Antiferromagnetic Superexchange Effect

Abstract

The delocalization effect recently proposed by Anderson as the principal contribution to the antiferromagnetic coupling of magnetic ions is analyzed by methods used in an earlier paper to derive the Heisenberg exchange operator formalism for both ferromagnetic and antiferromagnetic coupling. It is shown that Anderson's effect can be included, together with the superexchange correlation effect treated previously, as contributions to the Heisenberg exchange integral for ions of arbitrary spin. In the case of the antiferromagnetic oxides MnO, FeO, CoO, and NiO these two effects are found to be of comparable size, although the correlation effect is larger and determines the qualitative behavior of the Néel temperatures, which increase with decreasing spin through this sequence of transition metal ions. Agreement with the experimental Néel temperatures is significantly improved by including both effects.