Valence and spin states, and the metal-insulator transition in ferromagnetic La2-x Srx MnNiO6 (x=0,0.2)

Electronic structures of a ferromagnetic insulator La2 MnNiO6 and Sr-doped La1.8 Sr0.2 MnNiO6 have been investigated by employing soft x-ray synchrotron-radiation spectroscopies, first-principles band-structure calculations, and ligand-field multiplet calculations. Our study provides evidence that La2 MnNiO6 has the [high-spin (HS) Mn4+ - A3 2 Ni2+] configuration while La1.8 Sr0.2 MnNiO6 has [HS Mn4+ - A3 2 Ni2+ low-spin Ni3+]. It is directly observed that the spin moments of Mn and Ni ions are coupled ferromagnetically. The metallic spectral weight is observed for x=0.2 in 01s x-ray absorption spectroscopy, which is consistent with the half-metallic ground state predicted by band-structure calculations.