Approaching Ferrite-Based Exchange-Coupled Nanocomposites as Permanent Magnets

TL;DR

This study investigates the reduction process of CoFe2O4-based nanocomposites, revealing the formation of monoxide intermediates and their impact on magnetic properties, using in situ synchrotron PXRD and neutron diffraction.

Contribution

It introduces a novel in situ reaction cell for monitoring reduction and provides detailed insights into phase evolution and magnetic softening mechanisms in ferrite-based nanocomposites.

Findings

Monoxide forms as an intermediate during reduction.

Magnetic softening increases with soft phase content.

Co-rich alloy formation causes Co deficiency and influences magnetic properties.

Abstract

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.