Phase transformation and exchange bias effects in mechanically alloyed Fe/magnetite powders
Highlights
► 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.
Introduction
Granular systems of Fe nanoparticles embedded in iron oxide matrix have attracted a great deal of interest due to the observed magnetic exchange bias effect in such systems [1], [2], [3]. The exchange bias effect, defined as the shift of the hysteresis loop in ferromagnetic (FM)–antiferromagnetic (AFM) systems upon cooling down below the Néel temperature of the AFM phase [4], [5], has been shown to occur in granular ball-milled Fe/Fe oxide powders [1], [2]. It usually arises due to the coupling between FM and AFM grains at the interfaces, but has been revealed also in systems containing ferrimagnetic (FI) phases and disordered phases with spin-glass behaviour [3]. Therefore, the degree of structural disorder induced by the ball milling and the nanograin size distributions within the granular mixed FM/AFM powders are key factors for the exchange bias in granular systems. Mechanically alloyed mixtures of Fe and magnetite (Fe3O4) [6], and Fe and hematite (Fe2O3) [7] were shown to result in formation of wüstite (FeO) via a chemical reduction reaction occurring during the ball milling. Upon annealing FeO may decompose into the constituent powders [6]. The evolution of phase composition in such mixtures has a strong influence on the magnetic properties and the exchange bias effect, in particular and therefore needs to be strictly controlled during annealing, if the granular systems are to be used for technological applications.
We have processed a non-equimolar mixture of iron and magnetite by ball milling and we have studied the phase transformation that occurs during dynamic annealing in such systems via a unique tool of in situ characterization: temperature dependent X-ray diffraction of synchrotron radiation. Magnetic behaviour of as-milled and annealed samples are presented and interpreted in correlation with the phase composition in the samples.
Section snippets
Experimental
The Fe/Fe3O4 mixed powder (2:8 mass%) has been prepared by ball milling in a Retzsch PM 400 planetary ball mill with 4 vials. The total amount of the sample was 10 g, and the powders used were of high purity (99.99+%). The experimental conditions were: sun wheel frequency 200 rpm, sun wheel/vial frequency ratio 1.5, the loading constant 8; ball size 20 mm, ball-to-powder mass ratio 1/40 and number of tungsten carbide balls used was 8. The powder and balls were sealed together with hexane as
Structural phase transformation
In Fig. 1 we present the synchrotron XRD patterns taken at several, selected temperatures, from 45 °C to 450 °C. The pattern at 45 °C presents broad peaks, typical for nanostructured materials. We are able to identify the main Bragg reflections of the α-Fe, FeO (wüstite) and Fe3O4 (magnetite). As the initial mixed powders were α-Fe and Fe3O4 the observation of the Bragg reflections of the wüstite is the proof of an un-complete redox reaction that develops during the milling, in agreement with
Conclusions
The temperature evolution of the phase structure of a mixture of Fe and Fe3O4 (magnetite) powders, obtained by ball milling for 50 h at room temperature, has been studied by energy-dispersive in situ X-ray diffraction of synchrotron radiation. The nanocomposite powder is proven to undergo during preparation an incomplete redox reaction with formation of FeO. This reaction is favored due to the high energy developed during the milling and alloying. Two different stages of phase transformation are
Acknowledgements
The authors wish to acknowledge the help of Radu Nicula and Christian Lathe from MAX 80 beamline at HASYLAB during the XRD measurements. The financial support in the frame of the Marie Curie Host Development project HPMD-CT-2001-00089 as well as from Romanian Ministry of Education and Research via PN II project 12-129 is gratefully acknowledged.
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