Assessment of the relationship between magnetotransport and magnetocaloric properties in nano-sized La0.7Ca0.3Mn1−xNixO3 manganites

Abstract

The effect of Ni2+ substitution on the magneto-transport properties and the concomitant relationship with the magnetocaloric function of nano-sized La0.7Ca0.3Mn1−xNixO3 (x = 0, 0.02, 0.07, and 0.10) perovskite manganites are reported. All the samples were synthesized using the auto-combustion method. X-ray diffraction analysis studies confirmed the phase purity of the synthesized samples. The substitution of Mn3+ ions with Ni2+ ions in the La0.7Ca0.3MnO3 lattice was also verified using this technique. Rietveld analysis indicated that the volume of the unit cell increased with increasing Ni2+ content. Zero-field-cooled and field-cooled magnetization showed that all samples underwent a paramagnetic-ferromagnetic phase transition, which was concomitant with a metal-insulator transition. A small deviation between the zero-field-cooled and field-cooled magnetization curves was observed when the measurements were carried out in a field of 1000 Oe. The Curie temperature decreased systematically from 264 K for x = 0 to 174 K for x = 0.10. Probably the doping at the Mn3+ sites with Ni2+ ions in the La0.7Ca0.3MnO3 lattice weakened the Mn3+-O-Mn4+ double exchange interaction, which led to a decrease in the transition temperature. The metal-insulator transition also shifted to lower temperatures upon Ni2+ substitution, and the value of the resistivity increased. Different conduction mechanisms were found in different temperature regions. Important physical parameters such as the polaron activation energy were obtained from the fit of the models to the experimental data. Arrot’s plots revealed a second-order nature of the magnetic transition for all the samples, which was also confirmed by Landau’s theory and universal curves. The second-order character of the magnetic phase transition observed in the pristine La0.7Ca0.3MnO3 sample may be attributed to effects of the downsizing of the particle. Interestingly, a notable increase in the value of the magnetic entropy change was observed at Ni2+ doping levels as low as 2%. The magnetoresistance underwent a great change near the magnetic transition temperature, suggesting a close relationship between the magnetocaloric effect and magnetotransport properties in La0.7Ca0.3MnO3 manganites. Such behavior can be attributed to the spin order/disorder feature, which plays a crucial role in both effects. On the other hand, the value of the magnetoresistance of the pristine La0.7Ca0.3MnO3 sample increased upon Ni2+ doping, which is probably related to the downsizing of the particles.