Spin-dependent magnetism and superparamagnetic contribution to the magnetocaloric effect of non-stoichiometric manganite nanoparticles

Despite extensive research on manganites owing to their potential use in modern technologies, some aspects of their behaviour associated with changes in spin, valence, and charge states remain unclear. In this work, we investigated the structural, magneto-transport, and magnetocaloric properties of La0.8-x□xNa0.2Mn1+xO3-Δ manganite nanoparticles with strong spin-electron coupling. It was found that the perovskite structure contained different valence manganese ions, MnB3+, MnB4+, and MnA2+, even in the pristine compound. With an increase in x, increases in the Curie temperature TC, spontaneous magnetization, and magnetic entropy change ΔSM, as well as a decrease in the metal-semiconducting temperature Tms, were detected. The change in the spin value of Mn ions during the transition from ferromagnetic-metallic (μFMexp < μFMtheor) to the paramagnetic-semiconducting (μPMexp > μPMtheor) state owing to the localization–delocalization process of eg-electrons on the Mn sites was found to indicate spin-dependent magnetism. The complex critical behaviour is manifested near TC with a second-order phase transition. An additional influence of the super-paramagnetism of nanoparticles on the magnetocaloric effect was determined.