\textit{Ab initio} study of the CE magnetic phase in half-doped manganites: Purely magnetic versus double exchange description
Roland Bastardis, Coen de Graaf, and Nathalie Guih\'{e}ry

The leading electronic interactions governing the local physics of the CE phase of half-doped manganites are extracted from correlated \textit{ab initio} calculations performed on an embedded cluster. The electronic structure of the low-energy states is dominated by double exchange configurations and O-2$p_{\sigma}$ to Mn-3$d$ charge transfer configurations. The model spectra of both a purely magnetic non-symmetric Heisenberg Hamiltonian involving a magnetic oxygen and two non-symmetric double exchange models are compared to the \textit{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.

© 2008 The American Physical Society.