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.
Highlights
Over the last few decades, the increasing world requirements for more efficient and cheaper electronic devices have highlighted the need for advanced functional materials with energy-efficient fabrication and integration processes
The sintering conditions might favor partial ferric cation reduction to the spinel lattice and cobalt de-mixing, leading to a chemical composition closer to Co2+1-xFe2+xFe3+ 2O4 than the initial Co2+1Fe3+2O4. This metallic contamination was observed in the composite ceramic XRD patterns and was assumed to originate from spinel de-mixing of transition metal elements
Even if the nano-structuration is conserved, the high energy involved in the Spark-Plasma Sintering (SPS) process, along with the presence of a pulsed DC current and reductive atmosphere, induces atomic diffusion of all the phases
Summary
Over the last few decades, the increasing world requirements for more efficient and cheaper electronic devices have highlighted the need for advanced functional materials with energy-efficient fabrication and integration processes. The integration of magnetically contrasted composite materials in such devices opens up the possibility of designing new systems with enhanced electromagnetic properties. The exchange interaction between F and AF nanomaterials, known as exchange bias (EB) coupling, can enhance the thermal stability of small F structures[1,2], overcoming their superparamagnetic limit[3]. In general both effects are observed simultaneously, due to structural defects or grain size dispersion, which produce local variations of the AF anisotropy
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