Structural, magnetic and magnetocaloric properties of La0.65Ca0.35-xNaxMnO3Na-doped manganites

In this work, we report the effect of Na doping on the structural, magnetic and magnetocaloric properties in La0.65Ca0.35-xNaxMnO3 powder samples. Our samples have been synthesized by the solid-state reaction method at high temperature. The parent compound La0.65Ca0.35MnO3 crystallizes in the orthorhombic system with a Pbnm space group. Na doping induces a structural transition from orthorhombic (Pbnm) to rhombohedral (R (3) over barc) symmetry. With increasing Na amount, the unit cell volume decreases in the orthorhombic phase (x <= 0.1) and increases in the rhombohedral one. Magnetization measurements versus temperature in a magnetic applied field of 50 mT showed that all our investigated samples display a paramagnetic-ferromagnetic transition with decreasing temperature. The Curie temperature T-C increases with increasing Na content from 248 K for x = 0-315 K for x = 0.2. The critical exponent gamma defined by M-sp(T) = M-sp(0)[1 - T/T-C](gamma)deduced from the magnetization measurements as a function of the magnetic field up to 7T at several temperatures is found to be 0.34 for x = 0.1. A large magnetocaloric effect has been observed in all samples, the maximum entropy change, vertical bar Delta S-m vertical bar(max), shifts to higher values with increasing Na content. vertical bar Delta S-m vertical bar(max) reaches the highest value of 3 and 5.8J/KgK for a magnetic field change of 5T for x = 0.05 and 0.2. (C) 2008 Elsevier B.V. All rights reserved.