Magnetocaloric effect in polycrystalline La0.65Ba0.3M0.05MnO3 (M=Na, Ag, K) manganites

The effects of monovalent doping on the crystallographic, magnetic and magnetocaloric properties of La_0.65Ba_0.3M_0.05MnO₃ (M=Na, Ag, K) powder samples, elaborated using the solid state reaction method at high temperature, have been investigated. In our three samples the Mn⁴⁺ amount remains constant equal to 40%. The Rietveld refinement of the X-ray powder diffraction shows that all our synthesized samples are single phase and crystallize in the distorted rhombohedral system with R3¯c space group. All our studied samples undergo a paramagnetic–ferromagnetic transition with decreasing temperature. Using the Arrott plot, the second-order transition Curie temperature T_C for M=Na, Ag and K is found to be 310, 300 and 290 K, respectively. The magnetic entropy change, deduced from isothermal magnetization curves, exhibits a maximum |ΔS_M^Max| of about 2.65, 2.82 and 2.66 J/kg K for M=Na, Ag and K, respectively, in a magnetic applied field change of 5 T. Although these values are modest, the magnetocaloric effect extends over a large temperature range leading to an important value of the relative cooling power (RCP). The RCP values exhibit a nearly linear dependence with the magnetic applied field. The refrigeration capacity in a magnetic applied field of 1 T is found to be 28.8, 27.8 and 25.6 J/kg for M=Na, Ag and K compounds.