Ab initio study of the CE magnetic phase in half-doped manganites:: Purely magnetic versus double exchange description

The leading electronic interactions governing the local physics of the CE phase of half-doped manganites are extracted from correlated ab initio calculations performed on an embedded cluster. The electronic structure of the low-energy states is dominated by double exchange configurations and O 2p(sigma) to Mn 3d charge transfer configurations. The model spectra of both a purely magnetic nonsymmetric Heisenberg Hamiltonian involving a magnetic oxygen and two nonsymmetric double exchange models are compared to the ab initio one. While a satisfactory agreement between the Heisenberg spectrum and the calculated one is obtained, the best description is provided by a double exchange model involving excited non-Hund atomic states. The energy eigenvalues of this refined double exchange model are derived as a function of the local lattice distortions, and it is shown that in the absence of non-Hund states, an incorrect ground state is obtained.