions in paramagnetic materials. The theory is based upon the conservation of total, i.e., spin plus lattice, angular momentum. Rotational invariance of the Hamiltonian of the coupled system leads to new contributions to the spin-phonon interaction. These new contributions are determined by the anisotropy of the crystal field acting on the spin system. © 1972 The American Physical Society.