Investigation on the structural, magnetic, magnetocaloric and magnetotransport behaviour of La0.7Sr0.3MnO3 manganites synthesised by different routes

Abstract

In the present investigation, we present the comparison of the structural, magnetic, magnetocaloric and magnetoresistance behavior of solid state and sol–gel derived La_0.7Sr_0.3MnO₃. X-ray diffraction together with Rietveld refinement confirms the rhombohedral structure of the synthesised samples with the Rc space group. The ferromagnetic-to-paramagnetic transition temperature decreases from 360 K to 346 K for the nanocrystalline sample. The XPS measurements confirm the presence of Mn³⁺ in the synthesised samples. Furthermore, the polycrystalline sample exhibits a considerable −ΔS_M of 4.68 J kg⁻¹ K⁻¹ at 360 K for a field change of 50 kOe and a relative cooling power (RCP) of 205 J kg⁻¹. A −ΔS_M of 1.14 J kg⁻¹ K⁻¹ was obtained for the nanocrystalline sample at 346 K with an RCP of 83 J kg⁻¹. Critical exponent analysis has also been performed on both samples to establish the universality class. Both samples exhibit a distinct metal-to-insulator transition, which increases with grain size from 187 K to 334 K as a result of grain growth and decreased grain boundary. As the grain size increases, the resistivity decreases and shifts towards high temperatures with increasing magnetic fields. The itinerant electron model (IEO) which is based on the hopping of O 2p itinerant electrons has been used to explain the resistivity behaviour of the samples. It is found that the negative magnetoresistance also increases with a decrease in grain size where the highest %MR of 26% can be observed for the nanocrystalline sample. These results make La_0.7Sr_0.3MnO₃ a suitable candidate for multifunctional applications.