Physics of manganites

In the chemical formula AMnO₃, oxygen has valence of -2. Thus manganese is trivalent when the ion on A site is +3 valenced, e.g. La³⁺. Substitution of ions on A sites by divalent ions (Ca²⁺, Sr²⁺, etc) results in mixed valenced manganese ions: Mn³⁺ and Mn⁴⁺. The trivalent and tetravalent Mn ions have very different properties in the lattice. The electronic configuration of a Mn³⁺ ion is 1s²2s²2p⁶3s²3p⁶3d⁴. Therefore, the 3d shell, being the only partially filled shell, is of interest. When placing the 4 electrons in the five 3d (l=2, m_l= -2, - 1, 0, 1, 2) orbitals, a few relevant interactions, within current context, need to be considered. The first is the on-site Coulomb repulsion energy U between electrons on the same site and same orbital (same m_l). U was estimated to be 5.2±0.3 eV and 3.5±0.3 eV for CaMnO₃ and LaMnO₃respectively, by Park et al.  [95], using photo-emission techniques. The second is the Hund (ferromagnetic) coupling between electrons on different orbitals. The exchange constant J_H was suggested to be larger than 1 eV from both experimental and theoretical studies [96,97,98]. Thus, as a result of the strong on-site coulomb repulsion, the four 3d electrons will have their spins lined up in parallel, and occupy 4 of the 5 orbitals, resulting in a total spin of S=2. In the case of Mn⁴⁺, we can simply remove one of the four electrons, resulting a total spin of S=3/2.

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Subsections

• Crystal field effect
• Jahn-Teller distortion
• Double exchange mechanism
• Semi-covalent exchange and the Goodenough-Kanamori rule
• Summary