Spin-Spin Interaction in the Paramagnetic Salts

The spin-spin interaction via phonon exchange is proposed as a new mechanism of the so-called exchange interaction between the electron spins of the paramagnetic ions in crystal which has been overlooked hitherto. The interaction Hamiltonian derived by Van Vleck to account for the spin-lattice relaxation phenomenon is used for the numerical estimate. It is shown that the interaction through phonon exchange may have the same order of magnitude as the dipolar coupling in some of the paramagnetic salts. This interaction is temperature independent and at large distances falls off as inverse cube power of the inter-atomic distance. In nickel Tutton salts, the magnitude of the interaction is about 10 -3 cm -1 at the distance 12A which is comparable to that of the dipolar interaction.