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Despite being difficult to identify, extremely dilute oxygen vacancies have been widely reported to play an important role in enhancing magnetism in ZnFe 2 O 4. The mechanisms underlying this enhanced magnetism have not been well understood for a long time and remain controversial because the formation of oxygen vacancy-rich ZnFe 2 O 4 can be accompanied by changes in the chemical/physical characteristics, especially the composition, particle size, surface morphology and cation distribution, which can significantly affect the magnetization. An open and important question is whether and to what extent the enhanced magnetization can be attributed only to oxygen vacancies. In this study, the relationship between the magnetization and oxygen vacancies in ZnFe 2 O 4 was definitively determined by using a carefully designed "shake-and-heat" treatment to prepare vacancy-rich samples while keeping the other crystal/surface parameters constant. Compared to the nearly vacancy-free paramagnetism samples, the vacancy-rich samples exhibited a higher magnetization of approximately 5 emu/g at both 300 K and 2 K. The Fe3+-O2--Fe3+ superexchange paths broken by oxygen vacancies then resulting in the Fe3+-Fe3+ ferromagnetism configuration. Meanwhile, the oxygen vacancy is highly diluted then the ferromagnetism configuration is confined in a single super-cell, favoring a short-range magnetic ordering at room temperature. The concentration of oxygen vacancies was calculated to be 0.68% by magnetization measurement. Our results may shed a light on how oxygen vacancies affect magnetism. [ABSTRACT FROM AUTHOR]

Bi 25 FeO 39 microcrystals were synthesized via a one-pot hydrothermal process without additives, where their hierarchical structures could be designed from tetrahedron to orchid and marigold by the increment of pH values and reaction time. For all types of as-prepared Bi 25 FeO 39 , a high crystallinity and purity was confirmed in XRD and XPS analysis. A good thermal stability up to 1000 °C was revealed by DSC-TG with two endothermal peaks corresponding to decompositing and melting. A magnetic transition between ferromagnetism and paramagnetism around 36.3 K was distinguished from thermomagnetic curves as well as hysteresis loops at 5 K and 300 K respectively. In contrast to tetrahedron and orchid shaped Bi 25 FeO 39 , marigold shaped Bi 25 FeO 39 exhibited a higher magnetization up to 4.8 emu/g at 5 K and enhanced photocurrent density around 0.37 μA/cm2. The special crystal arrays and increased concentration of Fe should be responsible for it. This work paves an attractive way for facile preparation and potential applications of sillenite materials. [ABSTRACT FROM AUTHOR]