Study of Correlation Effects in the High Formal Oxidation State Compound Sr${}_2$CoO${}_4$

Abstract

Two recent reports confirm that the newly synthesized Sr${}_2$CoO${}_4$ (formal oxidation state Co${}^{4+}$) shows a high Curie temperature (~ 250 K), but they report different moments of 1.8 ${\mu }_B$ and 1 ${\mu }_B$ per Co. Using both commonly used functionals in the correlated band approach (LDA+U) as well as the local density approximation (LDA),the combined effects of correlation and hybridization with O 2p states are calculated and analyzed. Sr${}_2$CoO${}_2$ is already ferromagnetic within LDA (M=1.95 ${\mu }_B$). Increasing U from zero, the two LDA+U schemes affect the moment oppositely out to a critical value $U_c$=2.5 eV, at which point they transform discontinuously from different states to the same large U state. Fixing U at $U_c$, fixed spin moment calculations show similar behavior out to a minimum at 1${\mu }_B$ (a half metallic state), beyond which the fully-localized-limit scheme jumps to a state with energy minimum very near 2${\mu }_B$ very close to the LDA moment). Although the energy minima occur very near integer values of the moment/Co (1${\mu }_B,2$μ_B$), the strong 3d-2p mixing and resulting 3d orbital occupations seem to preclude any meaningful S=1/2 or S=1 assignment to the Co ion.

10 pages and 10 embedded figures

References in corpus (1)

• Disproportionation Transition at Critical Interaction Strength: Na${}_{1/2}$CoO${}_2$

Cited by in corpus (1)

• Anisotropy and Magnetism in the LSDA+U Method