Influence of post-annealing, defect chemistry and high pressure on the magnetocaloric effect of non-stoichiometric La0.8-xK0.2Mn1+xO3 compounds

Abstract

In view of the need to limit harmful emissions of chlorofluorocarbons into the atmosphere and sharp increase in life support areas that require cooling, the search for new magnetocaloric materials is very urgent at the present time. In this work, we suggest the ways for improving the magnetocaloric properties of the promising La0.8-xK0.2Mn1+xO3 (0 ≤ x ≤ 0.2) perovskites and compare the functional properties of nanocrystalline samples of about 40 nm with polycrystalline samples of about 1 μm. The structure, morphology, magnetic properties and magnetocaloric effect of these compounds were studied as a function of manganese doping level. The main crystalline phase of all samples is a rhombohedral R3‾c perovskite structure. The maximum doping level of excess manganese for preserving single-phase structure of the nanocrystalline La0.8-xK0.2Mn1+xO3 compounds is x = 0.10. As x increases the Curie temperature TC is increased slightly. The best composition with the highest entropy change ΔSMmax = −3.629 J/(kg⋅K) under 3 T at TC = 332 K is the La0.8-xK0.2Mn1+xO3 with x = 0.05. On the other hand, the La0.8-xK0.2Mn1+xO3 polycrystalline manganite with x = 0.10 exhibits the highest ΔSMmax = −4.176 J/(kg⋅K) under 3 T at TC = 274 K. Additionally, the effect of hydrostatic pressure P up to 1.4 GPa has been investigated for the La0.7K0.2Mn1.1O3 polycrystalline sample that leads to increasing TC to 287 K and reducing ΔSMmax to −3.145 J/(kg⋅K) under 3 T. The obtained results demonstrate the ways for controlling magnetocaloric properties of the La0.8-xK0.2Mn1+xO3 compounds under internal and external conditions.