On the 90° Exchange Interaction between Cations (Cr3+, Mn2+, Fe3+ and Ni2+) in Oxides

Some empirical relations between the bond angle (of M–O 2- –M) and the asymptotic Curie temperature (\ (\) ) or Néel temperature (T N) are derived from data for oxides containing Cr 3+, Mn 2+, Fe 3+ or Ni 2+ ions. Further, these relations seem to be explained through assuming the presence of two types of exchange interactions; one is a 90° superexchange interaction which obeys the rule proposed by Kanamori and Goodenough, the other a direct exchange interaction which is sensitive to the overlap between neighboring two d ε orbitals. In oxides containing Cr 3+ or Mn 2+ the above two types of exchange are effective, while in oxides containing Fe 3+ or Ni 2+ only the former (superexchange) effective.