On the limits of Reactive-Spark-Plasma Sintering to prepare magnetically enhanced nanostructured ceramics: the case of the CoFe2O4-NiO system

Abstract

Magnetic materials are crucial for the efficiency of the conversion-storage-transport-reconversion energy chain, and the enhancement of their performance has an important impact on technological development. The present work explores the possibility of preparing hetero-nano-structured ceramics based on magnetic oxides, by coupling a ferrimagnetic phase (F) with an antiferromagnetic one (AF) on the nanometric scale. The field-assisted sintering technique or SPS (Spark-Plasma Sintering), adopted at this purpose, ensures the preservation of nano-sized crystals within the final solid structure. The aim is to establish how exchange bias may affect the resulting nano-consolidates and to investigate the potential of this process to increase the total magnetic anisotropy of the CoFe2O4 grains, and thus their coercive field, while keeping the saturation magnetization the same. The structure, microstructure and magnetic properties of the ceramics obtained were studied by several techniques. The results show that the sintering process, along with its typical reductive atmosphere, modifies the composition of the constituents. A new metallic phase appears as a consequence of the reciprocal diffusion of Co and Ni cations, leading to a change in the amount and structure of the AF phase. We propose a schematic representation of the atomic movements that hinder an exchange bias effect between the F and AF phases.

On the first evidence of exchange-bias feature in magnetically contrasted consolidates made from CoFe2O4-CoO core-shell nanoparticles

“…The spectra are consistent with the signature of two main contributions, a sextet (major) and a doublet (minor) with an average isomer shift < δ > of 0.40 mm.s −1 and an average hyperfine field (<B hyp > of 49.0 T) (see Figure SI-4 in the supporting information section). The sextet is quite consistent with ferric cations involved in the ferrimagnetic spinel structure but some of them with a partial electron delocalization (explaining the inner wing of the left outer line), while the quadrupolar doublet must be assigned to paramagnetic ferrous species: they result probably from a small contamination by a paramagnetic phase non detected by XRD, such as siderite FeCO 3 (that would be compatible with the observation of traces of carbon as often reported on SPS sintered ceramics 44,45 ) and/or the replacement of the CoO phase by its solid solution Co 1-x Fe x O (x close to zero) (that would be compatible with the diffusion of iron cation from spinel to adjacent rock-salt grains because an interatomic diffusion in SPS sintered nanocomposites was also reported 34 ).…”

“…Indeed, SPS operates at moderate temperatures for short periods of time, favoring ultrafine grained and highly dense microstructures 26,27 . It is also a powerful solid-state chemical route for the preparation of single 28–31 and multi-phases 32–34 . The SPS technique could really overcome the limits highlighted above.…”

Exchange bias behavior in cobalt ferrite-cobalt oxide CoFe2O4/CoO nanocomposites for data storage applications

“…The exchange bias phenomenon is typically found in nanocomposites where antiferromagnetic (AFM) material is coupled with a ferromagnetic (FM) material. The coupling between two materials produces a shift in the hysteresis loop along the magnetic field axis [23][24][25]. This effect is significantly influenced by the shape of the nanoparticles.…”

Rapid Synthesis of W_18O_49 via Reactive Spark Plasma Sintering with Controlled Anisotropic Thermoelectric Properties

“…Furthermore, under the influence of uniaxial pressure, preferential grain orientation is likely to occur, which could yield extraordinary results for materials with highly anisotropic crystal structures 23,24) It has been reported that the dc pulse current during SPS is highly suitable for initiating and facilitating the reactions of precursors, yielding nanograined product 25) . This so-called reactive SPS can also synthesize materials within much shorter reaction time compared to conventional methods [25][26][27] . Thus, here we report onestep direct synthesis and simultaneous consolidation of W18O49 from commercially purchased precursors via reactive SPS.…”