We have systematically investigated the magnetic properties of (Fe 1Ày Zn y ) 2 Mo 3 O 8 (0 y 1). The ground state of the parent compound, Fe 2 Mo 3 O 8 (y ¼ 0), is an antiferromagnetic (AFM) insulator. The substitution of magnetic Fe 2þ with nonmagnetic Zn 2þ gives rise to ferromagnetic (FM) correlation. With increasing y, the FM correlation evolves and becomes the most enhanced with y ¼ 0:5. One of the possible explanations for the origin of the FM behavior is that Zn 2þ ions prefer the octahedral oxygen coordination to the tetrahedral one. In addition, the magnetizations when y ¼ 0:125 and 0.25 show a stepwise behavior at low temperatures, implying that several magnetic phases are competing with one another.

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We have investigated the electronic structure of Sr 3 Fe 2−x Co x O 7−δ using photoemission and soft xray absorption spectroscopy (XAS). From the analysis on the O 1s, Fe 2p, and Co 2p core level photoemission spectra and the line-shape analysis on the Fe and Co L 2,3 edge XAS spectra, we concluded that the valence of Fe and Co are both ∼3.85+, the expected value. From the analysis of 2p−3d resonant photoemission spectra, we found that the electron density of states near the Fermi level has a dominant Fe 3d character and the Co 3d states are located below the Fe 3d states by ∼1 eV, in agreement with the smaller charge-transfer energy of Co 4+ than that of Fe 4+ .