Approaching Ferrite-Based Exchange-Coupled Nanocomposites as Permanent Magnets

Area of Science:

• Materials Science
• Nanotechnology
• Magnetism

Background:

• CoFe₂O₄ (hard)/Co-Fe alloy (soft) magnetic nanocomposites are typically made by partially reducing CoFe₂O₄ nanoparticles.
• Monoxide byproducts (FeO or CoO) are often detected, but their formation timing during reduction is unclear.

Purpose of the Study:

• To investigate the in situ formation of monoxide during the reduction of CoFe₂O₄ nanoparticles.
• To correlate the reduction stages with changes in magnetic properties.
• To determine the elemental composition of the phases formed during reduction.

Main Methods:

• In situ synchrotron powder X-ray diffraction (PXRD) with sequential Rietveld refinements.
• Vibrating sample magnetometry (VSM) for magnetic property measurements.
• Ex situ high-resolution PXRD and neutron powder diffraction (NPD) for elemental composition analysis.

Main Results:

• Monoxide was confirmed as an intermediate phase formed during the reduction process.
• Magnetic softening was observed with increasing soft phase content, exceeding predictions based solely on soft material introduction.
• The Co-Fe alloy formed was cobalt-rich, leading to cobalt deficiency in the remaining spinel phase.

Conclusions:

• The in situ PXRD study elucidates the intermediate role of monoxide in the reduction of CoFe₂O₄.
• The observed magnetic softening is significantly influenced by the cobalt deficiency in the spinel phase, not just the presence of the soft alloy.
• Understanding these formation dynamics is crucial for tailoring magnetic nanocomposite properties.