Covalency and exchange polarization in MnCO3

Abstract

A theory is given of the spin density distribution of the ferromagnetic component in canted antiferromagnets having a molecular ligand of the type AX3. It is found that for the well-known types, such as CO32-, NO3-, no appreciable spin is covalently transferred to the central ion. However, a great exchange polarization of the AX3 radical will occur and produce a negative spin density around the A ion. The theory is compared with experimental neutron scattering data on MnCO3. The analysis shows that f, the spin transfer parameter for Mn2+ in this salt, has a value of 6%; this is to be contrasted with values of the order of 1% observed in salts of a less covalent character than MnCO3. The analysis also shows that the magnetic moments on the oxygen and the carbon ions are of roughly equal magnitude, but of opposite sign; this is in qualitative agreement with the theoretical discussion of the exchange polarization in the CO32- radical. The existence of the aspherical Kaplan spin distribution is discussed, and it is concluded that the experiments with this salt are not yet precise enough to give evidence of the effect. The analysis is based only on the neutron data and shows a fair agreement with the magnetization data for the total canted moment.