Suchergebnisse

During the past decade, CoFe2O4 (hard)/Co–Fe alloy (soft) magnetic nanocomposites have been routinely prepared by partial reduction of CoFe2O4 nanoparticles. Monoxide (i.e., FeO or CoO) has often been detected as a byproduct of the reduction, although it remains unclear whether the formation of this phase occurs during the reduction itself or at a later stage. Here, a novel reaction cell was designed to monitor the reduction in situ using synchrotron powder X-ray diffraction (PXRD). Sequential Rietveld refinements of the in situ data yielded time-resolved information on the sample composition and confirmed that the monoxide is generated as an intermediate phase. The macroscopic magnetic properties of samples at different reduction stages were measured by means of vibrating sample magnetometry (VSM), revealing a magnetic softening with increasing soft phase content, which was too pronounced to be exclusively explained by the introduction of soft material in the system. The elemental compositions of the constituent phases were obtained from joint Rietveld refinements of ex situ high-resolution PXRD and neutron powder diffraction (NPD) data. It was found that the alloy has a tendency to emerge in a Co-rich form, inducing a Co deficiency on the remaining spinel phase, which can explain the early softening of the magnetic material. ; The authors thank financial support from the European Commission through the NANOPYME (FP7-SMALL-310516) and AMPHIBIAN (H2020-NMBP-2016-720853) projects. Financial support from the Danish National Research Foundation (Center for Materials Crystallography, DNRF-93) and the Danish Center for Synchrotron and Neutron Science (DanScatt) is gratefully acknowledged. Parts of this research were carried out at PETRA III at DESY, a member of the Helmholtz Association (HGF). We thank Jozef Bednarcik for assistance in using beamline P02.1. This work is based on experiments performed at the Swiss spallation neutron source SINQ, Paul Scherrer Institute, Villigen, Switzerland. This project has received funding from the European Union's Seventh Framework Programme for research, technological development, and demonstration under the NMI3-II Grant 283883. ; Peer reviewed