Ferromagnetic superexchange interactions

Moriya, T. 1960. Anisotropic superexchange interaction and weak ferromagnetism. Physical Review 120 91-98. 

EuO is an interesting oxide in the rocksalt family (Honig Van Zandt, 1975). It is a ferromagnetic T = 69 K) insulator when pure and stoichiometric. Unlike the M oxides, superexchange interaction in EuO is ferromagnetic because the charge transfer... 

The structure of KCrF3 is derived from the perovskite structure by a tetragonal distortion4. An antiferrodistortive coupling occurs between half-filled dz orbitals and empty d 2 yj orbitals. The superexchange interaction d°z y2 - p - dz2 leads to ferromagnetic layers, antiferromagnetically bound via the empty dx2 y2 orbitals. 

Fig. 86. Ferromagnetic T-M-T superexchange interactions, where T is a transition metal atom with half filled eg orbitals, M is an interstitial atom with np < 3. Excitation of an eg electron into a p orbital of M atom gives the three-electron configuration of Case 3, Figure 42. There is collective-electron spinpairing of the eQ and p electrons so that there is no localized moment on the intermediate metal atom.

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