Effect of Ni-doping on structural, magnetic and magnetocaloric properties of La0.6Pr0.1Ba0.3Mn1−xNixO3 nanocrystalline manganites synthesized by Pechini sol-gel method

The structural, magnetic and magnetocaloric properties of La0.6Pr0.1Ba0.3Mn1−xNixO3 (0 ⩽ x ⩽ 0.3) nanocrystalline manganites have been studied systematically. Samples were prepared using the Pechini sol-gel technique at 1223 K. Rietveld analysis of XRD shows the formation of pure crystalline phase with orthorhombic Pnma structure. Refinement results show that the unit cell volume and the average grain size decrease with increasing Ni content. An increase of the Ni content leads to the formation of a larger proportion of Mn4+ with respect to Mn3+. This reduces magnetization and the paramagnetic (PM)-ferromagnetic (FM) phase transition temperature (TC) from 215 K (for x = 0) to 131 K (for x = 0.3). The magnetocaloric effect has also been assessed by means of magnetic entropy change ΔSM which is basically determined from magnetic field dependences of magnetization at different temperatures M(H, T) near TC. Interestingly, the doping of a high amount of Ni in La0.6Pr0.1Ba0.3Mn1−xNixO3 nanocrystalline samples reduced the maximum magnetic entropy |ΔSMmax| from 1.97 J kg−1 K−1 (for x = 0) to 0.65 J kg−1 K−1 (for x = 0.3) for a field change ΔH = 5T. Respectively, the relative cooling power (RCP) decreases with increasing Ni content reaching 62 J kg−1 for x = 0.3 (ΔH = 5T). In order to study the nature of the PM-FM phase transition, a master curve behavior for the temperature dependence of ΔSM measured for different maximum fields is proposed.