Preparation and magnetic study of the CoFe₂O₄-CoFe₂ nanocomposite powders

Abstract

Ferri-ferromagnetic nanocomposites CoFe 2 O 4 CoFe 2 were prepared via reduction of the cobalt ferrite CoFe 2 O 4 into a hydrogen atmosphere and by heat treatments. This preparation method yielded powders with relative volume fraction of CoFe 2 O 4 and CoFe 2 in the range 0-0.91. The structure and the room temperature magnetization of the samples were analyzed by X-ray diffraction, scanning electron microscope (SEM) and transmission electron microscope (TEM), and by vibrating sample magnetometry, respectively. It was found that the saturation magnetization of the nanocomposite powders increases with the volume fraction of the ferromagnetic phase while their coercivity decreases. The highest value (5 kJ/m 3 ) for the maximum energy product ( ) max was obtained for the sample containing 91% in volume of CoFe 2 . This ( ) max is 19% higher than the value measured in pure CoFe 2 O 4 . The magnetic field dependence of the magnetization did also behave as the nanocomposite powders were single-phase materials. This result indicates that the hard ferrimagnet CoFe 2 O 4 and the soft ferromagnet CoFe 2 are effectively exchange coupled and that their magnetization reverses cooperatively in the range of compositions investigated.

Ferrite‐Based Exchange‐Coupled Hard–Soft Magnets Fabricated by Spark Plasma Sintering

“…Since BH max is the figure of merit for the permanent magnets obtained in this study, it will be discussed in the following paragraph. As mentioned, there are not many reports on the BH max values of hard–soft magnetic composites. According to the available data on the BH max values, the composites displaying the highest BH max were the ones prepared by Roy and Kumar .…”

“…Nevertheless, there are some reports that mention the maximum energy product of composites consisting of ferrites. BH max values of 5–10 kJ/m 3 have been reported for CoFe 2 shells on CoFe 2 O 4 cores and values of 10 kJ/m 3 for Sr‐ferrite/Fe 3 O 4 nanocomposites . In these cases the BH max was determined for the composites in a powder form, rather than for compacts or solid ceramics.…”

Approaching Ferrite-Based Exchange-Coupled Nanocomposites as Permanent Magnets

“…9−14 In particular, the CoFe 2 O 4 (hard)/Co–Fe alloy (soft) composite has been assiduously studied over the past few years. 15−24 Besides cobalt–iron alloys having the largest M s known at room temperature, 5 the system has drawn special attention because it can be prepared by partial reduction of CoFe 2 O 4 . This chemical route directly leads to coexistence of the two magnetic phases, and consequently, a greater crystallographic coherency between them is expected compared with mixing independently synthesized species to make the composite.…”

“…15−19 Other reduction agents have been used, e.g., activated charcoal 20 or CaH 2 . 21 These composites have also been made in the shape of dense ceramic materials by means of spark plasma sintering (SPS).…”

On the origin of remanence enhancement in exchange-uncoupled CoFe2O4-based composites

“…In the last years, research on this class of composites has proliferated and successful exchange-coupling and magnetic improvements have been claimed. [12][13][14][15][16][17][18][19] However, the physical mechanisms behind the phenomenology and the extent of effective exchange-coupling are not fully elucidated. 12,13,20 Given that CFO/FeCo composites have been mainly studied on polycrystalline powder form, which are complex systems that can accommodate broad particles size distributions and different compositions and interfaces within the same sample, it must be kept in mind that structural and compositional variations of the individual uncoupled compounds may be responsible for phenomenology that could mistakenly be interpreted as the fingerprint of exchange-spring.…”