Energy Product Enhancement in Imperfectly Exchange‐Coupled Nanocomposite Magnets

Abstract

Interfacial exchange coupling is known to improve the permanent magnetic performance (i.e., maximal energy product) in composites of magnetically hard and soft particles. The prevailing strategy, employed in a plethora of compositions, consists in maximizing the coupling between the hard and soft phases and optimizing material parameters such as particle size or phase composition. In CoFe2O4–FeCo nanocomposites, it is experimentally shown that interparticle uncoupling in combination with the sizes of the soft phase grains below the single‐domain threshold leads to enhanced magnetic properties at room temperature, while maximizing exchange coupling implies a collapse in coercivity and hence in the maximal energy product. The results are corroborated by micromagnetic calculations and the origin of the exchange‐induced softening is discussed. It is emphasized that engineering interfaces in order to optimize, rather than maximize, the degree of exchange coupling are a necessary requirement to improve the energy product in nanocomposite magnets and to successfully develop advanced rare‐earth‐free permanent magnets.

Approaching Ferrite-Based Exchange-Coupled Nanocomposites as Permanent Magnets

“…Moreover, the monoxide (paramagnetic at room temperature, i.e., nonmagnetic) has previously been suggested to play a role in the H c of CoFe 2 O 4 /Co–Fe composites, acting as pinning sites for the domain wall. 19 The H c decreases down to 75(2) kA/m for the composite that follows in alloy wt % (i.e., 400 °C). This value is still above that of the nonreduced material, but the loss of 25 kA/m in H c seems excessive for the very small difference in alloy wt % between these two composites (alloy = 12.39(4) wt % for 350 °C and 14.80(3) wt % for 400 °C).…”

“…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).…”

Development of novel exchange spring magnet by employing nanocomposites of CoFe2O4 and CoFe2

“…For sample reduced at 500°C, in addition to the CoFe 2 O 4 phase, the peaks related to the FeO phase (marked with asterisks) are clearly observed. During the reduction process, the initial CoFe 2 O 4 spinel structure transforms to a metallic FeCo bcc phase through the formation of an intermediate monoxide (FeO) fcc structure [9]. For sample reduced at 550°C, the formation of metallic CoFe 2 is observed, yielding three phase coexistence: CoFe 2 O 4 -CoFe 2 -FeO.…”

“…Parameters such as grain shapes, grain (crystallite)-size distribution, or relative orientations of crystallites, which are difficult to quantify and control, play a decisive role [8]. In particular, the structural requirements associated with the effective inter grain coupling, such as interfacial coherency and sizes of soft grains of the order of a few nanometers, are often hard to meet in large scale production methods as it is difficult to maintain control of the material structure on a nanoscale [9]. The exchange spring behavior was found in the hard-soft ferrite composite magnets, such as BaFe 12 [13] and CoFe 2 O 4 / CoFe [14][15][16][17][18].…”

“…The exchange spring behavior was found in the hard-soft ferrite composite magnets, such as BaFe 12 [13] and CoFe 2 O 4 / CoFe [14][15][16][17][18]. Nanocrystalline CoFe 2 O 4 -based spring magnets has attracted considerable attention due to the fact that a partial reduction of cobalt ferrite leads to FeCo (soft)-CoFe 2 O 4 (hard) composites where crystallographic coherence, mandatory for effective coupling, is more likely than in systems fabricated using two dissimilar materials [9].…”