Effects of sodium doping on physical properties of La0.75Sr0.25−xNaxCoO3 (0⩽x⩽0.2) cobaltites

The effects of Na doping on the crystallographic, magnetic, electrical and magnetocaloric properties of La0.75Sr0.25−xNaxCoO3 (0⩽x⩽0.2) samples, elaborated using the solid-state reaction method at high temperature, have been investigated. X-ray powder diffraction studies reveal that all our synthesized samples crystallize in the rhombohedral structure (R3¯c space group). Na substitution leads to an enhancement of the Co-O bond length and a reduction in the Co-O-Co bond angle and consequently to a weakening of the double exchange interaction between Co3+ and Co4+ ions. The zero field cooled (ZFC) and field cooled (FC) magnetization curves at 50 mT show thermomagnetic irreversibility. At low temperatures, with increasing Na amount, the samples change from ferromagnetic-like behavior (x=0, 0.05 and 0.1) to spin-glass one (x=0.15 and 0.2). The ferromagnetic-paramagnetic transition temperature decreases with increasing Na amount from 235 to 200 K (x=0.1). The critical exponent value, associated to the spontaneous magnetization, increases with increasing Na amount from 0.37 for x=0 to 0.57 for x=0.1. The temperature dependence of the electrical resistivity reveals three different regimes as a function of Na content: (i) for x=0, a metallic behavior is observed in the whole temperature range, (ii) for x=0.05, a metal-semiconducting transition is observed around 80 K and (iii) for x⩾0.1, we observe a semiconducting behavior in the whole temperature range. The parent compound (x=0) exhibits a maximum entropy change |ΔSMMAX| of 0.84 J kg−1 K−1 at 220 K in a magnetic applied field of 2 T. |ΔSMMAX| decreases to 0.47 J kg−1 K−1 at 185 K for x=0.1 in the same field magnitude. Well above TC, |ΔSMMAX| shows a linear dependence as a function of H2 indicating the existence of spin fluctuations inside ferromagnetic clusters.