EPR of Mn2+pairs in MgO and CaO

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Abstract
Nearest neighbour Mn2+ pairs in MgO and CaO have been studied using EPR. The isotropic exchange interaction measured in MgO cannot be written in the simple form Jsi.sj 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=J0 exp(-19r/r0), 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.