Magnetic and orbital order in the A-phase of manganites

The A-phase of La1-xCaxMnO3, stable for small x, is studied using a mean-field slave-boson formulation for eg-electrons in two orbitals with excluded multiple occupancy. The classical t2g-spins are ordered ferromagnetically (F) in the xy planes, which are antiferromagnetically (AF) coupled along the z-direction. All 3d-electrons are coupled F via Hund's rule. The F double-exchange, competing with the AF superexchange, can lead to a lattice distortion via Jahn-Teller coupling of the eg-electrons to the lattice degrees of freedom. Conditions for staggered orbital long-range order are discussed.