Crystal structure, magnetic and transport properties, and electronic band structure ofA2Mn2O7pyrochlores(A=Y,In,Lu,and Tl)

Abstract

Crystal structures, magnetic and transport properties, and electronic band structures for four compounds with the pyrochlore structure, ${\mathrm{Tl}}_2{\mathrm{Mn}}_2{\mathrm{O}}_7,$ which exhibits colossal magnetoresistive behavior, and insulating $A_2{\mathrm{Mn}}_2{\mathrm{O}}_7\hspace{0.5em}(A=\mathrm{Y},\mathrm{}\mathrm{In}$ and Lu), are discussed. The cubic unit cell dimensions increase with increasing ionic size of the $A^{3+}$ ions. The ferromagnetic behavior is consistent with a superexchange interaction of ${\mathrm{Mn}}^{4+}$ spins with an intermediate Mn-O-Mn bond angle of about $130°.$ In ${\mathrm{In}}_2{\mathrm{Mn}}_2{\mathrm{O}}_7$ and ${\mathrm{Tl}}_2{\mathrm{Mn}}_2{\mathrm{O}}_7,$ strong hybridization of the $\mathrm{In}\mathrm{}\left(5s\right)/\mathrm{Tl}\mathrm{}\left(6s\right)\mathrm{}$ orbital with $\mathrm{O}\mathrm{}\left(2p\right)\mathrm{}$ and $\mathrm{Mn}\mathrm{}\left(3d\right)\mathrm{}$ may enhance the ferromagnetic interaction, leading to high transition temperatures over 120 K. The metallic behavior of ${\mathrm{Tl}}_2{\mathrm{Mn}}_2{\mathrm{O}}_7$ also originates from the hybridization among $\mathrm{Tl}\mathrm{}\left(6s\right),\hspace{0.5em}\mathrm{O}\mathrm{}\left(2p\right),$ and $\mathrm{Mn}\mathrm{}\left(3d\right).$ The free-electron-like minority down-spin band crosses the Fermi energy, leading to spin-polarized conduction. At the ferromagnetic Curie temperature, where majority local spins align, spin-polarized conduction electrons are produced: Thus, a ferromagnetic-and-metallic state appears in the ${\mathrm{Tl}}_2{\mathrm{Mn}}_2{\mathrm{O}}_7$ pyrochlore.