In the present study, iron-containing pyrochlores of the A 2 FeBO 7 type (where A=Y, Gd, Eu and Dy; B=Sb and Nb) were synthesized by solid-state reaction of precursor oxides. The compounds were structurally and magnetically characterized by X-ray diffraction, Mössbauer spectroscopy and magnetic measurements. The results revealed that the crystallographic structures and magnetic properties depend, basically, on the type of B atoms.
Metal-oxide composites were synthesized by high-energy ball milling of metallic iron ͑␣-Fe͒ and alumina ͑␣-Al 2 O 3 ͒ powders, varying the starting relative concentration and the milling time. The samples were characterized by scanning electron microscopy, x-ray diffraction, and Mössbauer spectroscopy. The results revealed the formation of a FeAl 2 O 3+W spinel phase (hercynite) and of iron (super)paramagnetic nanoprecipitates, in addition to residual magnetic iron and alumina. We also observed that the relative amounts of nanoprecipitates and hercynite for isochronally milled samples were correlated with the sample nominal concentration x, with the precursor iron being relatively more converted in those phases for low x values. Particularly for x = 10 milled sample, the relative amounts of the (super)paramagnetic and spinel phases were observed to increase linearly with the milling time. An x = 20/ 24 h milled sample was annealed in H 2 atmosphere and revealed the reduction of hercynite, with iron phase separation.
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