EPR of Mn²⁺pairs in MgO and CaO

Nearest neighbour Mn 2+ pairs in MgO and CaO have been studied using EPR. The isotropic exchange interaction measured in MgO cannot be written in the simple form Js i .s j and it is shown that although a small contribution due to intrinsic biquadratic exchange is possible, the anomalous effect arises mainly as a result of exchange striction. This exchange striction is found to be consistent with a variation of J with ionic separation r given approximately by J=J 0 exp(-19r/r 0 ), and this variation gives good agreement with exchange interactions deduced for MnO. Exchange striction effects also appear in the crystal field interactions and in the magnetic dipole-dipole interactions for both pair systems.