Magnetocaloric properties of La0.67Ca0.33MnO3 produced by reactive spark plasma sintering and by conventional ceramic route

The structural properties of ferromagnetic La0.67Ca0.33MnO3 perovskite ceramics, produced by reactive spark plasma sintering (R-SPS) and conventional solid state reaction (SSR) methods, have been investigated by combined x-ray diffraction (XRD), Mn K-edge x-ray absorption near-edge structure (XANES) and magnetometry. All the samples are single phases and crystallize in the orthorhombic structure with a Pbnm space group. A slight unit cell dilatation is observed in the R-SPS sample compared to the SSR sample. The qualitative and quantitative analyses of the XANES spectra of both samples allow us to attribute this discrepancy to a manganese electronic state change in relation to the used experimental reaction conditions: the Mn3+/Mn4+ atomic ratio is found to be equal to 0.74/0.26 and 0.66/0.34, for the R-SPS and SSR samples, respectively, the former being significantly different from that expected for stoichiometric manganite. R-SPS material processing conditions affect also the microstructure of the considered manganite. The produced ceramic is dense and fine grained, with an average grain size of about 100 nm. Consequently, the measured values of the Curie temperature, T-C, and of the saturation magnetization, M-s, at low temperature decrease significantly in the former compared to the latter. The maximum of the magnetic entropy change, -Delta S-M(Max), is lower in the R-SPS sample than in the SSR sample, but the thermal variation of -Delta S-M is broader, resulting in a higher relative cooling power (RCP) in the former.