Theoretical study on the electronic and magnetic properties of double perovskite La_(2-x)Sr_xMnCoO_6 (x = 0,1,2)

摘要

In this paper, the electronic and magnetic properties of double perovskite La_(2-x) Sr_x MnCoO_6 (x = 0,1,2) have been studied using the local-spin-density approximation + U method. For the three compositions investigated, the low symmetry P2_1 structure yields consistently lower energy than that of the high symmetry Fm3?m structure. The strong electronic correlation and the orbital polarization of Co-d electrons play crucial roles. In agreement with experiments, we find that La_2MnCoO_6 is a ferromagnetic insulator with both Mn and Co ions in their high-spin states. The tilting of oxygen octahedrons is most significant in this case and is responsible for its insulating behavior; for LaSrMnCoO_6, the ground state remains a ferromagnetic insulator with Mn and Co ions in their high-spin states. The optimized P2_1 and Fm3?m crystal structures are nearly the same, and the P2_1 structure is stabilized by the spontaneous layer-wise antiferro-orbital ordering of Co-d electrons. We also predict that Sr_2MnCoO_6 is a ferromagnetic metal, and its electronic structure can be viewed as a rigid band shifting from that of LaSrMnCoO_6. Due to the strong covalency between transition metal and oxygen ions, the valences of Mn and Co ions differ considerably from those derived from purely ionic model. Also, doping induced holes mainly go to oxygen sites though the density of states near the Fermi energy has strong mixed character. This feature, together with the orbital ordering phenomenon, should be observable via the X-ray near-edge absorption spectroscopy and the polarized X-ray diffraction spectra.