Approaching Ferrite-Based Exchange-Coupled Nanocomposites as Permanent Magnets

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.

Expanding the tunability and applicability of exchange-coupled/decoupled magnetic nanocomposites

“…This observation is in agreement with previous in situ investigations on this system, where the reduced phases were observed to appear in a Co-rich form, to later incorporate Fe and evolve towards Co:Fe = 1:2. 28 At the higher temperatures, CoFe coexists with other alloy phases, i.e., Co 2 Fe in {2h@4001C} and Co 2 Fe 3 in {2h@6001C}, showing that the Fe-content increases as the temperature rises. A similar phase segregation may be occurring at 300 1C, although the effect remains hidden under the broader diffraction peaks derived from the smaller crystallite sizes at this temperature, and in that case, the refined unit cell parameter should be understood as the weighted average of all the phases present.…”

“…At 300 1C, the growth of the soft phase crystallites remains relatively controlled (r30.4(2) nm) regardless of the dwell time. Increasing the treatment temperature accelerates the reduction process, 28 thus, 2 h at 400 1C led to lower CoFe 2 O 4 content than 2 h at 300 1C. The monoxide content also decreased substantially at 400 1C.…”

“…Unfortunately, the Co : Fe ratio could not be extracted from the refinements, because Co and Fe are next-neighbors in the periodic table and therefore, practically indistinguishable using X-rays (see ESI in ref. 28).…”

“…27 Moreover, CoFe 2 O 4 can be easily reduced to a Co-Fe alloy in the presence of a small concentration of H 2 gas and moderate temperatures (E300 1C). 28 Both facts make this compound interesting, as an incomplete CoFe 2 O 4 reduction directly leads to coexistence of hard (CoFe 2 O 4 ) and soft (Co-Fe) magnetic phases. This is an excellent tool from the material science viewpoint, as it offers the potential to fine tuning the soft/hard magnetic behavior of the produced material by means of controlling the composite composition.…”

“…For the above reasons, numerous studies on the CoFe 2 O 4 (hard)/Co-Fe (soft) composite are found in the literature, including composites prepared as powders, 29 dense pellets, 30 or thin films. 31 Some works have set the main focus on the preparation process (in situ studies), 28,32 while others have taken care of an in-depth structural characterization of the produced composites using spectroscopic techniques such as Raman 33 or Mössbauer spectroscopy. 34,35 Others have put great efforts on studying the inter-particle coupling from different perspectives, both using transmission electron microscopy (TEM), and measuring dm curves (Henkel plots).…”