Abstract
The influence of lattice distortion, canting of magnetic moments, and doping on the exchange coupling and the electronic structure of La1−xCaxMnO3 was studied using a generalization of the linear muffin-tin orbital (LMTO) method for noncollinear magnetic ordering1 and the LMTO–Green function technique in single-site approximation.2 Jahn–Teller type lattice distortions (f), including rotation and tilting of the oxygen octahedra, were varied from the ideal orthorhombic phase (f=0) to the observed structure3 (f=1). The canting angle (Θ) is considered also as a simulation parameter which is varied from Θ=0° ferromagnetic (FM) phase to Θ=180° antiferromagnetic (AFM). The effect of doping was simulated for x=1.,0.5,0 ordered alloys. We found that (i) doping and change of the magnetic ordering (from FM to AFM) promotes the competition between positive eg and negative t2g contributions to the exchange coupling between ferromagnetic sheets (J⊥); (ii) the J⊥ exchange is strongly of non-Heisenberg type, the bilinear contribution changes sign from positive to negative at f∼0.75; (iii) the metal–semiconductor transitions occur for Θ⩾120° and for distortions f⩾0.6.
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