Magnetic phase diagram of Sr3Fe2O7−δ

Magnetometry, electrical transport, and neutron scattering measurements were performed on single crystals of the Fe${}^{4+}$-containing perovskite-related phase Sr${}_{3}$Fe${}_{2}$O${}_{7\ensuremath{-}\ensuremath{\delta}}$ as a function of oxygen content. Although both the crystal structure and electron configuration of this compound are closely similar to those of well-studied ruthenates and manganates, it exhibits very different physical properties. The fully oxygenated compound ($\ensuremath{\delta}=0$) exhibits a charge-disproportionation transition at ${T}_{D}=340$ K, and an antiferromagnetic transition at ${T}_{N}=115$ K. For temperatures $T\ensuremath{\le}{T}_{D}$, the material is a small-gap insulator; the antiferromagnetic order is incommensurate, which implies competing exchange interactions between the Fe${}^{4+}$ moments. The fully deoxygenated compound ($\ensuremath{\delta}=1$) is highly insulating, and its Fe${}^{3+}$ moments exhibit commensurate antiferromagnetic order below ${T}_{N}\ensuremath{\sim}600$ K. Compounds with intermediate $\ensuremath{\delta}$ exhibit different order with lower ${T}_{N}$, likely as a consequence of frustrated exchange interactions between Fe${}^{3+}$ and Fe${}^{4+}$ sublattices. A previous proposal that the magnetic transition temperature reaches zero is not supported.