Phase transformation and exchange bias effects in mechanically alloyed Fe/magnetite powders

Nanostructured powders processed by ball milling of a mixture of Fe and Fe3O4 at room temperature are shown to undergo an incomplete redox reaction with formation of FeO during the milling process. This reaction is favored by the high energy introduced during the mechano-alloying process. Concurrent effects of milling such as grain refinement down to the nanometre scale lead at the end of the milling processes to a mixed multiphase powder of nanograins, with Fe and Fe oxide grains inter-dispersed. We show that in the as-milled Fe/Fe3O4 powder, during milling process, wüstite (FeO) is formed as a consequence of the redox reaction. Moreover, with increasing temperature, the system undergoes an inverse phase transformation towards the initial Fe and Fe3O4 phases until about 450 °C. Above this temperature the reduction reaction Fe + Fe3O4 = 4FeO is reinitiated, resulting in sharp decrease of Fe and Fe3O4 content from about 550 °C and almost complete disappearance of these phases at about 900 °C. This transformation was investigated via an energy-dispersive in situ X-ray diffraction experiment using the synchrotron radiation. This study allows direct collection of X-ray patterns after few minutes exposure, at selected temperatures, ranging between 20 °C and 1000 °C. The structural and magnetic characterizations of the nanograin powders, as-milled and annealed at several temperatures, are studied using XRD, SEM and magnetic measurements. Such ferromagnetic–antiferromagnetic composites are extensively studied as they exhibit exchange bias effect, with a large impact in technological applications. The magnetic behaviour and intrinsic mechanisms leading to the occurrence of exchange bias effects are discussed and related to the samples microstructural features. A significant exchange bias effect, related to FeO content, is observed for as-milled sample, the effect being less pronounced upon annealing the nanograin powder.

► Phase evolution in Fe/Fe3O4 nanopowder monitored by synchrotron X-ray diffraction.
► Fe/Fe3O4 nanopowder undergo an incomplete redox reaction with formation of FeO.
► FeO decomposes gradually into initial constituents at T up to 500 °C.
► At higher T the redox reaction is reversible, at 900 °C only FeO is observed.
► For the first time a strong exchange bias effect, related to FeO content, is observed.