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Magnetocaloric functional properties of $Sm_{0.6}Sr_{0.4}MnO_3$ manganite due to advanced nanostructured morphology

The magnetocaloric effect (MCE) is the key concept to produce new, advanced, freon-like free, low cost and environmental friendly magnetic refrigerators. Among several potential materials, $Sm_{0.6}Sr_{0.4}MnO_3$ manganite presents one of the highest MCE value in comparison to all other known manganites; however, its studied was only concentrated on the bulk material. To overcame this lack of the information we successfully produced advanced nanostructures, namely nanoparticles and nanotubes of that highlighted manganite by using a sol-gel modified method. High resolution transmission electron microscopy revealed nanoparticle and nanotube diameters of 29 nm and 200 nm, respectively; and, in addition, this technique also showed that the wall of the nanotube is formed by the nanoparticles with 25 nm of diameter. The magnetocaloric potentials, $ΔS_M$ versus T curves, of the nanostructures were obtained and they are broader than the their bulk counterpart. This increases the useful temperature range of a magnetic refrigerator. But also an undesired M-shape profile for the nanotube sample was observed, due to the rising of a superparamagnetic behavior. These results also evidenced the existence of a nanoparticle size threshold below which the advantage to make the transition wider is no longer valid.