Exchange-bias features in nanoceramics prepared by spark plasma sintering of exchange-biased nanopowders

Abstract

Conventional methods for sintering magnetic biased granular solids are generally known to induce severe grain growth and coarsening, thus making any evidence of exchange bias (EB) hard to detect in the resulting consolidates. This work explores the possibility of preparing a hetero-nanoconsolidate starting from an exchange-biased nanopowder and using Spark Plasma Sintering (SPS), in order to overcome that microstructural limitation. It also investigates the capability of the resulting solid to establish EB. The nanopowder consists of an intimate nanocrystalline mixture of the antiferromagnetic NiO and ferrimagnetic CoFe₂O₄ phases. It exhibits a significant broadening of the hysteresis loop at room temperature when measured under field cooling (FC) conditions. X-ray diffraction coupled to high resolution transmission electron microscopy and ⁵⁷Fe Mössbauer spectrometry shows that sintering induces a drastic atomic diffusion leading to the replacement of the starting NiO and CoFe₂O₄ phases by their Ni_1−xCo_xO and Co_1−xNi_xFe₂O₄ solid solutions (x value close to 0.25). As a result, exchange-bias is hindered at 300 K but always expressed at lower temperatures, far below the Néel temperature of the new antiferromagnet. The maximum EB value for the ceramics is measured at 5 K and is calculated about 48 mT under a cooling magnetic field of 7 T. This value decreases when the temperature increases, but it remains different from zero up to 200 K.