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

In the present investigation, we present the comparison of the structural, magnetic, magnetocaloric and magnetoresistance behavior of solid state and sol-gel derived La0.7Sr0.3MnO3. X-ray diffraction together with Rietveld refinement confirms the rhombohedral structure of the synthesised samples with the R3̄c 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 -ΔSM 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 -ΔSM 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 La0.7Sr0.3MnO3 a suitable candidate for multifunctional applications.

Observation of short range ferromagnetic interactions and magnetocaloric effect in cobalt substituted Gd5Si2Ge2.

We report on the observation of double transition - a first order and a second order transition in Gd5Si2-xCoxGe2 with x = 0, 0.1, 0.2 and 0.4 with the appearance of short-range ferromagnetic correlations. The first order phase transition is due to a combined magnetostructural transition from monoclinic paramagnetic phase to orthorhombic ferromagnetic phase on cooling while the second order transition arises from an orthorhombic paramagnetic to ferromagnetic phase on cooling. Structural studies show that the substituted compounds crystallize in a combination of Gd5Si2Ge2 and Gd5Si4 phases. Low-temperature X-ray diffraction measurements confirm the complete transformation from monoclinic to orthorhombic phase. DC magnetization measurements reveal an anomalous low field magnetic behaviour indicating a Griffiths-like phase. This unusual behaviour is attributed to the local disorder within the crystallographic structure indicating the presence of short-range magnetic correlations and ferromagnetic clustering, which is stabilized and enhanced by competing intra-layer and inter-layer magnetic interactions. The magnetostructural transition results in entropy changes (-ΔSM) of 9 J kg-1 K-1 at 260 K for x = 0.1, 8.5 J kg-1 K-1 at 245 K for x = 0.2 and 4.2 J kg-1 K-1 at 210 K for x = 0.4 for a field change of 50 kOe. Co substitution induces compelling crystallographic and magnetoresponsive effects in the Gd-Si-Ge system, which could be useful for potential and smart applications such as solid-state magnetic refrigeration and sensitive magnetic switching from paramagnetic to ferromagnetic state. Universal curve analysis has been carried out on the substituted samples to study the order of the magnetic transition.