A Theory of Perovskite-Type Manganites (La,M(II))MnO₃
Goodenough, John B.
Semicovalence and its effects on indirect magnetic-exchange interaction are reviewed and applied to the manganites. These considerations lead to qualitative predictions which are in complete accord with the following experimentally observed facts, where x is taken as the percentage of manganese ions which are Mn[superscript 4+]: (1) At x = 0, LaMnO₃ is an orthorhombic perovskite-type lattice with a₁ = a₃ > a₂. The ratio a₂/a₃ increases to 1 at x = 1/4, and for 0.4<x<0.75 the lattice is tetragonal. For x>0.75 the lattice is cubic. (2) At x = 0 the lattice is composed of ferromagnetic layers parallel to a (110) face perpendicular to the a₂ axis; these layers are stacked antiferromagnetically. As x increases to 1/4, the lattice becomes ferromagnetic. For x>0.4 various antiferromagnetic phases form, the magnetic configurations varying with x. (3) In the range 0.25≤x≤0.35 the saturation moment corresponds to ferromagnetically coupled spin-only values of the manganese ions; it drops off sharply toward zero around x = 0.1 and x = 0.5. (4) The electrical resistivity is a minimum and the Curie temperature a maximum at x≈0.3. (5) The maximum Curie temperature increases markedly from (La,Ca)MnO₃ to (La,Sr)MnO₃ and (La,Ba)MnO₃.